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Earthquakes «Ñaltech.Edu»

Fri, 01 Sep 2017 20:54:29 +0000
Modeling the Effects of Wastewater Injection
Earth scientists develop a model to estimate the largest possible quake in a given location that could be caused by the disposal of water used in hydraulic fracturing
News Writer: 
Robert Perkins
Rupture
Wastewater injected in an underground reservoir layer crossed by a fault triggers an earthquake. The earthquake rupture grows larger than the zone pressurized by water injection.
Credit: Galis et al., and Thomas Willard/Caltech Graphic Resources

In work that offers insight into the magnitude of the hazards posed by earthquake faults in general, seismologists have developed a model to determine the size of an earthquake that could be triggered by the underground injection of fluids produced as a by-product of hydraulic fracturing.

Hydraulic fracturing, or "fracking," is a petroleum-extraction procedure in which millions of gallons of water (as well as sand and chemicals) are injected deep into underground shale beds to crack the rock and release natural gas and oil. According to the United States Geological Survey, fracking itself does not typically trigger earthquakes. Instead, the increased risk for seismicity is more strongly linked with the subsequent injection of the wastewater from fracking and other oil-extraction processes into massive disposal wells that are thousands of feet underground.

Previous attempts to model the relationship between injection of wastewater and the triggering of earthquakes suggested that the maximum magnitude of the seismic activity induced in this way would be proportional to the volume of the fluids injected. However, this interpretation fails to account for the fact that earthquakes can grow beyond the area impacted by fluid pressure, says Jean Paul Ampuero, professor of seismology at Caltech and co-author of a new study on the topic that appears in the journal Science Advances on December 20.

Combining theory and computer simulations of dynamic earthquake ruptures, Ampuero and his colleagues developed a model that explains how the size of injection-induced earthquakes depends on not only the volume of fluid being injected but also the energy stored on nearby faults. The result is a model that quantifies the distance that an earthquake can propagate beyond an injection site—which in turn predicts the maximum magnitude of an induced seismic event.

"Earthquakes induced by human activities involving underground injection of fluids or gas are a growing concern, a hazard that needs to be controlled in order to develop a safer and cleaner energy future," Ampuero says.

This induced seismicity has been the subject of significant research in recent years and is also attracting researchers who, like Ampuero, are primarily interested in unraveling the physics of natural earthquakes. "This may be the closest researchers will ever get to a large-scale controlled earthquake experiment," Ampuero says. For the new work, Ampuero teamed up with Martin Galis, postdoctoral researcher at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia.

It is important to note that the new model only predicts the maximum possible magnitude of an earthquake rather than what the earthquake magnitude will actually be, the researchers say. It defines upper limits based on the amount of pent-up energy in the earth's crust prior to fluid injection.

The new model offers insight into natural earthquakes, creating a framework for understanding what causes earthquakes to stop shaking. Earthquakes can be triggered by the pressure and disturbance caused by fluid injection, but they may grow beyond the zone immediately impacted by the wastewater injection by tapping into tectonic energy that is already stored nearby. As is the case for induced seismicity, natural earthquakes can start in small areas of the earth's crust where that energy is concentrated. How large they grow is determined by the amount of energy in surrounding regions.

The paper is titled "Induced seismicity provides insight into why earthquake ruptures stop." Ampuero and Galis's co-authors include Paul Martin Mai of KAUST and Fr


Tue, 15 Aug 2017 20:38:22 +0000
The Massively Big Picture
Mark Simons is part of a movement to add precise and panoramic perspectives to previously limited geographic observations.
Mark Simons
Mark Simons

The 2011 Japanese earthquake was a defining moment for Mark Simons. The devastating 9.0-magnitude quake and its subsequent tsunami, which took nearly 16,000 lives, spurred efforts around the globe that will shape how nations predict and prepare for future natural disasters and motivated new approaches to basic earthquake science that are applicable to seismic events large and small.

Read more on the Break Through campaign website.


