Tag Archives: San Andreas fault

Gravitational pull causes some earthquakes on California’s San Andreas Fault

Some of the earthquakes that take place on the San Andreas Fault – which extends approximately 800 miles through California – are the result of the gravitational pull between the sun and the moon, according to a new study.

A 3-D perspective view of the San Andreas Fault that extends through California. Credit: Wikipedia

A 3-D perspective view of the San Andreas Fault that extends through California. Credit: Wikipedia

For years, scientists have been fascinated by the gravitational tug of war between the sun and moon and the effects that it has on the Earth. Just like sea levels, the surface of the Earth goes up and down with the tides due to this gravitational pull. In the current study, the team found that during certain phases of this cycle, low-frequency earthquakes are more likely to occur

“It’s kind of crazy, right?” said Nicholas van der Elst, a U.S. Geological Survey (USGS) geophysicist and first author of the study. “That the moon, when it’s pulling in the same direction that the fault is slipping, causes the fault to slip more – and faster. What it shows is that the fault is super weak – much weaker than we would expect – given that there’s 20 miles of rock sitting on top of it.”

Van der Elst and his team examined around 81,000 low-frequency earthquakes that took place between 2008 and 2015 along the Parkfield region of the San Andreas Fault and looked for a connection to the “fortnightly tide,” which is the fault’s two-week tidal cycle. The data revealed that these particular earthquakes occurred more often during the waxing period – the time when the tide is increasing in size at the fastest rate.

Tides within the surface of the Earth are strongest when the sun and moon and aligned and these forces cause stretching and compression of its crust. Some faults are more susceptible to tidal forces than others, and numerous additional factors such as the orientation of the fault and its distance to the Earth’s crust affect this susceptibility.

Examination of low-frequency earthquakes and their connection to tidal forces can help geologists better understand the San Andreas Fault and predict the possible outcomes of larger earthquakes. Furthermore, they also provide information on regions of the fault that extend as far as 20 miles underground and would otherwise be impossible for researchers to reach.

“It’s almost like having a lot of little creep meters embedded in the fault,” said David Shelly, a USGS seismologist and co-author of the study. “We can use these low-frequency earthquakes as measurements of, at least in a relative sense, how much slip is happening at each little spot on the deep part of the fault where we see these events. When we don’t see them, we don’t know what’s happening; we don’t know whether it’s slipping silently or not slipping at all.”

Journal Reference: Fortnightly modulation of San Andreas tremor and low-frequency earthquakes. 18 July 2016. 10.1073/pnas.1524316113

California faces tsunami risk – L.A. specifically threatened

It’s not just the San Andreas fault – a new study published in the Journal of Geophysical Research reports that there are several long faults on the U.S. West coast which can cause significant earthquakes, as well as tsunamis.

This map shows the California Borderland and its major tectonic features, as well as the locations of earthquakes greater than Magnitude 5.5. The dashed box shows the area of the new study. Large arrows show relative plate motion for the Pacific-North America fault boundary. Mark Legg

“There are many active faults offshore southern California which could produce greater then magnitude 7 quakes and tsunamis,” Mark Legg, who runs a Southern California consulting firm called Legg Geophysical and is the lead author of the study, said.

Geologists gave a collective criticism to the big-budget San Andreas movie, labeling it as a “classic” disaster movie with little science behind it. But that doesn’t mean that California isn’t threatened by seismic activity; the San Andreas area is way overdue for a major earthquake, and it’s likely gonna be big. The surveys of the region show a “complicated logjam” of faults produced by the movement of the Pacific Plate, sliding in relative motion to the North American Plate.

“What they were searching for are signs, like those seen along the San Andreas, that indicate how much the faults have slipped over time and whether some of that slippage caused some of the seafloor to thrust upwards,” the American Geophysical Union, which publishes the journal, said, in a press release.

Larger imageSCEC Community Fault Model. This map shows the 3-dimensional structure of major faults beneath Southern California. Image via Earthquake Country.

Legg said that while most geophysical studies (and movies) focus on the inland San Andreas fault, offshore faults still hold a great potential for damage. A geological fault is a planar fracture or discontinuity in a volume of rock, accompanied by major displacement. Most major faults emerge at areas of tectonic pressure, usually at the edge of tectonic plates. He and his colleagues gathered seafloor bathymetry data which revealed that two of the largest faults (the Santa Cruz-Catalina Ridge Fault and the Ferrelo Fault) have advanced in recent decades and are now connected to the smaller faults in the Borderland. Connected faults can be a major problem because they cause a domino-like effect, where movements on one fault trigger further movement and displacement on other faults.

A schematic block model of Southern California showing the motion of the Pacific and North American plates, and the big bend of the San Andreas fault where the plates squeeze together. Image via Earthquake Nation.

