US Earthquake Early Warning System Gets Major Upgrade

The ShakeAlert earthquake early warning system in California, Oregon and Washington, operated by the U.S. Geological Survey (USGS), now uses real-time satellite data in addition to seismometers to detect ground movement. This update improves the accuracy of earthquake magnitude measurements—a key factor in emergency response.

“This can tell us how strong an earthquake is sooner, which ultimately means alerts can be sent to people faster,” said Robert DeGroot, ShakeAlert operations team lead.

Previously, ShakeAlert relied solely on data from broadband seismometers and strong motion meters. These instruments measure the size and shape of seismic waves, including those generated by earthquakes. Seismologists have developed algorithms to convert this data, along with information about the acceleration and speed of the waves, into an estimate of the earthquake’s magnitude.

The automated systems collect continuous, real-time data from a network of more than 1,500 seismometers located across the West Coast of the United States. When a strong earthquake occurs, the ShakeAlert system automatically sends a message to its alert delivery partners, including Google, NASA’s Jet Propulsion Laboratory, and transportation authorities. This triggers public announcements, mobile notifications, trains slowing or stopping, and other actions aimed at protecting infrastructure and giving people time to take shelter before the shaking reaches their location. The larger the earthquake, the larger the area is likely to experience shaking and receive a ShakeAlert.

Broadband seismometers are designed to detect weak tremors caused by small or very distant earthquakes. But they can’t measure large earthquakes accurately because the seismic waves generated by these shocks are too large for the instruments to record—the larger waves are clipped from the top of the record.

De Groot likens the problem to a punk rock singer screaming into a microphone—at some point, the louder the screaming gets, the harder it is for the audience to understand what he’s saying. Broadband seismometers exceed, or saturate, around 7.0 on the Richter scale.

On the other hand, the powerful seismometers, present at all ShakeAlert stations, can record the very large seismic waves that occur during large-magnitude earthquakes.

But even a powerful seismometer directly above the epicenter may not be able to pinpoint the magnitude of the quake because the shaking itself is difficult to distinguish from strong quakes, said Sarah Minson, a geophysicist with the U.S. Geological Survey who is not directly involved in ShakeAlert operations. “There’s not a lot of difference between the shaking in a magnitude 8 earthquake and a magnitude 9 earthquake,” she said.

For very strong earthquakes, the magnitude of the quake is accurately calculated from seismometers located far from the epicenter. But that calculation isn’t instantaneous, Minson said, in part because the waves take time to travel through the crust. ShakeAlert improves the speed and accuracy, so the first estimates of the magnitude come from stations closer to the epicenter.

That means ShakeAlert initially underestimates the magnitude of larger, more destructive earthquakes, said David Mincin, a geodesist at EarthScope, a nonprofit seismic research consortium. EarthScope partners with the U.S. Geological Survey and others to manage ShakeAlert satellite data.

U.S. officials recommend people lie down on the ground, hide under a sturdy table or desk, cover their heads and necks, and hold on during an earthquake. Copyright: Earthquake Country Alliance Measuring Big Movement

Global navigation satellite systems can measure ground displacement—one of the factors used to calculate an earthquake’s magnitude (moment magnitude)—in real time. Unlike seismometers, these sensors can easily distinguish magnitudes because they do not measure vibrations.

Each GPS sensor “doesn’t care about the acceleration and velocity of the seismic waves,” as seismometers do, De Groot said. “It doesn’t try to do all these calculations. It just says, ‘The ground moved this much.’” That displacement can be used to determine the magnitude of the earthquake. For larger earthquakes, he said, GPS stations provide more accurate magnitude measurements than seismometers.

Although ShakeAlert currently uses only the US GPS satellite network, the system could expand to include satellites from other countries, De Groot said.

Accurate measurements of earthquake magnitude are crucial to emergency response. The distance at which alerts are sent to the epicenter depends in part on the magnitude of the earthquake—so underestimating the magnitude of an earthquake means that some people who might feel a strong shaking may not be warned in time or at all. Accurate measurements of earthquake magnitude ensure that those at risk can take action to prepare for an impending earthquake—for example, by hiding under a table or pulling over to the side of the road.

“Underestimating the problem can have disastrous consequences.”

“If we underestimate the earthquake, we underestimate the area affected,” says Yehuda Bock, a geodesist at the Scripps Institution of Oceanography at the University of California, San Diego, who is not part of the ShakeAlert team. “Underestimating the size of an earthquake can have catastrophic effects.”

For example, the 9.1-magnitude Great Tohoku Earthquake in 2011 killed more than 18,000 people. Japan had an early warning system similar to ShakeAlert, and warnings were issued. But at the time, the system relied on seismometers that didn’t determine the true magnitude until about 20 minutes after the initial shock. “A lot of things happen in 20 minutes,” Mincin says.

Scientists have been thinking about adding GNSS sensors to earthquake early warning systems for more than a decade, Bock said. In a 2009 study he co-authored, they built a prototype earthquake early warning system using GPS data and found benefits to adding the sensors. “GPS and seismic devices can be mutually beneficial,” the authors wrote. “The weaknesses of one system are offset by the strengths of the other.”

De Groot said it took a long time to develop an algorithm to integrate GNSS and seismometer data into a precise operational system for ShakeAlert. “When people’s lives are at stake, when a safety system needs to perform, you don’t want to make a mistake,” he said.

“I'm glad this finally happened,” said Bock.

—Grace Van Deelen (@GVD__), Staff Writer

Citation: van Deelen, G. (2024), US earthquake early warning system gets major upgrade, Eos, 105, https://doi.org/10.1029/2024EO240363. Published August 13, 2024. Text © 2024. AGU. CC BY-NC-ND 3.0 Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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