Fri, 28 Apr 2017 23:19:13 +0000
How Friction Evolves During an Earthquake
By simulating quakes in a lab, engineers study the way that friction changes along a fault during a seismic event
News Writer: 
Robert Perkins
Wind Tunnel
The "seismological wind tunnel" at Caltech. The white square block in the center is a plastic known as homalite that acts as an analogue for rock. It contains a miniature fault that is triggered via a nickel-chromium wire fuse to produce a tiny simulated earthquake.
Credit: Courtesy of Vito Rubino

By simulating earthquakes in a lab, engineers at Caltech have documented the evolution of friction during an earthquake—measuring what could once only be inferred, and shedding light on one of the biggest unknowns in earthquake modeling.

Before an earthquake, static friction helps hold the two sides of a fault immobile and pressed against each other. During the passage of an earthquake rupture, that friction becomes dynamic as the two sides of the fault grind past one another. Dynamic friction evolves throughout an earthquake, affecting how much and how fast the ground will shake and thus, most importantly, the destructiveness of the earthquake.

"Friction plays a key role in how ruptures unzip faults in the earth's crust," says Vito Rubino, research scientist at Caltech's Division of Engineering and Applied Science (EAS). "Assumptions about dynamic friction affect a wide range of earthquake science predictions, including how fast ruptures will occur, the nature of ground shaking, and residual stress levels on faults. Yet the precise nature of dynamic friction remains one of the biggest unknowns in earthquake science."

Previously, it commonly had been believed that the evolution of dynamic friction was mainly governed by how far the fault slipped at each point as a rupture went by—that is, by the relative distance one side of a fault slides past the other during dynamic sliding. Analyzing earthquakes that were simulated in a lab, the team instead found that sliding history is important but the key long-term factor is actually the slip velocity—not just how far the fault slips, but how fast.

Rubino is the lead author on a paper on the team's findings that was published in Nature Communications on June 29. He collaborated with Caltech's Ares Rosakis, the Theodore von K


Thu, 30 Jun 2016 18:43:58 +0000
Earthquakes Can Make Thrust Faults Open Violently and Snap Shut
Experiments reveal a new mechanism that could explain the source of a destructive feature of the 2011 Tohoku earthquake.
News Writer: 
Robert Perkins
Earthquake
Illustration based on computer model shows how the hanging wall (right) of a thrust fault can twist away from the foot wall (left) during an earthquake.
Credit: Harsha Bhat

It is a common trope in disaster movies: an earthquake strikes, causing the ground to rip open and swallow people and cars whole. The gaping earth might make for cinematic drama, but earthquake scientists have long held that it does not happen.

Except, it can, according to new experimental research from Caltech.

The work, appearing in the journal Nature on May 1, shows how the earth can split open—and then quickly close back up—during earthquakes along thrust faults.

Thrust faults have been the site of some of the world's largest quakes, such as the 2011 Tohoku earthquake off the coast of Japan, which damaged the Fukushima nuclear power plant. They occur in weak areas of the earth's crust where one slab of rock compresses against another, sliding up and over it during an earthquake.

A team of engineers and scientists from Caltech and


Wed, 03 Feb 2016 20:15:30 +0000
Community Seismic Network Detected Air Pulse From Refinery Explosion
Tight network of low-cost detectors improve resolution of seismic data gathering and could offer city inspectors crucial information on building damage after a quake
News Writer: 
Robert Perkins
Building
Models of mode shapes for building motion
Credit: Image courtesy of M. Kohler/Caltech

On February 18, 2015, an explosion rattled the ExxonMobil refinery in Torrance, causing ground shaking equivalent to that of a magnitude-2.0 earthquake and blasting out an air pressure wave similar to a sonic boom.

Traveling at 343 meters per second—about the speed of sound—the air pressure wave reached a 52-story high-rise in downtown Los Angeles 66 seconds after the blast.

The building's occupants probably did not notice a thing; the building shifted at most three-hundredths of a millimeter in response. But the building's seismometers—one is installed on every floor, as well as on the basement levels—noted and recorded the motion of each individual floor.

