The Limits of Earthquake Early-Warning Alerts

The magnitude 5.3 earthquake that shook parts of Southern California on Thursday may have been the region’s strongest in many years but fortunately only caused minor damage close to its epicenter offshore near the Channel Islands. It also provided a good opportunity for an earthquake early-warning system that’s been under development to show how seismic alerts can be relayed to the public in the years to come.

Thursday’s quake was centered west of Los Angeles on a strike-slip fault adjacent to Santa Cruz Island, and was felt over a wide swath of L.A., Santa Barbara and Ventura counties but due to the epicenter’s distance from populated areas, most only felt light shaking.

After the earthquake, Alissa Walker, a Los Angeles-based editor for Curbed who has been testing a quake-warning app from Santa Monica-based Early Warning Labs, reported that she had 34 seconds of lead time before the shaking reached her location.

The system used to relay the seismic warnings is built upon efforts by the U.S. Geological Survey and West Coast universities to develop ShakeAlert, which relies on networked seismometers to rapidly pinpoint the location of where an earthquake starts and transmit warnings ahead to cities and populated areas faster than damaging seismic waves can travel.

The relayed warnings, when tied into infrastructure, can automatically trigger fire station garage doors to open, prompt trains to slow down or stop and close valves on water storage tanks, though those quake-resiliency tech innovations on the West Coast have not yet been widely implemented, as they have in Japan.  

Emergency management officials in California and elsewhere on West Coast envision ShakeAlert notifications reaching the public at large through cellphones and other applications, giving people a warning in the moments before serious shaking reaches them—that could be enough time to shelter under a sturdy table to ride out the earthquake or move away from a brick-and-masonry building facade that could crumble during a major seismic event.

These early warning alerts are used in Japan and in Mexico City, where they’re are tied into public address systems in many buildings and public spaces. The U.S. has lagged in developing similar warning systems, but major progress has been made to build out the ShakeAlert network by the U.S. Geological Survey and its partner universities in West Coast states.

John Vidale at the Southern California Earthquake Center told the Los Angeles Times on Thursday that he heard the alert go off at his office at the University of Southern California, "maybe 10 seconds ahead, enough for us to puzzle about it before the shaking hit."

Although ShakeAlert isn’t yet available beyond a group of beta testers and partner organizations before the system becomes more publicly available, earthquake early-warning technology has been viewed with great promise in seismically vulnerable areas of the West Coast. In the federal spending package approved by Congress and the White House last month, champions of the earthquake early-warning network celebrated $23 million in new funding that will support continued development of the technology and its public safety applications. (The Trump administration had proposed to cut funding for more work on ShakeAlert.)

Last November, the city of Los Angeles put out a request for proposals for an app designed to relay earthquake early-warning alerts to the public with the aim of having something available by the end of this year.

For all the promise the ShakeAlert has, the system does have its limits.

Earthquake early-warning notifications are less useful the closer a recipient is to an epicenter of an earthquake. So in the case of Los Angeles, an early warning can be considerably helpful for massive quakes that start farther away from the city, like what’s expected for the next major “Big One” rupture of the southern San Andreas Fault near the Salton Sea. In that scenario, L.A. residents may get upwards of a one-minute warning before significant shaking reaches them. But L.A.’s lead time would be far shorter for quakes originating on faults in or closer to the city, like on the Hollywood Fault or in the Puente Hills southeast of downtown.

Even as ShakeAlert’s technology and ability to delivery earthquake early-warning alerts improves, there will be other challenges, especially on the user end, including potential “warning fatigue” from more frequent but smaller seismic events—not to mention accidental alerts, which have happened.

According to the U.S. Geological Survey, which late last month released a study on earthquake early-warning notifications:

When and if an advance warning is issued thus depends critically on the ground motion threshold set for alert notification. The time available to issue an alert depends on both a user’s distance to the rupturing fault and the minimum level of ground motion for which the user wants to be alerted. The amount of warning time depends most strongly on the ground-motion level that is used to trigger alerts. Longer warning times are possible if alerts are issued at lower thresholds, when only weak ground motion is expected from the earthquake.

“Using the example of an earthquake on the San Andreas Fault, that starts in northern California and ruptures toward San Francisco, alerts issued when just light shaking is expected in San Francisco could provide warning times as long as about 48 seconds in advance of when the earthquake shaking is felt there,” said Elizabeth Cochran, a USGS seismologist and coauthor of the report. “In contrast, if you wait to alert San Francisco until very strong shaking is anticipated, only 8 seconds of warning are possible.”

The authors noted that if users are willing to receive alerts and take safety actions even when it is unlikely the ground shaking will grow to become damaging, they are more likely to receive timely information they can act on. In contrast, if users prefer to limit alerts to events during which ground motion is expected to be very strong, then warning times will be short, or perhaps even arrive too late to act.

Regardless of the system’s limits, the potential benefits of ShakeAlert may not be fully realized until the next significant quake strikes a major West Coast population center, whether that’s something in Southern California, the San Francisco Bay area, or the I-5 corridor in the Pacific Northwest.

As seismologists and emergency managers say, it’s not a matter of if, but when. So it’s a race to get this lifesaving technology into the hands of the public.

PREVIOUSLY on Route Fifty:

• Los Angeles Seeks to Build—and Share—an Earthquake Early Warning App
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