Government officials and Caltech scientists gathered at the Caltech Seismological Laboratory on October 17 to declare ShakeAlert—an earthquake early warning system for the three states along the West Coast—"open for business."
Caltech president Thomas Rosenbaum, Sonja and William Davidow Presidential Chair and professor of physics, led the midmorning press conference, which included U.S. Representative Adam Schiff; U.S. Representative Judy Chu; Tom Heaton, professor of engineering seismology; Lucy Jones, research associate in geophysics at Caltech and founder of the Dr. Lucy Jones Center for Science and Society; Doug Given, earthquake early warning coordinator for the United States Geological Survey (USGS); and Ryan Arba, seismic hazards branch chief at the California Office of Emergency Services.
"Caltech has worked for nearly 100 years with colleagues in government and other academic institutions to leverage the insights and tools of seismology against the risks of earthquakes," Rosenbaum said, announcing a new stage in the development of an earthquake early warning system for the West Coast. "Partner institutions can now use ShakeAlert to automatically slow trains; warn industrial sites to shut off gas lines; and warn personnel to drop, cover, and hold on."
Given added: "Today is important because we're making a large change from a production prototype in pilot mode to an open-for-business operational mode. Now, the system is not yet finished, it's not yet complete; there is a lot of work to be done. However, there is a lot of capability in the system as it exists today to the point that it can definitely be used."
Earthquake early warning systems like ShakeAlert consist of a network of sensors near faults that transmit signals to data-processing centers when shaking occurs. These data-processing centers use algorithms to rapidly determine the earthquake's location, magnitude, and the fault rupture length—determining the intensity of an earthquake and sending out an alert that can provide seconds or even minutes of warning. Paired with automated responses that will shut off gas before shaking starts, ShakeAlert could be instrumental in preventing the fires that typically damage cities after a major earthquake, Jones said.
Earthquake early warning systems do not predict earthquakes before they happen. Rather, they transmit a heads-up that an earthquake is happening; a heads-up that can arrive ahead of the seismic waves generated in the quake, potentially providing crucial time to allow individuals to take cover and for infrastructure to prepare for the quake (for example, for trains to halt operation). These warnings operate on the principle that seismic waves travel at just a few miles per second, but messages can be transmitted almost instantly. During an earthquake, several types of seismic waves radiate out from the quake's epicenter, including compressional waves (or P-waves), transverse waves (or S-waves), and surface waves. The weak P-waves move faster than the damaging S-waves and surface waves. With an earthquake early warning system in place, those P-waves will trigger sensors that can send out a warning ahead of the arrival of the S-waves and surface waves.
Though only half of the sensor network that ShakeAlert will need has been built out so far—primarily around major metropolitan areas—the state of California and the federal government have allocated funding that should allow the rest of California's portion of the network to be constructed over the next two years, Given said. In addition, an upgrade to the software that processes data from the sensor networks was deployed on September 28. This new software should reduce the number of mistakes and missed alerts, making ShakeAlert more reliable, Given said.
A key step now is for companies and institutions to help find ways to take advantage of these alerts to save lives, he said.
"This is a wonderful milestone," Schiff said. "We can now see the end, I hope, in two or three years where the system is fully built out and funded and in operation. And once people come to see the benefit, then the future of the system will be even brighter. Getting that kind of advance notice is going to be so meaningful in terms of making sure people get to a safe place."
Future iterations of the system will be able to send warnings to cell phones as well, Schiff said. Such alerts will need to be rolled out with public education to explain to individuals what to do when they receive such alerts—not to panic—and know that there could be false alarms.
Chu, whose district includes Caltech, said, "One of the reasons that I am so proud to be a representative from this area is our science. In our district, amazing advances are happening every day that will take us to Mars or bring us a better understanding of our environment. And the ShakeAlert that we are announcing today belongs in that pantheon of history-making innovations to come out of Caltech."
For Heaton, one of the fathers of ShakeAlert and a scientist who has been interested in earthquake early warning since 1979, this is a day that was a long time in coming.
"In those days, I could see that we could technically do it. But what I didn't really understand was what was involved to get 40 million people on the West Coast to get together to try and make this system a reality. What it really takes is leadership to do that," Heaton said.
Earthquake early warning systems already exist in Mexico and Japan, which have experienced recent and devastating earthquakes. But it has been difficult to find the political will to spend millions of dollars developing a system for the U.S. West Coast, which is long overdue for a serious earthquake.
ShakeAlert has been in development since 2006. In 2011, Caltech, along with UC Berkeley and the University of Washington received $6 million from the Gordon and Betty Moore Foundation for the research and development of the system; and in 2015, the USGS announced approximately $4 million in awards to Caltech, UC Berkeley, the University of Washington, and the University of Oregon for ShakeAlert's expansion and improvement.
Currently, ShakeAlert's infrastructures consist of the California Integrated Seismic Network (400 ground-motion sensors operated by Caltech in partnership with UC Berkeley, the USGS, and the State of California), and the Pacific Northwest Seismic Network (a similar regional network operated by the USGS, University of Washington, and the University of Oregon).
Over the past few years, ShakeAlert has detected thousands of earthquakes, including two that caused damage. It began sending alerts within four seconds of the beginning of the magnitude 5.1 La Habra earthquake in 2014, and gave users in Berkeley five seconds of warning before seismological waves arrived during the magnitude 6.0 South Napa earthquake, also in 2014. Beta-test users received these alerts as a pop-up on their computers; the pop-up displayed a map of the affected region as well as the amount of time until shaking would begin, the estimated magnitude of the quake, and other data.
Researchers with the Advanced Rapid Imaging and Analysis (ARIA) project, a NASA mission led by Caltech scientists, used new satellite data to produce a map of ground deformation on the resort island of Lombok, Indonesia, following a deadly 6.9-magnitude earthquake on August 5.
The false-color map shows the amount of permanent surface movement that occurred, almost entirely due to the quake, over a six-day period between satellite images taken on July 30 and August 5.
From the pattern of deformation in the map, scientists have determined that the earthquake fault slip was on a fault beneath the northwestern part of Lombok Island, and it caused as much as 10 inches (25 centimeters) of uplift of the ground surface. White areas in the image are places where the radar measurement was not possible, largely due to dense forests in the middle of the islands.
Through these maps, NASA and its partners are contributing important observations and expertise that can assist with response to earthquakes and other natural or human-produced hazards.
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
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.
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
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
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.
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?
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.
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.
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."
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.