Tag Archives: earthquake

Earth’s inner core may actually be mushy

In 1936, Danish seismologist Inge Lehmann performed a groundbreaking study showing that Earth’s iron-rich inner core is solid although it’s hotter than the sun’s surface. Since then, our understanding of the planet’s innermost layer has been constantly refined. Earlier this year, for instance, scientists in Australia showed that the inner core may be made of two distinct layers, which suggests perhaps two separate cooling events in Earth’s history. But that’s not all.

A new potentially textbook-altering study shows that the inner core may not be entirely solid — at least not in the sense that we image a solid material. Instead, scientists have found that the deepest layer of the Earth is made of a tangled bunch of solid surfaces that sit against melted or mushy iron.

Earth sounds more and more like a meat pie

Earthquake locations (red) and seismic stations (yellow). (Photo credit: Butler and Tsuboi, 2021.)

Although the inner core is obscured by more than 4,000 miles (6,300 km) of crust, mantle, and liquid outer core, scientists have a fairly clear picture of what goes inside the bowels of the Earth. How so?

Whenever a volcano erupts or an earthquake strikes, these events generate acoustic waves whose properties, such as direction, angle, and velocity, change predictably depending on the material they encounter.

There are multiple types of seismic waves, ignoring surface waves, which are responsible for the onslaught in the wake of some very powerful earthquakes. When studying the inner layers of Earth, geophysicists mainly focus on primary waves (P-waves) and shear waves (S-waves). P-waves travel through all types of mediums, whereas S-waves only travel through solid materials.

When seismic waves created by earthquakes hit the liquid outer core then travel through the inner core, seismic data gathered from stations across the world record an extra wave going off at right angles which can only be explained by a shear wave. This is how Lehmann showed that the inner core, which is about the size of the moon, is solid. It’s not all that different from how a doctor might use a CT scanner to image what’s inside your body without cutting it open.

Geophysicists are constantly learning new things about Earth’s inner layers as seismic data improves, helped by new tools such as machine learning algorithms and other AI machines. A new study led by Rhett Butler from the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology (SOEST), found that the inner core is not exactly solid. Instead, it’s a melange of liquid, soft, and hard structures. The heterogeneous composition is especially striking in the top 150 miles (240 km) of the inner core.

“In stark contrast to the homogeneous, soft-iron alloys considered in all Earth models of the inner core since the 1970s, our models suggest there are adjacent regions of hard, soft, and liquid or mushy iron alloys in the top 150 miles of the inner core,” said Butler. “This puts new constraints upon the composition, thermal history and evolution of Earth.”

The outer core is entirely liquid and much less controversial, with its molten iron in a constant churning movement, driven by convection as it steadily loses heat from the time Earth formed to the static mantle above. It’s this motion that generates our planet’s magnetic field like a dynamo, which cushions us from harmful radiation from the sun.

However, the outer core is influenced by the inner core. So having a better grasp of its true structure helps scientists form a better understanding of the dynamics between the inner and outer cores.

“Knowledge of this boundary condition from seismology may enable better, predictive models of the geomagnetic field which shields and protects life on our planet,” said Butler.

The findings appeared in the journal Science Advances.

Could fiber optic cables predict Iceland’s next volcanic eruption?

A team from ETH Zurich and the Icelandic Meteorological Office embarked on a seemingly bizarre mission. They deployed some 13 km (8 miles) of fiber optic cable on an active volcano in Iceland. The goal wasn’t to bring faster internet to the mountain trolls, but rather to see if the cables could sense slight tremors in the volcano — an indication that an eruption may be impending.

Image credits: ETH Zurich / Hildur.

The first earthquake detector was invented some 2,000 years ago, and while things have come a long way, the underlying principle is more or less the same: a ground tremor is passed through to a detector that records the movement (and its intensity) and then translates it into a readable measure. Early seismometers were analog, but in more recent times, they all use some form of digital recording.

But things have not stayed still in the world of seismology. In recent years, for instance, a new idea emerged among some researchers: what if we could use something else, something that wasn’t designed for earthquakes, to sense this movement? Something like, for instance, fiber optic cables.

The idea is not without precedent. Operators of critical infrastructure have long used these cables to monitor their facilities, and researchers thought they could use the same for earthquake study.

“The idea of using optical fibres for multiple purposes is nothing new,” says Andreas Fichtner, a professor of geophysics in the Department of Earth Sciences at ETH Zurich. Together with Fabian Walter, a professor at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), he wants to use this technology to monitor and study glacial earthquakes. “I’m particularly interested in tiny earthquakes that originate in the glacier bed.”

The key to this approach is the cable itself. The way fiber optic cables work, pulses of light of a specific wavelength are directed through the cable continuously from one end of the cable to the other. If the cable is moved or shaken, this will change how the pulses come back to the receiver.

By analyzing the interference in the returning signals, researchers can calculate when and where the earthquakes happen, and how strong they are — and the quantity of data that can be accessed this way is enormous. “You’re basically replacing thousands of seismometers with a single cable,” says Fichtner.

Project manager Fabian Walter (at rear) and his colleague Małgorzata Chmiel check if the cable is fully functional. (Photo: Wojciech Gajek)

The problem is that despite this large volume of data, it is not exactly high-quality data we’re talking about. The cable is less sensitive than a modern seismometer, but researchers hope they can compensate for this with the sheer volume of measured points. But it won’t be easy.

“Analysing it will be a tremendous job,” Fichtner tells ETH with a smile. “We will have to come up with methods to cope with the sheer quantity of data.” The researchers expect that the first measurement campaign will produce around 20 terabytes of raw data.

But the method is promising. For their worksite, researchers chose the Grimsvoetn volcano in Iceland — an active volcano. The idea is that glacial temblors can help researchers estimate when a volcanic eruption may be coming. But this isn’t their first rodeo. A study published by the same authors on a previous test site, on the Rhône Glacier in the Swiss Alps, is already challenging some of the existing theories in the field.

The study found that the glacial quakes occur in clusters, especially at the boundary between the ice and the underlying rock. This would imply that the glacier isn’t sliding smoothly (which would produce a different type of earthquake), but rather moves forward in a jerky motion.

“That’s not what you would expect based on current theories,” explains Walter. “Glaciologists assumed that glaciers could slide because the glacier bed was well lubricated with meltwater.” Some of the mini quakes in the Rhône Glacier occur as often as once a second, and are relatively small.

“My new hypothesis is that the sliding motion of glaciers is comparable to that of tectonic plates,” adds Walter. Most of the quakes measured in the Rhône Glacier have a magnitude of −1 to −2. “That’s roughly equivalent to ice cracking when you skate on a frozen lake,” he says. “It’s not something that you can feel like a real earthquake.”

Image credits: ETH Zurich / Hildur.

The approach could be used to boost earthquake preparedness, since the infrastructure is relatively cheap — and researchers could even piggyback on existing infrastructure. But Fichtner hopes to use this for more than just measuring earthquakes.

Most of what we know about our planet’s deep geology, we know from earthquakes. When seismic waves propagate through the subsurface, they pass through different environments differently, and by picking up this information, researchers can make deductions about the subsurface. Fichtner envisions one day using the fiber-optic networks in big cities to study the geological subsurface. This approach could be doubly useful, as it could identify areas prone to failure, like faults or subsurface voids. He’s already set a test environment in the city of Bern, Switzerland, and would like to see similar setups in other cities.

“The fiber geometry was very simple – that’s one reason why Bern was the ideal test site,” Fichtner reflects.

It’s remarkable to think that infrastructure designed for something completely different could prove useful in so many different ways. Fiber optic cables are already starting to offer a relatively inexpensive way of measuring even the tiniest earthquakes. Soon enough, we may have cheap and capable seismic networks beneath our very feet — in the form of cables.

What causes earthquakes — and what you should do if hit by one

Earthquakes are one of the most striking and impactful phenomena in nature. Depending on their cause, depth, and energy, earthquakes can be more or less damaging. Most earthquakes are barely even felt, but every once in a while, a big one comes along. Well-documented throughout human history, earthquakes can hit seemingly out of nowhere, and depending on their magnitude, damage or even destroy entire cities.

Earthquakes are individually unpredictable (no one can truly predict an individual earthquake), but researchers can assess the likelihood of an earthquake striking an area. This is why we know, for instance, that an earthquake in the San Francisco area is quite likely in the near future.

Some areas are more prone to earthquakes than others (areas around countries such as Chile or Japan, for instance), and this has to do with their geology and the mechanisms that produce earthquakes.

What Causes an Earthquake?

