Alarm systems have been in place for many years in Mexico and Japan which carry out earthquake warnings but a comparable design has been long delayed for California. The first state-wide early warning system will officially be activated on Thursday. So far, large-scale alerts were only available in LA County, but have not yet been triggered.
Thursday marks the 30th year of the Loma Prieta terremble, as the LA Times says, when the northern edge of San Francisco was driven by energy from the tremor under the Santa Cruz mountains for 35 seconds.
While the ShakeAlert program of California can't predict earthquakes, it is sensed by the monitoring stations and sends out alert using the MyShake software (click here to download Android or iOS) or by Wireless Emergency Alerts (like AMBER Alert).
Also this week in the Bay Area was a 4.5 earthquake, and in central California a 4.7 earthquake. SFGate cites Richard Allen, Director of the UC Berkeley Seismological Laboratory, saying the median time from the identification to the arrival of alarms on sample devices is between 2.1 seconds and 1.6 seconds.
After several quakes in July have caused users to not be told LA, officials have lowered the bar for messages when at least magnitude 4.5 and magnitude 3 of the Modified Mercalli Intensity (MMI) are available.