Those sensors are part of the Community Seismic Network (CSN), a project launched at Caltech in 2011 to seed the Los Angeles area with relatively inexpensive seismometers aimed at providing a high level of detail of how an earthquake shakes the Southern California region, as well as how individual buildings respond. That level of detail has the potential to provide critical and immediate information about whether the building is structurally compromised in the wake of an earthquake, says Caltech's Monica Kohler, research assistant professor in the Division of Engineering and Applied Science.

For example, if building inspectors know that inter-story drift—the displacement of each floor relative to the floors immediately below and above it—has exceeded certain limits based on the building's size and construction, then it is a safe bet that the building has suffered damage in a quake. Alternately, if inspectors know that a building has experienced shaking well within its tolerances, it could potentially be reoccupied sooner—helping an earthquake-struck city to more quickly get back to normal.

"We want first responders, structural engineers, and facilities engineers to be able to make decisions based on what the data say," says Kohler, the lead author of a paper detailing the high-rise's response that recently appeared in the journal Earthquake Spectra.

The keys to the CSN's success are affordability and ease of installation of its seismic detectors. Standard, high-quality seismic detectors can cost tens of thousands of dollars and need special vaults to house and protect them that can easily double the price. By contrast, the CSN detectors use $40 accelerometers and other off-the-shelf hardware, cost roughly $300 to build, and require minimal training to install. Approximately 700 of the devices have been installed so far, mostly in Los Angeles.

However, the CSN sensors are roughly 250 times less sensitive than their more expensive counterparts, which is why the ability to successfully detect and quantify the downtown building's response to the ExxonMobil explosion was such an important proof-of-concept.

"It's a validation of our approach," says CSN's project manager, Richard Guy.

Sonic booms have been noted by seismic networks dozens of times before, beginning in the 1980s with the first detections of seismic shaking caused by space-shuttle reentries. The sonic booms, found Hiroo Kanamori and colleagues at Caltech and the United States Geological Survey, rattled buildings that, in turn, shook the ground around them.

"Seismologists try to understand what is happening in the earth and how that affects buildings by looking at everything we see on seismograms," says Kanamori, Caltech's John E. and Hazel S. Smits Professor of Geophysics, Emeritus, and coauthor of the Earthquake Spectra paper. "In most cases, signals come from the interior of the earth, but nothing prevents us from studying signals from the air. Though rare, the signals from the air provide a new dimension in the field of seismology."

The earlier sonic boom detections were made using single-channel devices, which typically record motion in one direction only. While this information is useful for understanding ground shaking, a three-dimensional record of the floor-by-floor motion of a building can reveal how much a building is rocking, swaying, and shifting; two or more sensors installed per floor can show the twisting of the structure.

"The more sensors you have in a small area, the more detail you're going to see. If there are things happening on a small scale, you'll never see it until you have sensors deployed on that scale," Kohler says.

Kohler and her colleagues found that the air pressure wave from the explosion had about the same impact on the high-rise as an 8 mile-per-hour gust of wind. A pressure wave about 100 times larger would have been required to have broken windows in the building; a wave 1,000 times larger would have been necessary to cause significant damage to the building.

The ExxonMobil blast was not the first shaking recorded by the building's seismometers. A number of earthquakes—including a magnitude-4.2 quake on January 4, 2015, with an epicenter in Castaic Lake, about 40 miles northwest of downtown Los Angeles—also were registered by the seismic detectors on nearly every floor of the building. But the refinery explosion-induced shaking was an important test of the sensitivity of the instruments, and of the ability of researchers to separate earthquake signals from other sources of shaking.

Other authors of the Earthquake Spectra paper, "Downtown Los Angeles 52-Story High-Rise and Free-Field Response to an Oil Refinery Explosion," include Caltech's Anthony Massari, Thomas Heaton, Egill Hauksson, Robert Clayton, Julian Bunn, and K. M. Chandy. Funding for the CSN came from the Gordon and Betty Moore Foundation, the Terrestrial Hazard Observation and Reporting Center at Caltech, and the Divisions of Geological and Planetary Sciences and Engineering and Applied Science at Caltech. 