Earthquakes on Earth are mostly driven by plate tectonics. Essentially, the Earth’s outer shell (the crust) is divided into large slabs of rock called “plates”, like a jigsaw puzzle. These plates glide on the planet’s mantle and very slowly slide by each other, and sometimes, they collide with each other.

The vast majority of earthquakes happens around the edge of these tectonic plates — so much so that if you look at a global map of earthquakes, you can immediately tell where most of these edges are.

Distribution of earthquakes across the world by magnitude. Credit: Wikimedia Commons.

This geologic movement also causes displacement and stress inside the tectonic plates, producing geological faults. So essentially, an earthquake is caused by tectonic movement — or at least, most of them are.

Earthquakes can also be caused by volcanoes or even human activity; in addition, some very deep earthquakes (which occur at hundreds of km below the surface) are caused by mineral phase changes, with a mineral common in the upper mantle (olivine) undergoing a transformation that weakens the whole rock temporarily, causing it to collapse.

Earthquakes send out different types of waves, both towards the surface of the planet, and to the depths. The first and fastest waves are called the Primary (or P) waves. Neither these, nor the secondary (S) waves are considered to be very dangerous. Instead, the following waves, called surface waves, tend to do the most damage on the surface. This is why we sometimes get a warning of a few tens of seconds when it comes to an earthquake: because special equipment can already sense the P waves, and the surface waves arrive a bit later, as you can see on the following seismogram.

How to Stay Protected from Earthquakes?

If you live in an area that is predisposed to earthquakes (or plan on moving to one), it pays to have at least some basic idea of how to protect yourself and your property. Here are some of the basic tips, as recommended by the CDC:

  • DROP down onto your hands and knees; do this before earthquake has a chance to knock you down. Keep in mind that you can still move in this position if necessary.
  • COVER your head and neck (and your entire body if possible) underneath a sturdy table or desk. If there is no shelter nearby, get down near an interior wall or next to low-lying furniture that won’t fall on you, and cover your head and neck with your arms and hands.
  • HOLD ON to your shelter (or to your head and neck) until the shaking stops. If the shaking shifts it around, move with it.
  • If you are inside, stay inside.
  • If you are in the kitchen and the stove is running, quickly close it before taking cover.
  • If possible, quickly move away from glass, hanging objects, bookcases, china cabinets, or other large furniture that could fall. Watch for falling objects.
  • If you are in bed and can’t reach shelter quickly, stay there and cover your head (or entire body) with a pillow.

Lastly, if your area is prone to earthquakes, you may also want to have a look at insurance. Since earthquakes are so unpredictable, insurance is the only way of ensuring that if damage does happen, you at least get reimbursed. Homeowner’s and renter’s insurance doesn’t typically secure the destruction of property caused by earthquakes. The only possible exception to this may be if your homeowner’s insurance policy covers fires caused by earthquakes.

Many individuals in densely populated areas deal with earthquakes, so the earthquake insurance cost generally isn’t very high – although the price will vary by region and property value.

Earthquakes can have a severe and long-lasting impact on people’s lives, and we’ve seen all sorts of disasters caused by earthquakes over the years. It’s important to be aware of any earthquake risks in your area, and try to choose earthquake-resistant buildings. If there are risks, take the necessary precautions and be on your toes should an earthquake actually happen.

Google expands its earthquake detection system to Greece and New Zealand

First launched in the US, Google is now expanding its Android-based earthquake detection and alert system to Greece and New Zealand. Users will get warnings of earthquakes on their phones, giving them time to get to safety. The earthquakes won’t be detected by seismometers but by the phones themselves.

Image credit: Flickr / Richard Walker

It’s the first time the tech giant will handle everything from detecting the earthquake to warning individuals. Mobile phones will first sense waves generated by quakes, then Google will analyze the data and send out an early warning alert to the people in the affected area. Users will get the alert automatically, unless they unsubscribe.

When it launched the service in California, Google first worked with the US Geological Survey and and the California Governor’s Office of Emergency Services to send out earthquake alerts. This feature later became available in Oregon and will now expand to Washington in May – and eventually to even more states in the US.

Mobile phones are already equipped with an accelerometer, which can detect movement. The accelerometer can also detect primary and secondary earthquake waves, acting like a “mini seismometer” and forming an earthquake detection network. Seismometers are devices used to detect ground movement.

Traditional warning systems use seismometers to interpret an earthquake’s size and magnitude, sending a warning via smartphone or loudspeakers to residents. Even if they come seconds before the quake hits, these warnings can buy valuable time to take cover. But seismometers can be difficult and expensive to develop.

That’s why a warning system that can rely on smartphones has a lot of potential. Richard Allen, a seismologist at the University of California, Berkeley, told Science that Google’s interest in building quake-sensing capabilities directly into Android phones was an enormous opportunity, or, as he calls it, a “no brainer.”

“It’d be great if there were just seismometer-based systems everywhere that could detect earthquakes,” Marc Stogaitis, principal Android software engineer at Google, told The Verge last year. Because of costs and maintenance, he says, “that’s not really practical and it’s unlikely to have global coverage.”

Earthquakes are a well-known threat in Greece and New Zealand, where Google’s service is being deployed. Greece is spread across three tectonic plates, while in New Zealand, the Pacific Plate collides with the Australian Plate. Neither country has deployed an operational warning system, which created an opportunity for the tech giant.

Caroline Francois-Holden, an independent seismologist who until recently worked at GNS Science, told Science that many earthquakes in New Zealand originate offshore, where few phones are found. This might make Google’s system less than ideal. “Any earthquake early warning system needs to be designed with that in mind,” she said.

There are other limitations, too. Those closest to the earthquake won’t get much advance warning since they’ll be the first ones to detect the quake. But their phones will help give a heads-up to others farther away, giving them crucial time to take shelter. But as Android is the leading OS system for smartphones, this service probably has a lot of room to grow.

Researchers detect a boomerang earthquake under the Atlantic Ocean

Earthquakes usually start small and then extended outwards, causing tremors in and around their path. They may have aftershocks, but after a while, it’s over. But sometimes, earthquakes can go ‘boomerang’, spreading away from the initial rupture and then returning back at higher speeds.

Now, for the first time, researchers have detected such a boomerang earthquake.

A reconstructed image of the Romanche fracture zone

An international group of researchers found evidence of an unprecedented boomerang earthquake that affected the seabed of the Atlantic Ocean back in August 2016. It took place in the Romanche fracture zone, which is located near the equator, mid-way between the east coast of Brazil and the west coast of Africa. The earthquake, detected by undersea seismometers in the region and by distant monitoring stations, had a 7.1 magnitude.

In a new study, researchers showed that the temblor went one way first but then turned around and came back for more, increasing its speed in the process, the authors argued. It was an ultra-fast earthquake.

“Whilst scientists have found that such a reversing rupture mechanism is possible from theoretical models, our new study provides some of the clearest evidence for this enigmatic mechanism occurring in a real fault,” said lead researcher and seismologist Stephen Hicks from Imperial College London in a press release.

Reviewing the seismic data, the authors argued that the earthquake had two phases. First, the rupture went upward and eastward to where the fracture zone meets the Mid-Atlantic Ridge. Then, it unusually expanded westwards, with the tremors going to the center of the fault at a speed of up to six kilometers per second.

via GIPHY

The explanations behind the phenomenon are only speculative so far. Nevertheless, the researchers believe that the first phase of the earthquake released enough fracture energy in order to start the reversal rupture in the westerly underwater land. Further studies would be needed to verify this theory.

It’s not the first-time seismologists looked at backward-propagating earthquakes, but until now the evidence was sparse and based on theoretical models. That’s why the researchers behind the study argue this is a first-of-its-kind type event, detecting a boomerang earthquake in the real world. Boomerang ruptures were also observed in the past in slow earthquakes, which progress slowly over days or months, Jean-Paul Ampuero of the Université Côte d’Azur in France told National Geographic. There were also hints of these events in other quakes. For example, the Tohoku earthquake may have had a boomeranged rupture.

But the circumstances of this particular earthquake are unique. The researchers believe that the study will now allow other scientists to find similar patterns in other earthquakes.

This will add new scenarios into their modeling and improve earthquake impact forecasts in the future, highly relevant to prevent damages.

“Studies like this help us understand how past earthquakes ruptured, how future earthquakes may rupture, and how that relates to the potential impact for faults near populated areas,” Kasey Aderhold, a seismologist with the Incorporated Research Institutions for Seismology, told National Geographic.

The study was published in Nature Geoscience.

The coronavirus-induced anthropause is now visible in seismic vibrations

Every step you take is a micro-earthquake — it produces tiny vibrations that propagate through the Earth. Other human activities, like construction and traffic, produce stronger vibrations that researchers can detect using instruments called seismometers.

