Quakes can't be sex g?i c?mpredicted. But unassuming Californians can now be alerted that a significant temblor has hit and it's time to promptly "Drop, Cover, and Hold On" before their world starts shaking.

On Thursday, California Governor Gavin Newsom announced the state's Earthquake Early Warning System, which means Golden State denizens from the northern Redwoods to the Salton Sea can be alerted of imminent shaking in two major ways.

The first part of the system is the new, free MyShake app (available on iOS and Android phones) which will notify the app's users about nearby quakes. The second element is Wireless Emergency Alerts which automatically blasts out text messages (the same way you receive an emergency Amber alert) after a quake has been detected. Both alerts, which can give people seconds to tens of seconds of warning, receive quake information from the U.S. Geological Survey's innovative, sensor-based, ShakeAlert system.

The new MyShake app, developed at the University of California, Berkeley, is a significant, though still prototypical part of California's quake warning plan. It works, "but it will improve over time," Richard Allen, director of UC Berkeley's Seismological Laboratory, said at a press conference Thursday. "We want every Californian to download the MyShake app," added Mark Ghilarducci, the director of California's Office of Emergency Services.

"There’s not really a downside," John Vidale, a seismologist and professor of Earth Sciences at the University of Southern California, said of the app. "Conceivably it could tell us before the shaking starts," said Vidale, who was not involved in creating the app but has tested earlier versions of quake alert apps.

What's more, the app uses smartphone sensors to detect significant earthquakes.

"In addition to alerting you about impending shaking [MyShake] turns your phone into an earthquake sensor," noted Wendy Bohon, a geologist at the Incorporated Research Institutions for Seismology. Bohon also had no role in developing California's early warning system.

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As the app's UC Berkeley creators note online, "Our testing has shown that most modern phones in use today can record earthquakes down to magnitude 5 within 10 km of the epicenter." (A magnitude 5 is a moderate quake "felt by nearly everyone" and will result in some broken dishes and windows, according to the U.S. Geological Survey.) MyShake's software will get progressively better at identifying quakes as more people use the app.

For now, text messages will be sent out to Californians who feel quakes with an intensity as low as 4.5, a light quake felt by many people indoors. But the MyShake app will send alerts at even lower quake intensities of 3, said UC Berkeley's Allen. This will match the current alert thresholds of the separate ShakeAlertLA app, released in January exclusively for Los Angeles denizens. The app angered many Angelenos in July when they felt a level 3 quake but didn't receive alerts. At the time, the L.A. app was set to notify users of shaking intensities of 4 or higher.

Finding the best alert levels for both text and app systems will require tweaking. "It will not be static," Governor Newsom said. And there'll certainly be ample opportunity for finding the ideal alert thresholds, as there will be plenty more quakes.

The big takeaways

Any "shake alert" received via text or the MyShake app is only a warning, albeit a potentially major warning.

"The most useful thing is to have your head up [figuratively] and know what’s happening," said Vidale. "We want people to do sensible things."

That means droppingto your hands and knees, covering your head and neck with an arm (ideally crawling under sturdy furniture if available), and holding on until the shaking stops.

"ShakeAlert can save lives and reduce the chance of injuries by giving people time to take protective actions like 'Drop, Cover and Hold on' before shaking from an earthquake arrives," said Bohon. "It will also potentially give them time to move away from dangerous or hazardous locations."

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But this early warning system comes with a major limitation, emphasized Bohon. If you're very close to an earthquake's epicenter, you may not receive an alert before shaking, or violent shaking, begins. "This is because the system needs time to determine the earthquake's size and likely shaking levels and it also needs time to distribute the alert," she said.

Most Californians live in quake country, so they should already have their plans together. "People should be sure to continue to prepare for earthquakes beforethey happen," said Bohon. "The few seconds to tens of seconds that ShakeAlert may provide is obviously not enough time to take critical actions like making an earthquake kit, securing heavy furniture to the walls, bolting the home to the foundation, etc."

When future quakes hit, Californians now have a new tool in their co-existence with a volatile, incessantly moving earth.

"Should it work well?" asked Bohom. "Hopefully, yes. ShakeAlert is an innovative technology that will improve over time."