Southern CA earthquakes show need for and limits of ShakeAlert

ShakeAlert is definitely useful when it gives even a few seconds of additional warning before a strong quake. What’s become apparent tho is that the app determines whether or not to issue a warning based on the strength of the quake where the device is located, and NOT based on the strength of the quake at its center. The two quakes in Southern California indicate tho that there’s a need to know both numbers — your local strength and the strength at the quake’s origin.

SoCal Earthquake: Why Everyone Is Talking About ShakeAlert

If you’ve been following news coverage about the 6.4-magnitude earthquake that hit Southern California on July 4, you’ve also been hearing about ShakeAlert.

ShakeAlert is a relatively new earthquake early warning system that is still being developed and tested by the U.S. Geological Survey (USGS) in partnership with a coalition of the California state government and California universities.

There is still no way to predict earthquakes before they happen, but ShakeAlert can detect that an earthquake has begun and can alert people even before they feel any shaking. The system is designed to give people tens of seconds of warning before shaking from a distant earthquake arrives at their location.

How does ShakeAlert work?

Over 1,600 seismometers that have been installed on the US West Coast send info to processing centers that rapidly determine location, magnitude, and estimated intensity of an earthquake.

During an earthquake, seismic waves travel in all directions, similarly to when a pebble hits the surface of a pond. As you may recall from science class, primary (P) waves travel faster than secondary (S) waves. ShakeAlerts can be sent faster than either wave.

After ground motion is detected by seismometers, the information is passed to an earthquake alert center where it is processed instantaneously. If the earthquake is large enough, eventually every mobile phone in the affected region – even the phones of non-residents – will receive an alert.

That’s the plan, but it’s still early days. At this time, there is no public smartphone app available that sends earthquake early warnings throughout all of California.

In January, a pilot program was launched in Los Angeles. Residents are able to download the ShakeAlertLA app, which sends a push alert in the event of an earthquake with a magnitude of 5.0 or greater. The app was developed by the city based off data in the ShakeAlert system and is available for Android and Apple smartphones.

Questions are surfacing as to why residents didn’t receive a ShakeAlert. — CBS News 8 (@CBS8) July 4, 2019

– via Twitter

Though the recent earthquake registered as a 6.4 at its epicenter in the Mojave Desert, ShakeAlertLA users did not receive a push alert, which led to much confusion when the shaking began. Officials said there was no alert because the earthquake’s magnitude in Los Angeles was only 4.5, with the epicenter well outside the predetermined warning area, the Los Angeles Times reported.

The #ShakeAlertLA app only sends alerts if shaking is 5.0+ in LA County. Epicenter was 6.4 in Kern County, @USGS confirms LA’s shaking was below 4.5. We hear you and will lower the alert threshold with @USGS_ShakeAlert — City of Los Angeles (@LACity) July 4, 2019

– via Twitter

After receiving questions and complaints from residents about the lack of push alerts from the app, the city of Los Angeles announced that officials will lowering the threshold, reported CNN and other outlets.

Most of the United States’ earthquake risk is concentrated on the West Coast. The Federal Emergency Management Agency (FEMA) has estimated the nationwide average annualized loss from earthquakes to be $6.1 billion, with 61% ($3.7 billion) coming from California and another 12% ($.8 billion) coming from Washington and Oregon combined, according the USGS’s ShakeAlert website.

In the next 30 years, California has a 99.7% chance of a magnitude 6.7 or larger earthquake and the Pacific Northwest has a 10% chance of a magnitude 8 to 9 megathrust earthquake, according to the USGS.

Later article for more info:

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