In a new study, researchers report a near-global 50% drop in this seismic noise. This is the first global study of the pandemic on the solid Earth beneath our feet.

These tiny seismometers were instrumental in the study. Image credits: Imperial College London.

The anthropause

I was but a wee student when our university group got to see a seismograph. It was an old unit, hidden in a dusty university room. As we were getting a brief lecture on how it works, the needle suddenly started to move, and we all freaked out a bit.

“That’s just the subway,” we were told.

Sure enough, a subway station was nearby, but the fact that human activities such as public transit can create seismic vibrations was an eye-opener.

In fact, not only is human activity detectable on seismic sensors, it’s an active problem. In some instances, humans produce so much noise that they distort information from actual earthquakes. For the last few months, however, that kind of stopped.

As the pandemic forced human activity to slow or shut down entirely, the noise lessened, says Dr. Stephen Hicks from Imperial’s Department of Earth Science and Engineering, co-author of the study:

“This quiet period is likely the longest and largest dampening of human-caused seismic noise since we started monitoring the Earth in detail using vast monitoring networks of seismometers.”

“Our study uniquely highlights just how much human activities impact the solid Earth, and could let us see more clearly than ever what differentiates human and natural noise.”

Example of activity drop in urban area. Image credits: Lecocq et al.

Although the earthquake activity has proceeded as normal, the seismic noise has dropped considerably, particularly in urban areas. We know this type of thing can happen as we’ve seen it briefly around New Years and Christmas when activity quiets down — but never on such a scale.

Some researcherrs are already calling this period the ‘anthropause’, the break in human activity. There are many ways to see the anthropause in action, from a drop in pollution levels to reduced mobility, and now, in seismic activity.

Do the Raspberry Shake

To get a clear picture of how the anthropause is ‘visible’ in the subsurface, researchers used data from 268 stations in 117 countries, even using some citizen seismometer stations such as Raspberry Shakes. Technology such as the Raspberry Pi (a credit-card sized computer, which the Raspberry Shake is also based on) has opened up a new world of scientific projects, including seismic research.

Researchers observed the effects of lockdowns starting in late January 2020 in China, and then expanding into Europe and the rest of the world from March onwards. The noise level reduction was, at times, larger than that observed over Christmas or New Years. The largest drop in vibrations was observed in the densest-populated areas such as Singapore and New York City, but the team also observed the anthropopause in remote areas such as Germany’s Black Forest and the town of Rundu in Namibia.

The global reduction in seismic noise. Image credits: Lecocq et al.

In addition to being an interesting view at the drop in human activity, this also highlights the problems of the day-to-day noise associated with things like traveling, drilling, and construction.

Researchers often monitor hazardous areas using seismic information. If our activity is concealing these natural signals, it may be increasingly difficult to predict impending hazards (such as landslides or volcanic eruptions).

“With increasing urbanisation and growing global populations, more people will be living in geologically hazardous areas,” explained seismologist and lead author Thomas Lecocq of the Royal Observatory of Belgium.

“It will therefore become more important than ever to differentiate between natural and human-caused noise so that we can ‘listen in’ and better monitor the ground movements beneath our feet. This study could help to kick-start this new field of study.”

The study’s authors hope that their work will inspire other further research on the seismic lockdown Dr. Hicks concludes:

“The lockdowns caused by the coronavirus pandemic may have given us a glimmer of insight into how human and natural noise interact within the Earth. We hope this insight will spawn new studies that help us listen better to the Earth and understand natural signals we would otherwise have missed.”

Seismic waves reveal surprisingly widespread blobs near the Earth’s core

Our planet’s core might be pockmarked with hot blobs. We don’t know what they are, we don’t know where they’re from, but according to a new study, they’re there.

The blobs in the core. Image credits: Doyeon Kim/University of Maryland.

Ever stopped and wondered just how we know so much about the Earth’s interior? Since we’re kids, we’re told that the Earth has a crust, a mantle, and a core, but how do we know this? The Earth’s radius measures thousands of kilometers, and the deepest hole mankind has ever dug only goes down to 10 km, so it’s not like we actually went there and saw what was going on.

Most of the information we have about the Earth’s structure comes from earthquakes.

When an earthquake takes place, it sends out seismic waves in all directions. These waves are essentially acoustic waves, propagating throughout the planet’s interior. Seismologists detect these waves using specialized stations placed all around the world, and by analyzing these waves, they can understand some of the properties of the planet’s structure, similar to an ultrasound. This is exactly what happened here.

Researchers looked at echoes generated by a specific type of wave. This particular type of wave travels along the core-mantle boundary and is called a shear wave. But looking for a single wave on a seismogram is very challenging — the wave from your earthquake needs to travel to the planet’s core and then back to the surface, where we can detect it. So instead, researchers tried a different approach.

Seismogram example from the 1906 San Francisco earthquake.

Using a machine-learning algorithm, they analyzed 7,000 seismograms from hundreds of big earthquakes around the Pacific Ocean from 1990 to 2018, looking for similarities and patterns in the data. A smudge in the seismograph might be a coincidence, but the same smudge in hundreds of seismograms has meaning — and in this case, researchers found quite a few smudges.

Correlation in smudges on different seismographs. Image credits: Doyeon Kim

The findings suggest that there are widespread areas around the Earth’s core where seismic waves travel at a lower-than-normal velocity. These low-velocity areas are thought to represent hot, molten blobs — and according to this study, the core is much more blobby than we thought.

In particular, the team found a lot of these hot blobs under the Marquesas Islands, a group of volcanic islands about halfway between South American and Australia.

“We were surprised to find such a big feature beneath the Marquesas Islands that we didn’t even know existed before,” said geologist Vedran Lekić of the University of Maryland.

“This is really exciting, because it shows how the algorithm can help us to contextualise seismogram data across the globe in a way we couldn’t before.”

The algorithm itself shows great promise. It’s called Sequencer and was designed to run through large astronomical datasets looking for patterns. Now that researchers have adapted it to different types of data, and this first find is already exciting.

“We were surprised to find such a big feature beneath the Marquesas Islands that we didn’t even know existed before,” said Vedran Lekić, an associate professor of geology at UMD and a co-author of the study. “This is really exciting, because it shows how the Sequencer algorithm can help us to contextualize seismogram data across the globe in a way we couldn’t before.”

Researchers knew that some of these can exist, but they turned out to be much more common than expected — potentially hinting that they may also be present in other areas of the planet’s interior.

“We found echoes on about 40% of all seismic wave paths,” Lekić said . “That was surprising because we were expecting them to be more rare, and what that means is the anomalous structures at the core-mantle boundary are much more widespread than previously thought.”

In addition, since the Sequencer algorithm has already proven to be quite robust, researchers say that it could potentially be adapted to other types of research as well.

“Exploring a large dataset with the Sequencer enables a data-driven analysis of seismic waveforms without any prior expectations. We anticipate this approach to be useful for many types of datasets beyond seismograms,” the researchers conclude.

Journal Reference: D. Kim, V. Lekić, B. Ménard, D. Baron and M. Taghizadeh-Popp. Sequencing Seismograms: A Panoptic View of Scattering in the Core-Mantle Boundary Region. Science, 2020 DOI: 10.1126/science.aba8972

NASA presents first insights from Martian earthquakes

A whopping 174 seismic events have been recorded by the lander’s seismometer on Mars, indicating an active plate with intriguing tectonics.

Artist’s rendition of NASA’s lander. Image credits: NASA / JPL.

If we once thought Mars was a barren and boring place, there’s no reason to believe that now. Not only did the Red Planet host liquid water and an atmosphere, it still seems to have some semblance of a tectonic system.

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (or InSight) mission was launched in 2018 to study the subsurface of the planet. The plan was fairly straightforward: send a lander on Mars, and deploy a few geological sensors, including a seismometer and a thermal probe. The goal is to study Mars’ early geological evolution, understanding how it formed and evolved, and whether it is active today.

InSight (shown as a star) landed on an ancient volcanic plain south of Elysium Mons and north of the Martian “equator”. Image credits: Banerdt et al, Nature Geoscience.

Seismic sensors offered invaluable information here on Earth. Most of what we know about the Earth’s depths comes from seismic information — the way seismic waves propagate through the crust, mantle, and core, can help us understand their size and physical properties. While a single sensor would be limited in scope, it would still offer an unprecedented view into the Martian depths.

“This is the first mission focused on taking direct geophysical measurements of any planet besides Earth, and it’s given us our first real understanding of Mars’ interior structure and geological processes,” said Nicholas Schmerr, an assistant professor of geology at UMD and a co-author of the study. “These data are helping us understand how the planet works, its rate of seismicity, how active it is and where it’s active.”

