An earthquake was widely felt across the Los Angeles area. We provide a quick summary of what happened.
By Temblor Team Members (@temblor)
At 1:49 p.m. local time on the afternoon of Wednesday, May 1, a magnitude 4.1 earthquake shook parts of the Los Angeles Area, including the cities of Corona, Anaheim, and Santa Ana. The epicenter was in the Santa Ana mountains just a few miles southwest of Corona. Because of the population density of the Los Angeles region, the earthquake was widely felt, although it caused little damage.
The broader region has experienced numerous small shocks in the past month (Figure 1). Wednesday’s quake struck close to the Elsinore fault, near the junction of the Glen Ivy (southeast), Whittier (northwest), and Chino (north) fault sections.
ShakeAlert
The earthquake triggered the ShakeAlert Earthquake Early Warning System, which is currently operational in California, Oregon, and Washington. Even though the quake was eventually determined to be magnitude 4.1 after human review, the automated ShakeAlert system estimated it as high as magnitude 4.5. That may not seem like a significant difference, but a magnitude of 4.5 is the minimum threshold an earthquake has to reach in order for alerting to occur on cell phones. Anyone within the octagon labeled MMI 3 in Figure 2 – in other words, light shaking on the Modified Mercalli Intensity scale – who had previously downloaded a ShakeAlert-powered app (like MyShake) is likely to have received an alert. In addition, anyone with an Android phone within that octagon should have received a push notification letting them know that there was a small earthquake nearby.
People receiving a warning from ShakeAlert likely would have had no more than a couple of seconds. People closer to the epicenter – folks in the city of Corona, for example – wouldn’t have received alerts until after they felt shaking. This is called the late alert zone (formerly referred to as the blind zone). In general, warning times are longer for larger earthquakes that impact a broader region.
The Wireless Emergency Alert (WEA) system is only triggered for earthquakes of magnitude 5 and larger, so no WEA messages were sent for this earthquake.
If you’re in the region and you felt this earthquake, let the USGS know. And don’t forget that if you feel shaking, the USGS recommends that you drop, cover and hold on.
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