Late-night phone alert tells Mendocino County to brace for 4.7 magnitude earthquake, but it was only 3.5. Here’s why

A phone alert late Sunday told more than 64,000 residents around Mendocino County to brace for a 4.7 magnitude earthquake.

Instead, it was a 3.5 magnitude temblor — and fully felt only by those within a half-mile of its epicenter, about 12 miles east of Cloverdale.

ShakeAlert, the U.S. Geological Survey’s system that uses sensors to detect ground motion, determined it was a 4.7 magnitude quake after initial rumbles, said Robert de Groot, ShakeAlert operations team lead.

“Using about one second worth of data — because we need to move quickly — we estimate the location of the earthquake, the size of the earthquake... and the amount of shaking that could happen around that earthquake,” he said Monday.

Push alerts were then sent out on the MyShake phone app and on Android Alerts, both of which are sponsored by the state.

Just after 11 p.m., 17,000 MyShake app users saw the estimated 4.7 magnitude on their screens with the message “drop, cover, hold on.”

Within seconds, the quake was downgraded to magnitude 3.5.

ShakeAlert’s misjudgment was a product of several factors. First, the station was about 4 miles from the quake’s epicenter, said Angie Lux, a project scientist at the Berkeley Seismological Laboratory.

Because it was so near the center of the quake, the sensor could have picked up primary and secondary waves at the same time, thus confusing the system into believing the quake was stronger.

“It got all of that information more or less simultaneously,” she said. “That’s probably why it created such a high magnitude estimate for that particular station.”

Another factor is the urgency with which the system acts in order to alert residents to an emergency. It’s a balance between speed and accuracy, de Groot said.

It’s not the first time de Groot has seen such a large discrepancy. For the 5.1 magnitude earthquake Aug. 20 in Southern California that accompanied Tropical Story Hillary, de Groot said he received an alert reporting an esimated 6.0 magnitude quake.

de Groot said he feels MyShake should indicate the initial magnitude is an estimate, but it still served its purpose to alert people to possible danger and give time for preparation.

“We hope people don’t sit there staring at their screens,” he said. “What you should do is get underneath the table.”

Lux said ShakeAlert is constantly being tweaked and will likely see a change based on how the system reported the Mendocino County quake.

“With every earthquake or event that happens, we learn something new,” she said. “That’s why our system is constantly evolving and updating.”

You can reach Staff Writer Madison Smalstig at madison.smalstig@pressdemocrat.com. On Twitter @madi.smals.