In a flurry of new papers, NASA researchers presented the first findings from InSight, including the identification and study of almost two hundred earthquakes.

Comparison between seismic waves from two marsquakes (top, brown) and two earthquakes (bottom, blue). Image credits: Banerdt et al, Nature Geoscience.

The waveforms of the seismic waves showed that most earthquakes were high-frequency and low-intensity.

However, over 20 earthquakes had a magnitude of 3-4, and several were low-frequency (potentially indicative of tectonic movement). Three showed wave patterns distinctly similar to tectonic quakes on Earth. Researchers believe that they might be able to identify the source of these earthquakes.

“These low-frequency events were really exciting, because we know how to analyze them and extract information about subsurface structure,” said Vedran Lekic, an associate professor of geology at UMD and a co-author of the study. “Based on how the different waves propagate through the crust, we can identify geologic layers within the planet and determine the distance and location to the source of the quakes.”

In addition to the seismometer, the mission also deployed the first ground-based magnetometer, capable of studying the planet’s crustal magnetic field. Although satellite missions have also measured crustal magnetization, land-based surveys can provide more detailed and precise information about the planet’s localized magnetic field. Measurements showed a stronger-than-anticipated magnetic field, consistent with an Earth-like ancient dynamo field that would have been capable of supporting an atmosphere.

This further emphasizes Mars’ past as an active, Earth-like planet.

The seismometer’s delicate sensors also provided important information about Martian weather. For instance, when strong winds lift the ground significantly, the seismometer registers a tilt in the substrate. Researchers found that these winds produce a distinct seismic signature, which, along with direct weather information, can paint a picture of daily meteorological activity.

The team reports that the winds start picking up steam at midnight, becoming stronger through the early morning, as cooler air rolls down from the highlands to the plain where the lander is carrying out its activity. This wind activity produces enough noise to mask the seismic activity during the day. From late evening until midnight, conditions become very quiet around the lander, and it’s during the night — and it’s in that period that almost all the seismic events were detected. It’s almost certain that the activity continues outside thous hours, but there’s just too much noise to detect the seismic waves.

“What is so spectacular about this data is that it gives us this beautifully poetic picture of what a day is actually like on another planet,” Lekic said.

The research papers, “First constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data,” P.Lognonné et al., and “Initial results from the InSight mission on Mars” by W. Banerdt et al., were published as part of a special issue of the journal Nature Geoscience released on February 24, 2020.

The Earth was humming because a huge underwater volcano was forming

Something was brewing underneath the Comoros archipelago, and the Earth was rumbling.

Image credits: James Tuttle Keane.

All around the world, researchers have installed seismometers that captures the Earth’s minutest vibrations. When an earthquake takes place, it might rumble the area closest to it — but echoes of this rumbling are spread all around the world and can be detected by precise equipment.

This is actually how we know what the interior of the planet looks like: vibrations spreading from one point on the Earth to the other are affected by the environment they travel in, and they carry “fingerprints” of these environments.

So when researchers picked up an unusual “humming” coming from the inside of the Earth, they took it very seriously.

Mayotte. Image in public domain.

It all started with an unusual amount of earthquakes from the island of Mayotte in the Indian Ocean — one of the areas in the Comoros archipelago between Africa and Madagascar. Over 7,000 earthquakes were detected, the most severe of which had a magnitude of 5.9.

To make matters even more mysterious, some earthquakes exhibited an unusual type of oscillations: low-frequency and almost-harmonic vibrations, almost like those from a large bell.

Example of seismic oscillations. Image credits: Cesca et al (2020), Nature Geoscience.

Unfortunately, there were no seismic monitors on the ocean floor in the area where earthquakes were occurring, so researchers had to rely on seismographs farther away. But after a year of hard work, they managed to piece together what had happened. Although there was no previous indication of volcanism in that area, the seismic sign is indicative of an emerging underwater supervolcano, says Simone Cesca from the German GeoForschungsZentrum (GFZ).

“We interpret this as a sign of the collapse of the deep magma chamber off the coast of Mayotte,” explains Eleonora Rivalta, co-author of the scientific team. “It is the deepest (~30 km) and largest magma reservoir in the upper mantle (more than 3.4 cubic kilometers) to date, which is beginning to empty abruptly.”

The existence of the volcano was also confirmed by a separate investigation, but this research could help piece together what happened as the volcano was forming, and could help us make sense of similar events that would take in the future.

Luckily, despite the significant earthquakes, there were no casualties and major property damage. Nevertheless, researchers will keep a close eye to see how the volcano continues to develop.

“Since the seabed lies 3 kilometers below the water surface, almost nobody noticed the enormous eruption. However, there are still possible hazards for the island of Mayotte today, as the Earth’s crust above the deep reservoir could continue to collapse, triggering stronger earthquakes,” says Torsten Dahm, head of the section Physics of Earthquakes and Volcanoes at the GFZ.

The study “Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation” has been published in Nature Geoscience.

Hurricanes trigger ‘stormquakes’ on the bottom of the ocean

Hurricane Irene was the first hurricane of the 2011 Atlantic season. When it struck the U.S. East Coast, its path crossed areas populated by more than 50 million Americans. It also generated thousands of ‘stormquakes’ in its wake. Credit: NASA Earth Observatory.

A hurricane and earthquake happening at the same time sound like an unlikely combo — but at the bottom of the ocean, it isn’t. According to a new study, it’s quite common for a powerful hurricane to cause the seafloor to rumble like a 3.5-magnitude earthquake.

Researchers call this phenomenon a ‘stormquake’. They occur when a large storm forms above the ocean, triggering a secondary wave that interacts with the seafloor. The interaction causes the seafloor to shake, primarily when it is flat and over a large continental shelf.

“During a storm season, hurricanes or nor’easters transfer energy into the ocean as strong ocean waves, and the waves interact with the solid earth producing intense seismic source activity,” said Wenyuan Fan, an assistant professor of Earth, Ocean and Atmospheric Science at Florida State University and lead author of the new study.

A new geophysical phenomenon

There’s no need to worry, though. Although powerful tsunamis such as those that struck Japan in 2011 are formed when an earthquake occurs on the seafloor, stormquakes are simply too weak to pose a threat. In fact, they represent a new useful seismic source that scientists can use to investigate the planet’s structure, especially in locations where we lack seismic instruments or earthquakes. Moreover, they can be used to study ocean wave dynamics during large storms, which ultimately should improve forecasting in the future.

“We can have seismic sources in the ocean just like earthquakes within the crust,” Fan said. “The exciting part is seismic sources caused by hurricanes can last from hours to days.”

The study, which was published was published in the journal Geophysical Research Letters, notes that 14,077 stormquakes have been recorded between 2006 and 2015. Hurricane Ike, which struck the Caribbean Islands and Texas in 2008, and Hurricane Irene, which washed on the East Coast, created the most stormquakes in their path.

But, not all hurricanes lead to stormquakes. The massively powerful Hurricane Sandy barely generated one, suggesting that local oceanographic features and seafloor topography are essential to their formation. No stormquake was detected off the coast of Mexico or the U.S. East Coast from New Jersey to Georgia.

“We have lots of unknowns,” Fan said. “We weren’t even aware of the existence of the natural phenomenon. It really highlights the richness of the seismic wave field and suggests we are reaching a new level of understanding of seismic waves.”

Ripping the desert apart: Stunning images show Ridgecrest earthquakes shattering the ground

The 2019 Ridgecrest earthquakes struck California on the 4th and 5th of July, with magnitudes of 6.4, 5.4, and 7.1, respectively. Millions felt the shaking, and even more were frightened by a potentially devastating earthquake. Now, satellite images show just how powerful the earthquake was.

Before-and-after satellite images show rock displacement following the 7.1 magnitude earthquake. Image credits: Sotiris Valkaniotis / Google Earth / Digital Globe.

California is pierced by the San Andreas fault, which extends roughly 1,200 kilometers (750 mi). The fault poses great seismic threat, with many seismologists suspecting that the fault is overdue for a major earthquake.

The Ridgecrest earthquakes should also be understood in the context of the San Andreas fault. Although the earthquakes caused relatively minor damage, the effects were felt across much of Southern California, as well as Arizona and Nevada, and even Mexico. It’s estimated that some 30 million people experienced the main shock.

However, the desert satellite images help convey the full power of the earthquake.

A long scar produced along a geological fault in the aftermath of the Ridgecrest earthquake of July 5. The dark stain is water leaking from a pipeline that had ruptured. Image credits: Sotiris Valkaniotis / Google Earth / DigitalGlobe.

