According to a project manager quoted by the LA Times, about 40 people received the alert, and it took 8.7 seconds for the alert to go out. In this case, that reached people in the town of Paso Robles, about 22 miles away in enough time to give them about one second of warning. However, that far away the intensity dropped to two on the scale, hardly enough to jostle anyone or cause noticeable damage. If you’re near a powerful enough quake, the warnings can come to you via the Wireless Emergency Alert system, or pushed via the MyShake app on iOS and Android.

Please note that while we only send out the early warning alerts for M4.5 and above earthquakes, the initial ShakeAlert estimate for this event was M4.8, so that is why the alert went out for the (now) M4.4 earthquake.

— MyShake (@MyShakeApp) December 17, 2019

This system doesn’t predict earthquakes, and after quake’s early this year failed to produce a notification on phones in Los Angeles, the threshold for a warning has been lowered to at least a magnitude 4.5 and Modified Mercalli Intensity (MMI) of 3, which is similar to a truck driving by outside your house. According to UC Berkeley, tests have shown alerts can reach users of the MyShake app in about 3.7 seconds, while the WEA (like severe weather notifications and Amber Alerts) had an average delivery time of 13 seconds — so probably go ahead and install that app.

In Mexico and Japan, warning systems that send out alerts for seismic events have been in place for years, but a similar setup for California has been long-delayed. Finally, on Thursday the first statewide Earthquake Early Warning System will go online. Until now, large-scale alerts have only been available in LA County, where they haven’t been triggered yet.

As the LA Times reports, Thursday is the 30th anniversary of the Loma Prieta earthquake, where energy from a tremor under the Santa Cruz mountains took up to 35 seconds to reach the northern edge of San Francisco. While the ShakeAlert system California will rely on can’t predict earthquakes, it can feel them thanks to monitoring stations and send out messages using the MyShake app (click here to install of Android or iOS) or Wireless Emergency Alerts (like an AMBER Alert) that can give people time to prepare before the quake’s energy reaches them.