Mon, 09 Nov 2015 18:26:17 +0000
White House Puts Spotlight on Earthquake Early-Warning System
News Writer: 
Katie Neith
The earthquake early warning (EEW) UserDisplay in action for a scenario M7.8 earthquake. The most intense colors correspond to very strong ground shaking. The banner on top shows expected shaking at the user site. The number "14" on the left indicates warning time, and the expected intensity at the user site is shown in roman numerals, VII. Other information indicates the epicenter and date/time of the earthquake.
Credit: Caltech

Since the late 1970s, Caltech seismologist Tom Heaton, professor of engineering seismology, has been working to develop earthquake early-warning (EEW) systems—networks of ground-based sensors that can send data to users when the earth begins to tremble nearby, giving them seconds to potentially minutes to prepare before the shaking reaches them. In fact, Heaton wrote the first paper published on the concept in 1985. EEW systems have been implemented in countries like Japan, Mexico, and Turkey. However, the Unites States has been slow to regard EEW systems as a priority for the West Coast.

But on February 2, 2016, the White House held the Earthquake Resilience Summit, signaling a new focus on earthquake safety and EEW systems. There, stakeholders—including Caltech's Heaton and Egill Hauksson, research professor in geophysics; and U.S. Geological Survey (USGS) seismologist Lucy Jones, a visiting associate in geophysics at Caltech and seismic risk advisor to the mayor of Los Angeles—discussed the need for earthquake early warning and explored steps that can be taken to make such systems a reality. 

At the summit, the Gordon and Betty Moore Foundation announced $3.6 million in grants to advance a West Coast EEW system called ShakeAlert, which received an initial $6 million in funding from foundation in 2011. The new grants will go to researchers working on the system at Caltech, the USGS, UC Berkeley, and the University of Washington.

"We have been successfully operating a demonstration system for several years, and we know that it works for the events that have happened in the test period," says Heaton. "However, there is still significant development that is required to ensure that the system will work reliably in very large earthquakes similar to the great 1906 San Francisco earthquake. This new funding allows us to accelerate the rate at which we develop this critical system."

In addition, the Obama Administration outlined new federal commitments to support greater earthquake safety including an executive order to ensure that new construction of federal buildings is up to code and that federal assets are available for recovery efforts after a large earthquake.

The commitments follow a December announcement from Congressman Adam Schiff (D-Burbank) that Congress has included $8.2 million in the fiscal year 2016 funding bill specifically designated for a West Coast earthquake early warning system.

"By increasing the funding for the West Coast earthquake early-warning system, Congress is sending a message to the Western states that it supports this life-saving system. But the federal government cannot do it alone and will need local stakeholders, both public and private, to get behind the effort with their own resources," said Schiff, in a press release. "The early warning system will give us critical time for trains to be slowed and surgeries to be stopped before shaking hits—saving lives and protecting infrastructure. This early warning system is an investment we need to make now, not after the 'big one' hits."

ShakeAlert utilizes a network of seismometers—instruments that measure ground motion—widely scattered across the Western states. In California, that network of sensors is called the California Integrated Seismic Network (CISN) and is made up of computerized seismometers that send ground-motion data back to research centers like the Seismological Laboratory at Caltech.

Here's how the current ShakeAlert works: a user display opens in a pop-up window on a recipient's computer as soon as a significant earthquake occurs in California. The screen lists the quake's estimated location and magnitude based on the sensor data received to that point, along with an estimate of how much time will pass before the shaking reaches the user's location. The program also gives an approximation of how intense that shaking will be. Since ShakeAlert uses information from a seismic event in progress, people living near the epicenter do not get much—if any—warning, but those farther away could have seconds or even tens of seconds' notice.

The goal is an improved version of ShakeAlert that will eventually give schools, utilities, industries, and the general public a heads-up in the event of a major temblor.

Read more about how ShakeAlert works and about Caltech's development of EEW systems in a feature that ran in the Summer 2013 issue of E&S magazine called Can We Predict Earthquakes?


Wed, 05 Aug 2015 23:53:00 +0000
Schools Help Researchers Understand Quakes
News Writer: 
Jessica Stoller-Conrad

In the event of a major earthquake in Los Angeles, first responders ideally would immediately have a map of the most intense shaking around the city—allowing them to send help to the hardest-hit areas first.

