ShakeAlert, the earthquake early warning system for California, Oregon, and Washington managed by the U.S. Geological Survey (USGS), now uses real-time satellite data in addition to seismometers to detect ground motion. This upgrade improves the accuracy of earthquake magnitude measurements—a key factor in emergency response.
“This can tell us how big an earthquake is sooner, which ultimately translates to getting alerts out to people faster,” said Robert de Groot, ShakeAlert operations team lead.
The ShakeAlert system previously relied solely on data from broadband and strong-motion seismometers. These instruments measure the size and shape of seismic waves, including those that are generated by earthquakes. Seismologists have developed algorithms to convert these data and information about waves’ acceleration and velocity into an estimated earthquake magnitude.
Automated systems gather continuous, real-time data from a network of more than 1,500 seismometers located throughout the U.S. West Coast. When a strong earthquake occurs, the ShakeAlert system automatically issues a message to its alert delivery partners, including Google, NASA’s Jet Propulsion Laboratory, and transportation authorities. That triggers public announcements, cell phone notifications, the slowing or stopping of trains, and other actions aimed at protecting infrastructure and giving people time to take cover before shaking reaches their location. The greater the earthquake magnitude is, the bigger the area that’s likely to experience shaking and receive a ShakeAlert is.
Broadband seismometers are tuned to detect weak shaking from small or very distant earthquakes. They cannot accurately measure large earthquakes because the seismic waves that these shocks generate are too large for these instruments to record—larger waves get clipped off at the top of the record.
De Groot likens the problem to a punk rock singer screaming into a microphone—at a certain point, the louder they scream, the harder it is for audience members to understand. Broadband seismometers top out, or saturate, above about magnitude 7.0.
On the other hand, strong-motion seismometers, which are present at all ShakeAlert stations, can record the very large seismic waves that occur during large-magnitude earthquakes.
But even a strong-motion seismometer located right above an earthquake’s epicenter may not be able to determine magnitude because shaking itself is also difficult to differentiate at high intensities, explained Sarah Minson, a geophysicist at USGS who is not directly involved in ShakeAlert operations. “There’s not actually much difference between shaking for a magnitude 8 and magnitude 9 earthquake,” she said.
For very strong earthquakes, an accurate magnitude is calculated from seismometers located farther from the epicenter. But this calculation is not immediate, in part because the waves take time to move through the crust, Minson said. ShakeAlert optimizes for speed and accuracy, so initial estimates of earthquake magnitude come from stations closer to the epicenter.
This means that for the largest, most destructive earthquakes, ShakeAlert initially underestimates magnitude, said David Mencin, a geodesist at EarthScope, a nonprofit seismological research consortium. EarthScope partners with USGS and others to manage the satellite data for the ShakeAlert system.
Measuring Big Movement
Global Navigation Satellite Systems (GNSS) can measure the displacement of the ground—one factor used to calculate an earthquake’s magnitude (moment magnitude)—in real time. Unlike seismometers, these sensors can easily differentiate between magnitudes because they do not measure shaking.
Each GNSS sensor “doesn’t care about acceleration and velocity in the seismic waves,” as seismometers do, de Groot said. “It’s not trying to make all those calculations. It’s just saying, ‘The ground moved this much.’” That displacement can be used to determine an earthquake’s size. For the largest earthquakes, GNSS stations provide magnitude measurements that are more accurate than measurements from seismometers, he said.
Though ShakeAlert is currently using only the U.S. satellite network (GPS), the system may expand to use other countries’ satellites, de Groot said.
Accurate magnitude measurements are crucial for emergency response. How far from an earthquake’s epicenter alerts are sent is based in part on the magnitude of the quake—so underestimating magnitude means some people who will feel strong shaking may not be warned in time or at all. Correct magnitude measurements ensure that those at risk can take action to prepare for an impending earthquake—for example, by ducking under a table or pulling their car over to the side of the road.
“Underestimating the magnitude can have catastrophic effects.”
“If you underestimate the earthquake, you underestimate the area that’s affected,” said Yehuda Bock, a geodesist at the Scripps Institution of Oceanography at the University of California, San Diego, who is not involved with the ShakeAlert team. “Underestimating the magnitude can have catastrophic effects.”
For example, the 2011 magnitude 9.1 Great Tōhoku Earthquake in Japan killed more than 18,000 people. Japan has an earthquake early warning system similar to ShakeAlert, and warnings were issued. But at the time, it relied on seismometers that didn’t determine the true magnitude until about 20 minutes after the initial shaking. “A lot of things happen in 20 minutes,” Mencin said.
Scientists have been considering adding GNSS sensors to earthquake early warning systems for more than a decade, Bock said. A 2009 study he coauthored created a prototype earthquake early warning system with GPS data and found benefits to adding the sensors. “GPS and seismic instruments can be mutually beneficial,” the authors wrote. “Weaknesses in one system are offset by strengths in the other.”
De Groot said developing an algorithm to integrate both GNSS data and data from seismometers into an operational and accurate system for ShakeAlert took significant time. “When people’s lives are at stake, when a safety system needs to perform, you don’t want to get it wrong,” he said.
“I’m happy it’s finally happened,” Bock said.
—Grace van Deelen (@GVD__), Staff Writer
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