Tag Archives: Seismics

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

Scientists find evidence of 1755 earthquake in pond sediment

To this day, the Cape Ann 1755 earthquake remains the largest earthquake in the history of Massachusetts. According to existing data, no one was killed, but it damaged hundreds of buildings in Boston and was felt as far north as Nova Scotia and as far south as South Carolina. Now, seismologists have found evidence for this earthquake in the unlikeliest of places: a pond.

A layer of sediment told researchers quite a bit about an old earthquake. Image in public domain, not from actual study.

Katrin Monecke of Wellesley College and her colleagues were able to identify a layer of light brown organic-rich mud within the core, deposited between 1740 and 1810. They concluded that the unusual sediment comes from an underwater landslide, probably caused by the Cape Ann earthquake.

Earthquakes are not common in New England, which lies on the inside of a tectonic plate. Earthquakes are much more common in places like California, which lie at the edge of tectonic plates. With few geological faults or other telltale clues of the earthquake, seismologists have gotten a bit more creative in their search for evidence.

“We don’t have any evidence of the fault that ruptured in 1755 for several reasons: the earthquake was probably not large enough to cause a surface rupture. Also, based on historical description of damage, the epicenter of the 1755 earthquake was most likely offshore,” Monecke told ZME Science in an email.

“It is important to see what an earthquake signature looks like in these sediments, so that we can start looking at deeper, older records in the region and then figure out whether 1755-type earthquakes take place for example, every 1000 years, or every 2000 years,” she added.

Earthquake forensics

Sluice Pond was in the area most strongly hit by the earthquake, which is why it was selected. Earthquakes need to be quite strong to be able to see deformation within the lake sediment. It has steep sides that would make it more susceptible to a landslide, and it has deep undisturbed layers of sediment for coring. Land clearing by settlers may have made the lake more susceptible to shaking.

“These were our main indicators that something had happened in the lake. We saw these near shore sediments and fragments of near-shore vegetation that appear to have been washed into the deep basin [by strong shaking],” said Monecke.

“It is important to see what an earthquake signature looks like in these sediments, so that we can start looking at deeper, older records in the region and then figure out whether 1755-type earthquakes take place for example, every 1000 years, or every 2000 years,” Monecke added.

The results could help to calculate the age of sediments from other ponds. You can learn more about the variation in the sediments and what they mean by also being able to know approximately when they happen. It would be interesting to look at the effect of earthquakes on the land in other areas. Researchers want to use these results as a sort of calibration and find older horizons in sediments for other earthquakes.

“This study can be seen as the calibration of earthquake-induced deformation in lake sediments in this area. With this information, we can now target deeper sediments that accumulated in Sluice Pond since the glaciers left the area and date back to about 16,000 years ago. Finding older earthquake horizons within these sediments will allow us to determine the recurrence intervals of 1755-type earthquakes. We also look at other lakes in the area, for example Walden Pond. It is the deepest lake in eastern Massachusetts and we expect to find an excellent sedimentary record here, too,” Monecke added in an email.

Although intraplate earthquakes are much rarer, they still do happen. Although they’re much rarer, they can still be quite massive.

“Earthquakes can occur in intraplate tectonic settings where strain is accumulating but at much slower rates compared to plate boundaries. This slow accumulation of strain causes large earthquakes to occur but with much longer recurrence intervals in the order of centuries to millennia. It is possible that preexisting, ancient fault lines get re-activated during these earthquakes,” Monecke concludes.

“The 1755 Cape Ann earthquake recorded in lake sediments of eastern New England –an interdisciplinary paleoseismic approach,” by Katrin Monecke et al, was published in Seismological Research Letters.

New Zealand’s earthquake pushes the sea floor 2 meters above ground

A magnitude 7.8 earthquake shook New Zealand on Monday night and left the locals of Kaikoura, South Island with an incredible surprise: areas of the sea floor were thrust up to 2 meters above ground, pushing up through the sand in lumpy slabs.

It’s geology under our noses. The pieces of sea bed rose so fast, they were still teeming with ocean critters by the time locals realized what had happened — and scientists say they’ve never seen anything like it before.

“I’ve never seen it before during an earthquake and it’s the first time we’ve seen something like this,” marine geologist Joshu Mountjoy from New Zealand’s National Institute of Water and Atmospheric Research told Stuff.

“It will take a while before this becomes normal again.”

“Man I should really stop drinking on Mondays”– this little crab. Image credits Anna Redmond / Facebook

“Man I should really stop drinking on Mondays”– this little crab.
Image credits Anna Redmond / Facebook

The quake sadly took a heavy toll on central New Zealand however, with reports of over 1,000 locals and tourists stranded by landslides, dozens of injuries and two reported dead.

Mountjoy sais the effects were so dire because of a phenomenon known as co-seismic movement. Basically, two almost simultaneous quakes have acted on fault lines across the South Island just after midnight, concentrating their energy. The unique nature of the event is probably behind the striking sea-bed movement in Kaikoura, with some pretty wide areas rising almost two meters above their initial position. And it all happened in a few minutes.

Nicola Litchfield, Earthquake geologist and head of the Active Landscapes Department at GNS Science told Michael Daly from Stuff that these movements had to have happened during the 90 to 120 seconds the quake lasted.

“It would have been amazing if it had been daylight and someone had seen it,” she said.

Researchers are now working on determining how the coast changed after the quake, already suspecting that rocks moved not only vertically, but horizontally as well.

“It’s not just a single rupture on one fault plain, it’s ruptured multiple fault strands all along the coast there,” said Mountjoy. “So that uplift will reflect that behaviour of different faults.”

GPS station installed on the island have revealed that Cape Campbell in the Marlborough region has moved north-east by 2 to 3 meters, and the tide gauge at Kaikoura rose a whopping 90 centimeters. Given these dramatic movements, it will take a while before the full extent of the earthquake’s effects are understood.