A new collaboration between Caltech researchers and schools of the Los Angeles Unified School District (LAUSD) provides a crucial step in the creation of such damage maps by vastly broadening the scope of a dense network of seismic sensors in the Los Angeles Basin.

To create an accurate shaking-intensity map, seismologists need to measure ground motion—which can vary from kilometer to kilometer because of differences in soil and earth structure—at many locations across the region. In 2011, Professor of Geophysics Rob Clayton and his colleagues, Professor of Engineering Seismology Tom Heaton and Simon Ramo Professor of Computer Science, Emeritus, K. Mani Chandy, began creating a web of such sensors via the Community Seismic Network (CSN), a program funded by the Gordon and Betty Moore Foundation.

The CSN consists of hundreds of small, inexpensive accelerometers—instruments that detect ground movements before, during, and after a seismic event—installed initially in the homes of volunteers in the greater Pasadena area. Since 2011, each device has been actively collecting and feeding seismic information to the CSN via its host house's Internet connection, allowing Clayton and his colleagues to create high-resolution maps of seismic activity in the western San Gabriel Valley. But the Caltech team wanted to find a way to expand the reach of the network throughout the earthquake-prone broader Los Angeles Basin. Their inventive solution? Integrate accelerometers into the infrastructure of L.A.'s public schools.

Through the efforts of Richard Guy, CSN project manager, sensors already have been installed in 100 LAUSD schools, covering an area ranging from northeast Los Angeles to downtown. CSN is now working to expand the project to include all of the district's more than 1,000 schools.

The new collaboration has the potential to help millions of people in Southern California when a big quake strikes. For example, data from the new network could be incorporated into the ShakeAlert early-warning system that is currently under development. Although no sensor can predict an earthquake, the accelerometers can detect an earthquake in one area of the L.A. Basin so quickly that an alert or warning could be sent to people in adjacent areas of the LA Basin before strong shaking arrives—potentially giving them enough time to find a safe spot.

The new dense network of sensors will also provide an improved map of shaking intensity for the whole region. The U.S. Geological Survey already provides a similar service called ShakeMap, which relies on sensors that are located several miles from one another and hence cannot provide a block-by-block resolution of shaking and possible damage. The new dense network of sensors has the potential to provide ShakeMap with a more accurate assessment of damage for response and recovery efforts.

"You can imagine a fire chief stepping out and saying, 'Wow. That was a big one. Now where do I go to help the community?' Obviously they want their focus to be where the maximum damage and danger is. They have other things to worry about too, but the best proxy for damage that we have is the level of shaking—and our dense network of sensors can provide that information," Clayton says.

But the new sensors do more than feed information into the network—they also provide valuable information to individual schools. "Principals have a particularly difficult problem in the event of an earthquake," Clayton says. "The first thing during a quake, of course, is to tell everyone to get under their desk. When the shaking stops, all of the kids are evacuated out of the school and into the schoolyard. And then what do principals do? At that point, they have to decide if it's safe enough to go back into the school, or if they should just send the kids home. But they do not know how badly the school is damaged."

The new school sensors could help inform this judgment call, Clayton says. Although they work in much the same way as those that were previously placed in volunteers' homes—recording ground accelerations and transmitting those data back to the researchers via an Internet connection—the sensors also contain an onboard computer that compares the event to a so-called fragility curve. Fragility curves provide predictions of the damage that a particular building would sustain under the shaking measured.

"Coupled with the fragility curve, the sensors could allow a school official to decide whether or not it is safe to reenter the school," Clayton says.

The Community Seismic Network's LAUSD collaboration was funded by the Gordon and Betty Moore Foundation. The network is a collaboration between Caltech's seismology, earthquake-engineering, and computer-science departments.


