A 4.1M earthquake centered in Isleton, in Sacramento County, prompted an Emergency Alert on phones in the Bay Area from the ShakeAlert system — though you might not have felt any shaking in SF.

At least we know the system works!

The earthquake struck at 9:29 a.m. Wednesday, according to the US Geological Survey (USGS), and it had a preliminary magnitude of 4.6, which likely prompted the wider alert. And even though we are scheduled for a big earthquake drill tomorrow — and today is one day after the Loma Prieta anniversary — this was not a test or a drill.

This is the third quake over 4.oM in recent days in Northern California. Early Monday morning, a 4.8M and a 4.1M quake rattled an area in Humboldt County, near Eureka.

The ShakeAlert Earthquake Early Warning system has yet to be put into full effect for a significant earthquake in San Francisco. Managed by the USGS, it launched in 2019 after over a decade's work on the project. Warnings can also come from the MyShake app, developed through UC Berkeley.

Warnings go out to an entire region through ShakeAlert and MyShake when a 4.5M or greater earthquake strikes, and you receive the alert if you are within a radius where shaking is likely to occur. The warning is meant to give residents of the area an extra few seconds to take cover before the shaking arrives.

In this case, for SF anyway, I didn't feel it. Did you?

Shaking was reported in the East Bay, and BART reported it was conducting track inspections before resuming regular service.

Otherwise, there's a lot of this going around:

Update: The USGS explains that the wide ShakeAlert went out due to the fact that the earliest estimates of the earthquake's magnitude put it at 5.7M. The agency says that because the quake was estimated over 5.0M, alerts went out not just through MyShake but through the national Wireless Emergency Alert system as well.

See the USGS tweets about it below.

Related: Earthquake Early Warning System Launches 13 Years After Project Began