A geological fault is essentially a crack in the Earth’s crust. Typically, major faults are associated with, Earth’s tectonic plates, but smaller faults emerge all around the world. In an active fault, the two sides of the fault tend to move relative to each other over time — this movement can cause earthquakes.

A fault on the scale of San Andreas isn’t neatly carved through California. It produces many other faults that slice up in ribbons. These secondary faults can be ruptured by earthquakes, causing further temblors. When this happens, the effects can be severe.

Note the displacement on the two sides of the fault. Image credits: Sotiris Valkaniotis / Google Earth / DigitalGlobe.

These images are among the best of their kind. For starters, the earthquakes occurred in the desert, where displacements can be followed with relative ease. There’s no vegetation and nothing to obscure the geologists’ (or the satellites’) eyes.

“I like to think about the desert as an unpainted canvas,” said Ken Hudnut at the US Geological Survey. “And the earthquake tore a big rip through the desert canvas.”

These stunning visual effects have been produced using openly available satellite imagery from Google Earth and DigitalGlobe by earthquake geologist Sotiris Valkaniotis, who is based in Greece. It should be said that while the earthquake and faults are the main cause of the displacement, other causes such as landslides or liquefaction can also cause displacement.

Stunning animation shows how Marsquakes look like

The Earth has lots of earthquakes, but it’s not the only place with temblors. While Martian quakes are much smaller in intensity, they do exist — here’s how the seismic waves propagate through the planet:

Seismic waves from a marsquake as they move through different layers of the Martian interior. Credits: NASA/JPL-Caltech/ETH Zurich/Van Driel

Seeing the inside

When NASA sent astronauts to the moon in the Apollo 11 mission, they also had them deploy scientific instruments — including seismometers. But it took a couple more decades until the agency’s InSight lander brought the first seismometer to Mars in late 2018. Called the Seismic Experiment for Interior Structure (SEIS), the seismometer made history in April 2019, when it detected the first marsquake.

The mission is led by researchers at ETH Zurich in Switzerland who monitor and analyze the data. Because Mars doesn’t have active plate tectonics, it also has much fewer (and less intense) earthquakes. These temblors pose no realistic risk whatsoever — the purpose of the seismometer is to help researchers better understand the inside of the planet.

InSight’s seismometer has a cozy shelter on Mars. Credits: NASA.

Much of what we know about Earth’s internal structure also comes from seismology. When an earthquake occurs, it spreads out energy in the form of seismic waves. There are different types of waves which propagate differently through the Earth’s inside. By calculating the time of arrival between different types of waves, their amplitude, and several other parameters, seismologists make certain deductions about the Earth’s structure.

In a way, it’s a bit like how an ultrasound reading can reveal a baby inside a mother’s womb, except the scale and accuracy of the procedures is very different. Ultrasounds and seismic waves are both acoustic waves and they get similarly reflected and refracted. But researchers didn’t stop here.

Feeling a marsquake

Researchers at ETH took things even further: they wanted to see how a marsquake feels, compared to one on Earth or on the moon.

Of course, since the marsquake is much weaker than its earthly equivalent, the signal is also weaker. The team had to amplify the marsquake signals by a factor of 10 million in order to make the barely-perceptible tremors comparable to earthquakes. Moonquakes were similarly amplified.

The reason why quakes on different types of planets can feel differently is that they are affected by the material the waves pass through. We’re still in the very early days of studying marsquakes but so far at least, the results are encouraging.

The 2020 Mars Rover will also feature an instrument that will help researchers “see” beneath the Martian surface. The ground penetrating radar will use electromagnetic waves to create a high-resolution visualization of a Martian subsurface, at depths of up to 10 meters.

Tides are turning in earthquake discoveries

It’s been suggested for a while that tides can have an impact on earthquakes that occur along mid-ocean ridges. However, no one knew why the frequency increased during periods of low tides. However new research might have found an answer.

A study published in Nature Communications by Christopher Scholz and Columbia University has found that it comes down to the magma below the mid-ocean ridges. The research was made possible by a network of seafloor instruments along the Pacific’s Juan de Fuca ridge — a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America.

“Everyone was sort of stumped, because according to conventional theory, those earthquakes should occur at high tides,” explained Scholz, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory. “It’s the magma chamber breathing, expanding and contracting due to the tides, that’s making the faults move.”

Since most mid-ocean ridges feature vertical faults — those featuring steeply inclined planes — scientists assumed earthquake-generated slips would most likely occur at high-tides since the upper block slides down with respect to the lower one during movement. But seismic data was actually showing that the opposite occurred; the fault slips down during low tide, when forces are actually pulling upwards “which is the opposite of what you’d expect,” said Scholz.

In the end, it all came down to the volcano’s magma chamber, a component no one had yet considered as part of this mechanism. The team realized that when the tide is low, there is less water sitting on top of the chamber, so it expands. As it puffs up, it strains the rocks around it, forcing the lower block to slide up the fault, and causing earthquakes in the process.

When the team charted the earthquake rate versus the stress on the fault, they realized that even the smallest amount of stress could produce an earthquake. The tidal data helped to calibrate this effect, but the triggering stress could be caused by anything — such as the seismic waves from another earthquake, or fracking wastewater pumped into the ground.

“People in the hydrofracking business want to know, is there some safe pressure you can pump and make sure you don’t produce any earthquakes?” said Scholz. “And the answer that we find is that there isn’t any — it can happen at any level of stress.”

Scholz also adds that the tidal earthquakes in this region are “so sensitive that we can see details in the response that nobody could ever see before.” Of course, the scale of things should also be considered: small stresses over a small area isn’t going to trigger a massive earthquake.

NASA detects first evidence of a Marsquake

Credit: Flickr, NASA.

On the 128th sol of the Martian lander Insight, researchers discovered a “Marsquake.” Scientists recorded the tremors with a French-made dome probe, the Seismic Experiment for Interior Structure (SEIS). While the event was too small to provide any useful information — if it had occurred on Earth, it wouldn’t have even registered — it was still the first quake recorded on Mars caused by forces inside the planet.

“We’ve been waiting months for our first marsquake,” said Dr. Philippe Lognonné, the Principle Investigator for SEIS. “It’s so exciting to finally have proof that Mars is still seismically active. We’re looking forward to sharing detailed results once we’ve studied it more and modeled our data.”

While Mars doesn’t have tectonic plates, which cause most of Earth’s quakes, both planets and the Moon experience the kind of quake caused by faults, or fractures in their crusts. As heavy masses and slow cooling add stress to the crust, it cracks, releasing energy.

Thousands of quakes were discovered on the Earth’s moon between 1969 and 1977 using five seismometers installed by Apollo astronauts.

“The Martian Sol 128 event is exciting because its size and longer duration fit the profile of moonquakes detected on the lunar surface during the Apollo missions,” said Lori Glaze, Planetary Science Division director at NASA Headquarters.

So far, the InSight team has yet to confirm the cause of the tremor, which was picked up on April 6. Three other signals, which occurred on March 14 (Sol 105), April 10 (Sol 132) and April 11 (Sol 133), could also be of seismic origin. The signals were far more enigmatic to the InSight team, but at least two of those do not appear to have been caused by wind or other unwanted sources of noise. Those signals were found to be much weaker than those on Sol 128 and were only detected by SEIS’s ultra-sensitive VBB sensors.

“InSight’s first readings carry on the science that began with NASA’s Apollo missions,” said InSight Principal Investigator Bruce Banerdt of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “We’ve been collecting background noise up until now, but this first event officially kicks off a new field: Martian seismology!”

 

Contour maps depict changes in gravity gradient immediately before the earthquake hits. Credit: Kimura Masaya.

New gravity earthquake detection method might buy more time for early warnings

Scientists from Japan, one of the most seismically active regions of the globe, claim that a new earthquake detection method based on gravity could provide an earlier warning than traditional methods.

Contour maps depict changes in gravity gradient immediately before the earthquake hits. Credit: Kimura Masaya.

Contour maps depict changes in gravity gradient immediately before the earthquake hits. Credit: Kimura Masaya.

In 2011, a magnitude-9 earthquake hit eastern Japan, along a subduction zone where two of Earth’s tectonic plates collide. The tremor came as a one-two punch, generating a huge tsunami in the process which led to the meltdown of the Fukushima Daiichi nuclear power plant. The effects of the powerful quake were devastating, with more than 120,000 buildings left in rubble and $235 billion-worth of incurred damage.