Mon, 03 Aug 2015 21:24:51 +0000
Caltech-Led Team Looks in Detail at the April 2015 Earthquake in Nepal
News Writer: 
Kimm Fesenmaier
John Galetzka, then of Caltech, and Sudhir Rajaure, of the Department of Mines and Geology in Kathmandu, install a high-rate GPS station in the Himalaya.
Credit: John Galetzka

For more than 20 years, Caltech geologist Jean-Philippe Avouac has collaborated with the Department of Mines and Geology of Nepal to study the Himalayas—the most active, above-water mountain range on Earth—to learn more about the processes that build mountains and trigger earthquakes. Over that period, he and his colleagues have installed a network of GPS stations in Nepal that allows them to monitor the way Earth's crust moves during and in between earthquakes. So when he heard on April 25 that a magnitude 7.8 earthquake had struck near Gorkha, Nepal, not far from Kathmandu, he thought he knew what to expect—utter devastation throughout Kathmandu and a death toll in the hundreds of thousands.

"At first when I saw the news trickling in from Kathmandu, I thought there was a problem of communication, that we weren't hearing the full extent of the damage," says Avouac, Caltech's Earle C. Anthony Professor of Geology. "As it turns out, there was little damage to the regular dwellings, and thankfully, as a result, there were far fewer deaths than I originally anticipated."

Using data from the GPS stations, an accelerometer that measures ground motion in Kathmandu, data from seismological stations around the world, and radar images collected by orbiting satellites, an international team of scientists led by Caltech has pieced together the first complete account of what physically happened during the Gorkha earthquake—a picture that explains how the large earthquake wound up leaving the majority of low-story buildings unscathed while devastating some treasured taller structures.

The findings are described in two papers that now appear online. The first, in the journal Nature Geoscience, is based on an analysis of seismological records collected more than 1,000 kilometers from the epicenter and places the event in the context of what scientists knew of the seismic setting near Gorkha before the earthquake. The second paper, appearing in Science Express, goes into finer detail about the rupture process during the April 25 earthquake and how it shook the ground in Kathmandu.


Build Up and Release of Strain on Himalaya Megathrust (caption and credit in video attached in upper right)

In the first study, the researchers show that the earthquake occurred on the Main Himalayan Thrust (MHT), the main megathrust fault along which northern India is pushing beneath Eurasia at a rate of about two centimeters per year, driving the Himalayas upward. Based on GPS measurements, scientists know that a large portion of this fault is "locked." Large earthquakes typically release stress on such locked faults—as the lower tectonic plate (here, the Indian plate) pulls the upper plate (here, the Eurasian plate) downward, strain builds in these locked sections until the upper plate breaks free, releasing strain and producing an earthquake. There are areas along the fault in western Nepal that are known to be locked and have not experienced a major earthquake since a big one (larger than magnitude 8.5) in 1505. But the Gorkha earthquake ruptured only a small fraction of the locked zone, so there is still the potential for the locked portion to produce a large earthquake.

"The Gorkha earthquake didn't do the job of transferring deformation all the way to the front of the Himalaya," says Avouac. "So the Himalaya could certainly generate larger earthquakes in the future, but we have no idea when."

The epicenter of the April 25 event was located in the Gorkha District of Nepal, 75 kilometers to the west-northwest of Kathmandu, and propagated eastward at a rate of about 2.8 kilometers per second, causing slip in the north-south direction—a progression that the researchers describe as "unzipping" a section of the locked fault.

"With the geological context in Nepal, this is a place where we expect big earthquakes. We also knew, based on GPS measurements of the way the plates have moved over the last two decades, how 'stuck' this particular fault was, so this earthquake was not a surprise," says Jean Paul Ampuero, assistant professor of seismology at Caltech and coauthor on the Nature Geoscience paper. "But with every earthquake there are always surprises."


Propagation of April 2015 Mw 7.8 Gorkha Earthquake (caption and credit in video attached in upper right)

In this case, one of the surprises was that the quake did not rupture all the way to the surface. Records of past earthquakes on the same fault—including a powerful one (possibly as strong as magnitude 8.4) that shook Kathmandu in 1934—indicate that ruptures have previously reached the surface. But Avouac, Ampuero, and their colleagues used satellite Synthetic Aperture Radar data and a technique called back projection that takes advantage of the dense arrays of seismic stations in the United States, Europe, and Australia to track the progression of the earthquake, and found that it was quite contained at depth. The high-frequency waves that were largely produced in the lower section of the rupture occurred at a depth of about 15 kilometers.