“The more connected the faults are, the more they can cause larger earthquakes,” said Paul Umhoefer, a geologist at Northern Arizona University. “The more detailed data that was gathered in this study is important for judging whether there is an earthquake and tsunami hazard.”

The good news is that even if a tsunami emerges, it won’t be as large as tsunamis generally get in subduction areas. Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. But even if a 2 meter tsunami is generated, it could cause massive damage on the coast.

Legg also warned that we can’t even understand what damage a potential tsunami might cause, because we don’t have enough bathymetry data on the U.S. West coast.

“We’ve got high resolution maps of the surface of Mars,” Legg said, in a statement. “Yet we still don’t have decent bathymetry for our own backyard.”


6.9 earthquake hits California, followed by aftershocks

A magnitude 6.9 earthquake off the coast of Northern California struck Sunday night, on the 9th of March. It was the largest on the West Coast since the 7.2 Baja California quake in 2010 and it was followed by a series of at least 13 aftershocks, the largest of which had magnitude of 4.6, according to the USGS.


Image via USGS.

The earthquake was shallow in depth (at approximately 6 km deep) and occurred 70 kilometers  west of Eureka in Humboldt County in the Pacific Ocean; however, the aftershocks were closer to the mainland, the biggest one being at approximately 20 km offshore.

Local authorities represented by Eureka Police Department Sgt. Brian Stephens said there had been no reports of significant damage or injuries.

“My car was rocking back and forth,” Stephens, who was on out on a call when the earthquake hit, told The Times. “I thought someone was shoving my car back and forth, looked around and nobody was there. Then I realized what was happening.”

The activity on the West Coast appears to intensify, and most seismologists seem to agree that California is way overdue for a major earthquake. In 2008, a report concluded that California has 99% chances of big earthquake in 30 years – which means that today, there’s a 99% chance of a major earthquake in 24 years – and the US has done little to prepare itself for this apparently inherent catastrophe on the San Andreas fault.

We’re talking a major earthquake, something over 7.5 magnitude. Of course, these things are literally impossible predict, but you could speak in term of probabilities (hence the report mentioned above). A team of researchers lead by Thomas Jordan of the University of Southern California has simulated an earthquake in 3-D, using the world’s fastest computer, to see how the ground would shake throughout Southern California and into Mexico in the event of a magnitude 8.0 earthquake centered northwest of Los Angeles. The results would be absolutely and horribly devastating – so why then are the US not better preparing themselves?

I don’t know. It may be a case of just closing your eyes and hoping the danger goes away (which it won’t happen), or maybe something more cynical, like keeping the price and the value of the area as high as possible, without panicking the population and lowering property value. Either way, they’d best try a little harder.

The next big earthquake

The massive 9.0 earthquake was the last in a series that devastated countries across the boundary of the Pacific plate. Even Japan, perhaps the country who was the most prepared in the world for this kind of seismic activity, was hopeless in front of the temblor and the tsunamis it created.

But after the earthquakes in Sumatra, Chile and New Zealand, another question arises: where is the next ‘big one‘ going to strike? An earthquake that takes place on a fault probably increases strain across the edge of the boundary (or bundaries), but there is no accurate way to predict earthquakes – anyone saying he is able to do so is a witch doctor. But talking about odds, that’s an entirely different story. We already know there’s a 99% percent of a major earthquake in California in the next three decades, but could it strike any time sooner ?

Many people are all over the place stating that the big one will arrive in a matter of months and that it will devastate absolutely anything and everything; is that true ? Not really, or at least there is nothing definitely pointing towards it. However, the San Andreas fault is way overdue for a major rupture, and the rupture will happen during our lifetimes and it will cause a major earthquake, but it is extremely unlikely that it is as powerful as the ones in Sumatra or Chile or Japan, and the area is not at a major tsunami risk.

The point is not to figure out where the next big one is going to happen – that’s impossible; we already know the most vulnerable points located on the edge of tectonic plates, so the point is to try and be prepared at every single moment, because the ‘big one’ is not going to give out warnings.

An earthquake wake up call… to the US

You don’t get any country better prepared for an earthquake than Japan. They know they are at risk, they are prudent, they have the money, the technology, and the work force; and yet, when hit by an earthquake of this magnitude, no matter how well prepared you are, you are in for some massive trouble.

It’s a well known fact that the San Andreas fault is way overdue for a major earthquake – an earthquake which will cause unthinkable damage, even though the state of California prides itself to be the leading edge in seismic technology. Very modern buildings have a fair chance of still standing, but everything other than that will become a carpet of rubble; after that, hot winds will undoubtably fan the fires that earthquakes always cause, pipelines will burst, and everything will be unstoppable.

