USA: When should we issue earthquake warnings? It’s complicated.
Seismologists need help from social scientists to know how people respond to earthquake alerts.
By Ula Chrobak
When back-to-back earthquakes shook Southern California in July, residents of Los Angeles were frustrated; Many felt the earthquakes, which originated over 100 miles away in Ridgecrest, but received no alert through their ShakeAlertLA phone apps. But the app didn’t err—it was designed to alert users when shaking was expected to reach the Modified Mercalli Intensity of 4, a level at which shaking is thought to be strong enough to cause some damage, such as a dish or book falling off a shelf.
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But geologists worry that these warning systems have been implemented without careful thought as to how people will use them. With regard to the California system, Elizabeth Cochran, USGS research geophysicist, says “I don’t believe there’s been a sufficient introduction to this system.”
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Early earthquakes warnings go out when seismic stations record a nearby fault slipping. Sensors measure the first wave of an earthquake, and then an automated system estimates how far the shaking will ripple out and how strong it will get. This system has about two seconds make the call, explains Cochran, and then it sends out an alert using cell phone towers in the area where the shaking will get to (or the shaking information is shared through an app).
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It’s what people do when they receive these alerts that seismologists worry about. Cochran grew concerned about hearing how people in Mexico City reacted after getting an earthquake warning. In the week following the 2017 Puebla earthquake, which claimed over 300 lives, residents got an early warning of another tremor. After getting the warning, two panicked residents had heart attacks and died, and one jumped out of a window. The earthquake itself ended up being very weak—most residents didn’t even feel it.
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