"That was good news for Kathmandu," says Ampuero. "If the earthquake had broken all the way to the surface, it could have been much, much worse."

The researchers note, however, that the Gorkha earthquake did increase the stress on the adjacent portion of the fault that remains locked, closer to Kathmandu. It is unclear whether this additional stress will eventually trigger another earthquake or if that portion of the fault will "creep," a process that allows the two plates to move slowly past one another, dissipating stress. The researchers are building computer models and monitoring post-earthquake deformation of the crust to try to determine which scenario is more likely.

Another surprise from the earthquake, one that explains why many of the homes and other buildings in Kathmandu were spared, is described in the Science Express paper. Avouac and his colleagues found that for such a large-magnitude earthquake, high-frequency shaking in Kathmandu was actually relatively mild. And it is high-frequency waves, with short periods of vibration of less than one second, that tend to affect low-story buildings. The Nature Geoscience paper showed that the high-frequency waves that the quake produced came from the deeper edge of the rupture, on the northern end away from Kathmandu.

The GPS records described in the Science Express paper show that within the zone that experienced the greatest amount of slip during the earthquake—a region south of the sources of high-frequency waves and closer to Kathmandu—the onset of slip on the fault was actually very smooth. It took nearly two seconds for the slip rate to reach its maximum value of one meter per second. In general, the more abrupt the onset of slip during an earthquake, the more energetic the radiated high-frequency seismic waves. So the relatively gradual onset of slip in the Gorkha event explains why this patch, which experienced a large amount of slip, did not generate many high-frequency waves.

"It would be good news if the smooth onset of slip, and hence the limited induced shaking, were a systematic property of the Himalayan megathrust fault, or of megathrust faults in general." says Avouac. "Based on observations from this and other megathrust earthquakes, this is a possibility."

In contrast to what they saw with high-frequency waves, the researchers found that the earthquake produced an unexpectedly large amount of low-frequency waves with longer periods of about five seconds. This longer-period shaking was responsible for the collapse of taller structures in Kathmandu, such as the Dharahara Tower, a 60-meter-high tower that survived larger earthquakes in 1833 and 1934 but collapsed completely during the Gorkha quake.

To understand this, consider plucking the strings of a guitar. Each string resonates at a certain natural frequency, or pitch, depending on the length, composition, and tension of the string. Likewise, buildings and other structures have a natural pitch or frequency of shaking at which they resonate; in general, the taller the building, the longer the period at which it resonates. If a strong earthquake causes the ground to shake with a frequency that matches a building's pitch, the shaking will be amplified within the building, and the structure will likely collapse.

Turning to the GPS records from two of Avouac's stations in the Kathmandu Valley, the researchers found that the effect of the low-frequency waves was amplified by the geological context of the Kathmandu basin. The basin is an ancient lakebed that is now filled with relatively soft sediment. For about 40 seconds after the earthquake, seismic waves from the quake were trapped within the basin and continued to reverberate, ringing like a bell with a frequency of five seconds.

"That's just the right frequency to damage tall buildings like the Dharahara Tower because it's close to their natural period," Avouac explains.

In follow-up work, Domniki Asimaki, professor of mechanical and civil engineering at Caltech, is examining the details of the shaking experienced throughout the basin. On a recent trip to Kathmandu, she documented very little damage to low-story buildings throughout much of the city but identified a pattern of intense shaking experienced at the edges of the basin, on hilltops or in the foothills where sediment meets the mountains. This was largely due to the resonance of seismic waves within the basin.

Asimaki notes that Los Angeles is also built atop sedimentary deposits and is surrounded by hills and mountain ranges that would also be prone to this type of increased shaking intensity during a major earthquake.

"In fact," she says, "the buildings in downtown Los Angeles are much taller than those in Kathmandu and therefore resonate with a much lower frequency. So if the same shaking had happened in L.A., a lot of the really tall buildings would have been challenged."

