When two earthquakes rattled the San Francisco Bay area on Monday and Tuesday, it was a hint that this might be an extraordinary week for ShakeOut, California’s annual earthquake drill held on the third Thursday of October.

It marked the debut of the state’s earthquake early warning system, which serendipitously fell on the 30-year anniversary of the Loma Prieta magnitude 6.9 earthquake that rocked San Francisco in 1989.

“This year ShakeOut was turbocharged,” said Sara McBride, a social scientist who studies earthquake preparedness with the United States Geological Survey (USGS). “We had this confluence of events that made it so much more relevant.”

The Oct. 17, 1989, quake, which unfolded during a live broadcast of a 1989 World Series baseball game at San Francisco’s Candlestick Park, left the region with nearly $6 billion in damages, 16,000 unlivable housing units, 63 fatalities and over 3700 injuries.

In the months that followed the 1989 tremor, as crews worked the wreckage of a collapsed Oakland freeway, scientists tested a bare-bones system to warn about Loma Prieta’s incoming aftershock waves — and it worked.

This work paved the way for California’s new earthquake early warning system unveiled by Gov. Gavin Newsom. “We’re announcing the nation’s first comprehensive early alert system for earthquakes,” he said at a Thursday news conference.

The MyShake app, now available for download to iOS and Android devices, allows quake warning messages to reach Californians statewide.

Earthquake early warning

“Earthquake early warning is about recognizing an earthquake that has begun, and the shaking that is coming,” said Richard Allen, director of the Berkeley Seismological Laboratory, who also spoke at the news conference.

In 2005, Allen and his collaborators began working to generate alert messages for strong earthquakes, the first steps toward what is now the USGS ShakeAlert warning network that covers the U.S. West Coast.

Then, in January 2019, the ShakeAlertLA app launched for Los Angeles County residents; it sounds a smartphone alarm if it detects a quake greater than a magnitude 5 (a threshold that was lowered to 4.5 after complaints that it did not warn users of the July 4 and 5 Ridgecrest earthquakes). But finding a way to deliver the product statewide was a final hurdle in the process.

Now, with the MyShake app, all Californians will be able to receive warnings before the next big quake. Most major cellphone carriers will also send the messages via Wireless Emergency Alerts, which are used for severe weather and other emergencies.

Earthquake early warning works because communications networks can receive and send messages faster than seismic waves travel.

In California, a dense network of 600 seismic stations record the leading P waves — the first seismic waves to arrive after a rupture begins. These compressional waves, which are usually felt as a “bump” but don’t typically cause damage, travel faster than the side-to-side shearing S waves and the damaging, rolling surface waves. And, in a big earthquake, P waves are the signal that more destructive waves will soon follow.

Because the MyShake app is a prototype, there is room for improvement. For now, users can expect to receive alerts before, during or after an earthquake. And the nature of earthquake warnings means that there can’t be much notice for those closest to the initial rupture.

“We recognize this is a work in progress that we’ll have to iterate over the next few years,” the governor said.

Scientists plan to expand the seismic network in the coming years because warning time is improved when sensors are located closer to an earthquake epicenter. And they hope that, with the new app, Californians’ mobile devices will become tiny sensors that can monitor seismic activity, and help to pinpoint a quake’s magnitude, location and intensity.

In the shadow of Loma Prieta

The first large urban earthquake since the 1906 San Francisco disaster, Loma Prieta left a legacy of lessons learned and work to be done.

One of those lessons was the degree to which loose sediments around the San Francisco Bay amplified shaking in parts of Oakland and San Francisco — some 55 to 60 miles away from the epicenter in the Santa Cruz mountains.

That intense shaking caused the collapse of Oakland’s double-decker Nimitz Freeway, a 1.6-mile section near the Bay Bridge known as the Cypress Viaduct, which was anchored in soft mud. In the subsequent six months, scientists radioed messages to crews working at the site whenever a strong aftershock was detected near the epicenter, sending 12 alerts in total.

If a far-reaching system like ShakeAlert had been in place in 1989, USGS scientists estimate that these heavily damaged areas around the bay could have received 15 to 20 seconds lead time to prepare for strongest shaking.

Two earthquakes, and a drill

This week’s two widely felt quakes in the San Francisco Bay area — Monday’s magnitude 4.5 northeast of Oakland and Tuesday’s 4.7 southeast of Santa Cruz — likely frayed nerves but were not damaging. But they are rare reminders that a larger, more destructive quake looms.

According to a 2015 report, the probability of a quake greater than magnitude 6.7 between 2014 and 2043 is 72 percent for San Francisco and 60 percent for Los Angeles.

That’s where the ShakeOut drill comes into play: It puts into practice experts’ top recommendations for staying safe during those dangerous events.

“The most frequent types of injuries have occurred because people were moving around during the shaking,” McBride said. “We are trying to get people to stop moving, drop to the ground and get under something sturdy.” McBride said that maneuver, also known as Drop, Cover and Hold On, should happen with every felt quake and every warning received.

“You have to have the training that underpins the value of getting the alert,” she said. “It’s this wonderful partnership: scientists provide the technology, but the community has to act on the alert. Once you get the alert, get into place immediately.”

In California, over 10 million people participated in the drill, practicing Drop, Cover and Hold On to protect themselves from falling debris during a large quake. And when the real big one strikes, earthquake early warning will give Californians enough time to do just that.