Japan handled the onslaught bravely and admirably. Thanks to its sophisticated network of sensors, Tokyo residents were given a minute warning via texted alerts on their cell phones before the city was hit by strong shaking. These sensors also recorded a wealth of data that is still keeping researchers busy with work that might lead to improved earthquake detection.

Exactly 8 years after the Tohoku earthquake, a team of researchers from the University of Tokyo’s Earthquake Research Institute (ERI) used some of this data to argue that a new detection method based on gravimeters could theoretically detect earthquakes earlier than seismometers.

Gravimeters are sensitive devices for measuring variations in the Earth’s gravitational field. They’re typically employed by industries to prospect subterranean deposits of valuable natural resources, including petroleum and minerals, but also by geodesists who study the shape of the earth and its gravitational field.

When an earthquake occurs at a point along the edge of a tectonic plate, it generates seismic waves that radiate outward at up to 8 kilometers per second. These waves transmit energy through the earth, thereby altering the density of the subsurface material they pass through. Denser material has a slightly greater gravitational attraction than less dense material, and since gravity waves propagate at the speed of light, it’s possible to measure these changes in density before the arrival of a seismic wave.

The Japanese researchers combined gravimetry and seismic data, which they fed into a complex signal analysis model. The results scored 7-sigma accuracy, meaning that there’s only a one-in-a-trillion chance that they are incorrect.

“This is the first time anyone has shown definitive earthquake signals with such a method. Others have investigated the idea, yet not found reliable signals,” ERI postgraduate Masaya Kimura said in a statement. “Our approach is unique as we examined a broader range of sensors active during the 2011 earthquake. And we used special processing methods to isolate quiet gravitational signals from the noisy data.”

TOBA prototype. Credit: Ando Masaki.

TOBA prototype. Credit: Ando Masaki.

At the moment, the researchers are working on a new kind of gravimeter called the torsion bar antenna (TOBA), which aims to be the first instrument specifically designed to detect earthquakes by gravity. A network of such devices could theoretically warn people 10 seconds before the first seismic waves arrive from an epicenter 100 km away. These precious extra seconds could mean the difference between life and death in many situations.

“SGs and seismometers are not ideal as the sensors within them move together with the instrument, which almost cancels subtle signals from earthquakes,” explained ERI Associate Professor Nobuki Kame. “This is known as an Einstein’s elevator, or the equivalence principle. However, the TOBA will overcome this problem. It senses changes in gravity gradient despite motion. It was originally designed to detect gravitational waves from the big bang, like earthquakes in space, but our purpose is more down-to-earth.”

The findings appeared in the journal Earth, Planets and Space.

Mermaids offer a rare view of our oceans’ subsurface

In this case, however, MERMAIDS are seismic sensors, deployed around the world’s oceans.

Floating seismometers dubbed MERMAIDs — Mobile Earthquake Recording in Marine Areas by Independent Divers — reveal that Galapagos volcanoes are fed by a mantle plume reaching 1,900 km deep. This photo shows one rising to the surface. Image credits: Yann Hello, University of Nice.

Most of what we know about the Earth’s inside comes from seismic studies. Just like a doctor analyzes your body using ultrasound, seismic stations can pick up waves from earthquake to image the interior of the planet or deduce some of its characteristics. The problem, however, is that most of our planet is covered in water — and we don’t have too many seismic stations in water.

“Imagine a radiologist forced to work with a CAT scanner that is missing two-thirds of its necessary sensors,” said Frederik Simons, a professor of geosciences at Princeton. “Two-thirds is the fraction of the Earth that is covered by oceans and therefore lacking seismic recording stations. Such is the situation faced by seismologists attempting to sharpen their images of the inside of our planet.”

With this in mind, Simons and Guust Nolet (now Professor of Geoscience and Geological Engineering) developed a new type of seismic sensor: a hydrophone. The hydrophone’s earthy cousin, the geophone, is routinely used in surveys to detect subsurface resource. Both types of sensors are essentially a very precise microphone, capable of picking up the sounds of distant earthquakes — or to be more technical, to pick up acoustic energy from earthquakes. The resulting hydrophone was fitted with a GPS and sensors for temperature and water salinity and was mounted on a platform called a MERMAID (Mobile Earthquake Recording in Marine Areas by Independent Divers).

MERMAIDs can dive down to depths as large as 3,000 m and are easily launched from even commercial or amateur vessels. They drift passively, usually at around 1,500 m deep, and whenever they detect an earthquake, they quickly rise to the surface to accurately gather GPS data and transmit data via satellite. They are currently the first marine instruments capable of transmitting seismic data in (almost) real time.

A bathymetric map of the Galápagos hotspot
region. Yellow dots show the location of MERMAIDs when the earthquakes were recorded. Image credits: Nolet et al.

The first fleet was launched in 2017 and now, an international team of researchers has presented the first scientific results.

For starters, MERMAIDs are useful to help scientists monitor abyssal currents — but that’s not even their main function. A more striking find is that the volcanoes on the Galápagos are fed by a so-called mantle plume: a magmatic source 1,200 miles (1,900 km) deep, connected to the surface volcanoes via a narrow conduit that is bringing hot rock to the surface.

These mantle plumes were first proposed in the 1970s as an important part of plate tectonics. Their existence has been confirmed in the meantime, but they have largely resisted attempts at seismic imaging because they are found in oceans, far away from seismic stations. The existence of this mantle plume was also indicated by abnormally high water temperatures.

In addition to filling in some missing puzzle pieces, the MERMAID network could also help geophysicists settle a long-lasting mystery about the Earth, which (thankfully) refuses to cool down.

“Since the 19th century, when Lord Kelvin predicted that Earth should cool to be a dead planet within a hundred million years, geophysicists have struggled with the mystery that the Earth has kept a fairly constant temperature over more than 4.5 billion years,” Nolet explained. “It could have done so only if some of the original heat from its accretion, and that created since by radioactive minerals, could stay locked inside the lower mantle. But most models of the Earth predict that the mantle should be convecting vigorously and releasing this heat much more quickly. These results of the Galápagos experiment point to an alternative explanation: the lower mantle may well resist convection, and instead only bring heat to the surface in the form of mantle plumes such as the ones creating Galápagos and Hawaii.”

The paper, “Imaging the Galápagos mantle plume with an unconventional application of floating seismometers,” by Nolet et al., has been published in Scientific Reports. doi: 10.1038/s41598-018-36835-w.

The main types of seismic waves: P, S, and surface waves

Generally speaking, there are two types of waves: body waves (which comprise of P or Primary waves and S or Secondary waves) and surface waves (Love and Rayleigh). But the long story is more complex — and much more interesting.

Seismic waves are produced by earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions. They are a form of acoustic wave, just like sound waves. The vast majority of them are associated with natural earthquakes.

What’s an earthquake, anyway?

In the broadest sense, an earthquake is just what the name suggests — any shaking of the Earth’s interior. Earthquakes can happen for a variety of reasons, but by far the most common cause is tectonic.

The Earth’s crust (the outermost layer) is split into rigid plates, all of which are moving relative to each other. The movement produces more and more stress on the ground until something eventually breaks along what’s called a geological fault. This is why, if you overlay a global tectonic plate map and a global earthquake map, you’ll see an almost perfect overlap between tectonic edges and temblors.

If you look on a world map of earthquakes (represented here), you can distinguish the tectonic plates

Volcanoes can also produce earthquakes, though they are generally less impactful than tectonic earthquakes.

Man-made explosions (for instance, atomic testing) can also produce earthquake-type features which produce seismic waves and can be detected — this is what allows remote monitoring of nuclear explosions.

For some earthquakes, the cause remains poorly understood, particularly in the case of intraplate tectonics (inside tectonic plates, not on the edges). Another cause of some very deep earthquakes is the so-called mineral phase change: atoms in minerals such as olivine can change their positions to become more tightly packed. Their chemistry remains the same, but their volume and density change dramatically if an equilibrium is reached. This requires very particular conditions to happen, but if it does happen, it creates a type of “anti-crack” and can generate massive, deep earthquakes.

Types of seismic waves

Seismologists like to split seismic waves into several categories, but the main types of seismic waves come in two categories — body waves (which move throughout entire bodies, such as the Earth), and surface waves )(which travel only on different surfaces, not through the whole body). The main types of seismic waves are the following:

  • Primary waves (P-waves). These are the “first” body waves — the ones that travel the fastest and through any type of medium (solid, liquid, gas). They propagate longitudinally on the propagation direction (think of an accordion) and are harmless in terms of earthquake damage.

  • Secondary (S-wave). These are shear waves, which arrive after the P-waves. They’re also body waves but they only propagate through a solid medium. They also rarely do any significant damage.