That points to one of the reasons it is important to understand how the land responded to the Gorkha earthquake, Avouac says. "Such studies of the site effects in Nepal provide an important opportunity to validate the codes and methods we use to predict the kind of shaking and damage that would be expected as a result of earthquakes elsewhere, such as in the Los Angeles Basin."

Additional authors on the Nature Geoscience paper, "Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake," are Lingsen Meng (PhD '12) of UC Los Angeles, Shengji Wei of Nanyang Technological University in Singapore, and Teng Wang of Southern Methodist University. The lead author on the Science paper, "Slip pulse and resonance of Kathmandu basin during the 2015 Mw 7.8 Gorkha earthquake, Nepal imaged with geodesy" is John Galetzka, formerly an associate staff geodesist at Caltech and now a project manager at UNAVCO in Boulder, Colorado. Caltech research geodesist Joachim Genrich is also a coauthor, as are Susan Owen and Angelyn Moore of JPL. For a full list of authors, please see the paper.

The Nepal Geodetic Array was funded by Caltech, the Gordon and Betty Moore Foundation, and the National Science Foundation. Additional funding for the Science study came from the Department of Foreign International Development (UK), the Royal Society (UK), the United Nations Development Programme, and the Nepal Academy for Science and Technology, as well as NASA and the Department of Foreign International Development.


Tue, 26 May 2015 18:11:03 +0000
Preparing for Earthquakes with ShakeAlert
News Writer: 
Lori Dajose
A ShakeMap for the 1994 M6.7 Northridge earthquake with a typical seismogram recorded in Pasadena superimposed.
Credit: Caltech

A few seconds may not seem like long, but it is enough time to turn off a stove, open an elevator door, or take cover under a desk. And before an earthquake strikes, a few seconds of warning can save lives. The U.S. Geological Survey aims to provide those seconds of warning with ShakeAlert, an earthquake early-warning system now being tested on the west coast of the United States. On July 30, the USGS announced approximately $4 million in awards to Caltech, UC Berkeley, the University of Washington and the University of Oregon, for the expansion and improvement of the ShakeAlert system.

"Caltech's role in ShakeAlert will focus on research and development of the system so that future versions will be faster and more reliable," says Thomas Heaton (PhD '78), professor of engineering seismology and director of Caltech's Earthquake Engineering Research Laboratory. "We currently collect data from approximately 400 seismic stations throughout California. The USGS grant will allow Caltech to upgrade or install new stations in strategic locations that will significantly improve the performance of ShakeAlert."

Earthquakes radiate two kinds of seismic waves: fast-moving and often harmless P-waves, followed by S-waves, which can cause strong ground shaking. A system of seismometers called the California Integrated Seismic Network (CISN) acquires data streams literally at the speed of light and uses several algorithms to quickly pinpoint the earthquake's epicenter and determine its strength. ShakeAlert analyzes the first P-waves in the CISN data streams to send out digital alerts, providing the "early warning" to a region before the slower, destructive S-waves arrive.

While predicting when and where an earthquake will occur is impossible, this early-warning system can give necessary seconds of preparation. Current beta-test users receive these alerts as a pop-up on their computers, displaying a map of the affected region, the amount of time until shaking begins, the estimated magnitude of the quake, and other data. In the future, alerts may be available through text messages and phone apps.

Though still technically in testing stages, ShakeAlert has already provided successful warnings. In August 2014, the system provided a nine-second warning to the city of San Francisco during a magnitude 6.0 earthquake in South Napa. In May, during a magnitude 3.8 quake in Los Angeles, an alert was issued before S-waves had even reached the earth's surface.

"With this new USGS funding, we will be able to add 20 new sensors to CISN, making coverage more robust and thus lengthening warning times," says Egill Hauksson, a research professor of geophysics and a principal investigator along with Heaton on the ShakeAlert project. "Caltech and its partners will be able to continue the high-quality seismological research that is such a necessary foundation for a reliable earthquake early-warning system."

In 2011, Caltech, along with UC Berkeley and the University of Washington, Seattle, received $6 million from the Gordon and Betty Moore Foundation for the research and development of ShakeAlert.



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