San Francisco after the 1906 earthquake

California should learn from the Japan experience that no amount of preparation can make you ready for something like this, when a major earthquake is coming.

“Everybody is playing a gamble that something like this won’t happen,” said Dana Buntrock, associate professor of architecture, at the University of California, Berkeley.

The thing is, the 1906 earthquake in San Francisco left the city devastated; everybody wanted to rebuild as fast as possible, and so, numerous buildings were made without adequate reinforcing steel, while homes and apartment complexes are extremely vulnerable themselves.

“The question is not if but when Southern California will be hit by a major earthquake — one so damaging that it will permanently change lives and livelihoods in the region,” according to a 2008 study by the United States Geological Survey study.

A study concluded that a 7.8 earthquake will probably cause 2.000 deaths and $200 billion damage, but these kind of studies are always optimistic. A 7.8 earthquake would be about 30 times smaller than the seismic event that struck Japan, but it is unlikely to be over 8, due to the tectonic of the region – but then again, so was the Japanese temblor. The thing is, there is a 99 percent chance of a 6.7 magnitude quake within three decades, and 46 percent chance of a 7.5 or greater temblor – talk about good odds. The odds for a 8 to 9 earthquake in the next 30 years were only at 10 percent.

California and Japan tend to look at each other when it comes to building earthquake-resistant buildings, but Japan has a different strategy, of tearing down older buildings and building newer ones instead, which makes them better prepared for this kind of event. But California isn’t the only area which the US has to worry when it comes to earthquakes. Frank Vernon, a geophysics professor and seismology specialist at the Scripps Institution of Oceanography at UC San Diego, speaks about these dangers:

“The most important lesson in the U.S. and North America is the reminder that we have a similar subduction zone called Cascadia up on the coast of British Columbia, Washington, Oregon and very northern California which could do the same thing,” he said. “Some day we will be having this same type of earthquake near our shores,” he said.

Picture sources: 1 2 3

3D supercomputer simulation predicts disastrous outcome for L.A. on the next “Big One”

The San Andreas fault is notorious for its unstable seismic nature, being the center stage for America’s most devastating earthquakes as well as the object of imagination for different artists. We’ve all marveled and even laughed at movies, magazines or comic books depicting scenes like a earthquake ravaged L.A. or a newly formed island state of California, but sadly one day these all might come true when the region will be seriously tried by nature – according to scientists that day may soon be upon us!

A team of researchers lead by Thomas Jordan of the University of Southern California has simulated an earthquake in 3-D, using the world’s fastest computer, to see how the ground would shake throughout Southern California and into Mexico in the event of a magnitude 8.0 earthquake centered northwest of Los Angeles. The output data is frightening.

As one can easily see in the presentation featured in the press video below, the seismic hazard from a magnitude 7.75 earthquake along the 170-kilometer section of the San Andreas fault between Tejon Pass and San Bernardino could have dramatic cataclysmic effects on the city of Los Angeles, considering the seismic tremblings will affect the city for a longer period of time (even up to two minutes). Of course, dire consequences would ensue. What’s also interesting to note is that this very unstable seismic fault was presumably met with a “big one” every 250-400 years, but now scientists have learned that the “big one” comes at a cycle between 45-144 years. Now, considering the last big earthquake hit the Southern California straight was 153 years ago, on 1857, a dramatic event might happen at any time.

3 million years inactive fault in Sierra Nevada may trigger quakes after all

The major fault in Sierra Nevada is believed to have been “quiet” for more than 3 million years has shown signs of becoming active and it is believed that it can trigger quakes that can reach magnitudes of up to 7. This comes as a rather unpleasant surprise to geologists, as well as the people living in the area.

“It came as a surprise to see that a long-inactive fault can produce significant quakes,” said geologist Elisabeth Nadin of Caltech.

Picture of the San Andreas fault

A fault is a fracture in a volume of rock, varying in size from a few centimeters to many kilometers. The thing is that these faults do not occur as single clean fractures; a big one generates numerous others, which is why the whole area is called a fault area. The mechanism through which they generate earthquakes is rather simple, but the effects are pretty devastating.

This particular fault emerged some 86 million years ago, according to evidence found by geologists in nearby rocks. Nadin, who just received her doctorate from Caltech reported finding a series of “fault scarps” – small jagged cliffs 6 feet or more high – that show little signs of erosion, which means of course that they were uplifted recently. She also found numerous rounded boulders that are estimated to come from the icy period that took place 12000 years ago. Although this is pretty much the same process from the San Andreas fault, it’s about 100 times slower, so while there is a need for caution and prevention, there is absolutely no reason to panic.