  • Surface waves — Rayleigh (R-wave). Surface waves (Rayleigh and Love) do by far the most damage. As opposed to body waves (S and P waves), they propagate on the surface and carry the vast majority of the energy felt on the surface — in other words, these are what you feel when you experience an earthquake. This happens because although they move slower than body waves, their particle movement is much more pronounced (see below). In the case of Rayleigh waves, the motion is of a rolling nature, similar to an ocean surface wave.

  • Surface wave — Love (L-wave). Contrary to their name, there’s nothing really lovable about the Love waves — they were named thusly after Augustus Edward Hough Love, a Professor for Natural Philosophy at Oxford University who first described the movement of the waves named after him. Love waves have a transversal (perpendicular) movement and are the most destructive outside the immediate area of the epicenter. Love waves can be devastating

Why seismic waves are important

Studying and understanding seismic waves is more than a theoretical pursuit — it’s very important for a number of reasons, which flow quite logically.

  • Detecting epicenters

If you detect an earthquake in at least three different locations, you can triangulate where the epicenter is.

There are numerous seismographs around the world, all of which measure the earthquake (seismic) waves to some extent. Because the different waves have different speeds, by detecting the arrival times at in different regions in the world, the position of the earthquake can be detected — the so-called hypocenter. Contrary to popular belief, the epicenter is not the place where the earthquake ruptures (that’s called the ‘hypocenter’), but rather is the projection of the earthquake on the surface, which can of course also be inferred from this data.

A depiction of how an earthquake is “felt” at different distances and in different geological structures.

A historical map of epicenters gives a good starting point to assess the likelihood of future earthquakes and can serve as a basic preparation, allowing city planners and residents to prepare for the likelihood of seismic events. Naturally, this leads to the next reason for studying seismic earthquakes.

  • Assessing hazards

Assessing hazards basically aims to predict the potential ground shaking intensity from future earthquakes. This can’t be done from studying seismic waves alone, it requires a lot of local geology input and external considerations (for instance, earthquakes can also cause indirect damage though processes such as landslides) — but seismology is the first step.

The U.S. Geological Survey’s 2014 earthquake hazards map indicates the hazard of shaking from earthquakes occurring during the next 50 years. Image credits: USGS.

Precise earthquake prediction (pinpointing the exact time and place of a future earthquake) is not possible and will not be possible for the foreseeable future due to the sheer complexity of the problem — but this doesn’t mean that we can’t make some guesses. Scientists predict earthquakes in odds and intervals, not in exact values. A noteworthy situation is the estimation of volcanic hazard: volcano eruptions are typically predicted by a swarm of small earthquakes, which is why most of the world’s active volcanoes are surrounded by seismic detectors.

  • Constructing better buildings

If you want to look for the best seismic engineers in the world, you’ll probably find them in places like Chile or Japan. Why? Because they need to be good, given that those are some of the seismically active places in the world.

Engineering seismology lays the bases for calculating seismic hazard, and it makes a big difference — for instance, Japan’s sophisticated engineering and strongly-enforced building codes have probably saved thousands of lives. The western coast of the US, for instance, is also quite earthquake-prone due to the San Andreas fault, and despite calls for better preparation, the area remains vulnerable.

  • 30-second warning

If you live in an earthquake-prone area, you probably have access to one type of “early” earthquake alert. Typically, these alerts can let you know when the earthquake is coming 30-60 seconds ahead of time — it’s not a lot, but in some cases, it could make all the difference.

In case you’re wondering how this is done, it has everything to do with the velocity of different seismic waves: if you recall, P-waves travel much faster than surface waves, but don’t do any real damage — the “30 seconds” are the interval between the arrival of the P waves and that of the surface (Love and Rayleigh) waves.

Can you guess where the surface waves come in? Hint: look for the biggest-looking waves.

  • Detecting explosions

If you’ve ever wondered why nations can’t just hide nuclear tests, it has a lot to do with seismic waves.

Man-made explosions generate types of waves which can be detected worldwide, and it’s essentially impossible to hide any massive explosion from the entire world (although seismic waves alone can’t reveal the nuclear or non-nuclear nature of the explosion).

Studying the Earth with seismic waves

Another completely different reason why it makes a lot of sense to study seismic waves is to study the Earth’s interior.

Since we’re kids, we’re taught that the Earth has a crust, a mantle, and a core… but how do we know that? The answer is, of course, through seismic waves.

Geologists use seismic waves to determine the depths and structures of different Earth layers. For instance, P waves travel through all types of medium, whereas S waves only travel through solid waves — this was used to deduct the fact that the mantle acts as a fluid (it’s not really a liquid, but it’s not exactly a solid either — think of it as extremely thick honey).

Seismic waves also get reflected and refracted when they travel from one medium to another. These transitions are governed by differences in density, which is why we know so much about the density of many structures deep inside the Earth. An interesting consequence of this property is that earthquakes have a “blind spot”: an area of the world where waves coming from them can’t be detected.

Much of what we know about the planet’s tectonics, the Earth’s deep structure, and even some features closer to the surface hinges on our understanding of seismic waves.[panel style=”panel-default” title=”Prospecting” footer=””]A few decades ago, people realized that they can mimic natural seismic waves through explosions or specialized machinery — at a much smaller scale. Similarly to how earthquake waves can reveal a lot about the subsurface at a large scale, these man-made waves are used to infer properties of at a smaller scale.

This is widely used as a prospection tool, particularly for oil and gas reservoirs, but to a smaller extent, also for mineral resources, water, and even environmental studies.[/panel]

Other types of seismic waves

If you’ve made it this far — first of all, congrats — you might be looking for a more detailed classification of seismic waves. Seismologists apparently love to draw up wave categories, not necessarily depicting different types of waves but rather describing where those waves have passed through. So while primary, secondary, Rayleigh, and Love waves are abbreviated by P, S, R, and L respectively, they can gain additional notations. For instance, a g notation indicates that the wave only travels through the crust, without any ocean floor in its path. Conversely, a indicates that the wave traveled or bounced on the ocean floor.

Going deeper, a J wave is an S wave in the outer core, while a K wave is a P-wave in the outer core. A c indicates that the wave reflects off the outer core, while an indicates that it bounces off the inner core.

A depiction of some wave paths.

In theory, there are an infinite of paths for waves to take — although in practice, their energy decays as the travel through the Earth. However, they can still reach an impressive number of bounces, and the notations add up. So you can end up with wave names such as PKiKP or SKS.

 

Earth’s inner core is solid, seismic waves reveal

A new study has sent ripples throughout the seismological world — a team of researchers from the Australian National University have found a new way to confirm that the Earth’s core is solid.

A simplified schematic of the Earth’s structure.

If you think about it, the very fact that we know so much about the Earth’s interior is stunning. The deepest hole we’ve dug is “only” around 12 km deep, whereas the radius of our planet is 6378 km — so how could we know so much about the Earth’s structure?

Seismic waves

Scientists use many bits and pieces of information to study this structure, but the most important clues come from seismic waves. Whenever an earthquake takes place, it sends out pressure waves in all directions — much like the acoustic waves we use to speak and hear sounds. These waves propagate through the subsurface, reflecting and refracting as they move from one environment to the other. Eventually, they reach the surface, where they are picked up by seismographs — devices that measure the movement of the earth.

Using seismographs, researchers can infer a surprisingly large amount of information. For instance, by measuring the arrival time of seismic waves at different seismograph stations, the position of the earthquakes’ epicentres can be triangulated — this is how we know where earthquakes happen.

But that’s just the start of it.

A typical seismogram.

More than just data about the earthquake itself, we can derive data about the rocks through which the seismic wave has traveled. For instance, generally speaking, there are three types of seismic waves:

  • P or Primary waves, which are longitudinal waves (these are the fastest and the first to arrive);
  • S or Secondary waves, which are shear waves; and
  • Surface waves, which are slower than both P and S waves, but have larger amplitude and more complex movements.

A simple simulation of an S wave structure.

P waves travel through any type of medium, whereas S waves only move through solid environments — not liquid. This was a huge indication that the Earth’s outer core is a liquid, as S waves don’t appear to pass through it. It’s not a liquid in a conventional sense (think of it more like viscous, molten lava, or thick honey), but it’s definitely not a solid.

A depiction of different seismic waves propagating through the Earth.

This also revealed that the inner core is solid, as S waves appear to be able to pass through it. However, the small size of the inner core makes detecting shear waves very difficult, and this theory could still use additional verification. This is where the new study comes in.

J waves

A J wave is nothing more than an S wave that passes through the inner core — seismologists love giving different names to waves depending on where they pass through, but don’t let it confuse you, it’s still a shear, S wave.

J waves are rather elusive because they have small amplitudes, and the Earth’s inner core has a relatively small volume, which means that many seismic waves simply go around it. The small, feeble amplitude is so problematic that detecting J-waves has sometimes been referred to as the “Holy Grail” of global seismology.

However, Associate Professor Hrvoje Tkalčić and PhD Scholar Than-Son Phạm believe they’ve found a new way to identify J waves. What they did is pretty creative: instead of looking directly at the wave as it comes, they simply ignore it for the most part and look at the wave signal hours after the largest rumbles have passed. By studying the similarities between these signals at two receivers after a major earthquake, they’re able to observe a correlation of patterns, and this correlation is the key to the J-wave identification. A similar approach was used by other scientists to study the thickness of Antarctic ice.

“Using a global network of stations, we take every single receiver pair and every single large earthquake – that’s many combinations – and we measure the similarity between the seismograms,” lead author Hrvoje Tkalčić explained.

“That’s called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram – a sort of fingerprint of the Earth.”

“We’re throwing away the first three hours of the seismogram and what we’re looking at is between three and 10 hours after a large earthquake happens. We want to get rid of the big signals,” Dr. Tkalčic added.

Using this approach, they confirmed that the inner core seems indeed solid, though it seems to exhibit some differences from current models. In fact, it’s a bit like two very familiar materials: gold and platinum.

“We found the inner core is indeed solid, but we also found that it’s softer than previously thought,” Associate Professor Tkalčić said.

“It turns out – if our results are correct – the inner core shares some similar elastic properties with gold and platinum. The inner core is like a time capsule, if we understand it we’ll understand how the planet was formed, and how it evolves.”

We should still wait for additional observations to confirm this study but, for now, it seems like the information about the nature of the inner core is becoming more and more solid.

This research was published in Science.

Amatrice, the epicenter of the 2016 Italian earthquake. Credit: Youtube, Wikimedia Commons.

AI can now predict where an earthquake’s aftershock will hit next

Amatrice, the epicenter of the 2016 Italian earthquake. Credit: Youtube, Wikimedia Commons.

Amatrice, the epicenter of the 2016 Italian earthquake. Credit: Youtube, Wikimedia Commons.

An aftershock is like an echo — a smaller earthquake that occurs soon after a larger one, hitting the same area as the main shock. Large magnitude quakes can generate aftershocks of varying magnitudes over a period of months. For people living and working around the site of an earthquake, the subsequent days and weeks are filled with anxiety — when will there be a new one?

Seismologists have crafted models that fairly accurately predict when an aftershock is going to take place and how violent (i.e. magnitude) it will be. Now, researchers at Google and Harvard have teamed up to produce an artificial intelligence system that can also predict where the aftershock will hit next.

The collaboration devised an AI that was fed a database of 131,000 earthquakes and the location of their subsequent aftershocks. The machine learning algorithm was instructed to spot the patterns in this complex landscape of variables upon variables.

There are a lot of things that shape a seismic event — from the composition of the ground to the interactions between tectonic plates to the ways seismic waves propagate through the Earth. Making sense of all the intricate layers upon layers can be maddening. However, this sort of high-volume pattern matching is what machine learning algorithms excel at. Such AIs are currently being used by tech giants like Facebook, Amazon, and Google to sell you virtual assistants or to show search results.

“After earthquakes of magnitude 5 or larger, people spend a great deal of time mapping which part of the fault slipped and how much it moved,” said Brendan Meade, a Professor of Earth and Planetary Sciences at Harvard University.

“Many studies might use observations from one or two earthquakes, but we used the whole database…and we combined it with a physics-based model of how the Earth will be stressed and strained after the earthquake, with the idea being that the stresses and strains caused by the main shock may be what trigger the aftershocks.”

Meade was first inspired to neural networks to predict aftershocks several years ago during his two sabbaticals at Google in Cambridge. At the time, deep learning algorithms were not as established as they are today but the idea immediately sounded too good to pass.

After years of work, Meade and colleagues came up with a model which has much better predictive power than anything before it. On a scale of accuracy from 0 to 1 — where 1 is a perfectly accurate model and 0.5 is essentially the accuracy of flipping a coin — the new AI system scored 0.849 while the previously most precise model scored only 0.583.

The neural network was able to work so well thanks a little quirk it managed to uncover all by itself. The complex calculations take into consideration a factor known as the “von Mises yield criterion”, which predicts when a material will break under a stress. It’s been mostly used by engineers in the field of metallurgy. Now, it has also found its place in earthquake science, the authors reported in the journal Nature.

“This is a quantity that occurs in metallurgy and other theories, but has never been popular in earthquake science,” Meade said. “But what that means is the neural network didn’t come up with something crazy, it came up with something that was highly interpretable. It was able to identify what physics we should be looking at, which is pretty cool.”

Another advantage of the new AI is that it works for different types of faults. Because it’s generalizable, the system can just as well predict aftershocks around slip-faults, such as those seen in California, or shallow subduction zones, as seen in Japan.

Before you get overly excited though, be aware that this AI has a number of important limitations. The system only works with aftershocks caused by permanent changes to the ground, so-called static stresses. Aftershocks, however, can also be triggered by dynamic stresses that do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone.

The AI is also too slow to work in real-time, which is a must-have considering that most aftershocks occur in the first day following an important earthquake.

Going forward, the researchers hope to overcome these challenges one by one. What’s more, Meade also has his mind set to predicting the magnitude of earthquakes themselves — something which is still considered highly esoteric and, perhaps, impossible to do.

“I think there’s a quiet revolution in thinking about earthquake prediction,” he said. “It’s not an idea that’s totally out there anymore. And while this result is interesting, I think this is part of a revolution in general about rebuilding all of science in the artificial intelligence era.

“Problems that are dauntingly hard are extremely accessible these days,” he continued. “That’s not just due to computing power — the scientific community is going to benefit tremendously from this because…AI sounds extremely daunting, but it’s actually not. It’s an extraordinarily democratizing type of computing, and I think a lot of people are beginning to get that.”

Scientists might soon be able to use underwater cables as seismometers

With the right setup, anything that moves the cables around can be detected — and that includes earthquakes.

Image credits: Marra et al / Science.

Detecting earthquakes is important both for assessing risks to the population and for understanding the inner structure of the Earth. Thankfully, we have enough seismometers on land to detect all but the smallest (and harmless) earthquakes — but in the sea, it’s a different story.

Over 70% of our planet’s surface is covered in water, and seismometer coverage is limited to a handful of permanent ocean bottom stations. It’s very expensive and logistically difficult to maintain permanent sensors underwater, so there are many gaps. Now, a team of researchers led by Giuseppe Marra of the UK’s National Physical Laboratory has an idea how to fill those gaps.

They discovered the solution accidentally while working on advanced fiber-optic cables. These cables are so fine-tuned that any vibrations can cause a distortion of the signal — which is generally a problem to be solved. But Mara and colleagues realized that one man’s problems can be another man’s solution when they found that one of the vibration sources are earthquakes.

When an earthquake happens, it sends seismic waves through the planet, and as these waves eventually pass through the fiber, they cause a slight delay in the signal. This measurable delay affects the oscillating lightwave can be studied and used to localize earthquakes.

[panel style=”panel-info” title=”Triangulating earthquakes” footer=””]Earthquakes generate several types of waves. The first ones are the primary or P waves, and these are the fastest. The second ones are the S waves, and lastly, the surface waves (Love and Rayleigh waves) arrive. Calculating the delay between the first waves and the subsequent ones is important in locating earthquakes.

Triangulating earthquakes requires — as you’d imagine — at least three seismometers. However, if underwater cables are long enough, opposite ends of the same cable could serve as different seismometers.[/panel]

Researchers tested their technique using several earthquakes, and found that if the cables are complemented with equipment that maintains a perfectly stable frequency of laser oscillations, they can get the job done. The add-on equipment is essential, as the data from the cable itself can’t be used as an earthquake-monitoring signal.

“We detected earthquakes over terrestrial and submarine links with length ranging from 75 to 535 km and a geographical distance from the earthquake’s epicenter ranging from 25 to 18,500 km. Implementing a global seismic network for real-time detection of underwater earthquakes requires applying the proposed technique to the existing extensive submarine optical fiber network,” researchers write.

Since the ocean bottom is riddled with these cables and the system requires only small amounts of power, researchers are confident that this technique can be widely applied. They point out that a similar approach could also be used for other purposes, such as studying noise pollution in the ocean or even tracking marine mammals as they migrate.

Journal Reference: Marra et al. “Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables.” Science, 2018. DOI: 10.1126/science.aat4458