The magnitude 5.1 earthquake that struck at 9:09 pm on Friday, March 28 located one mile east of La Habra was felt widely throughout the Orange County, Los Angeles, Ventura, Riverside, and San Bernardino counties.
It was preceded by two foreshocks, a magnitude 3.6 at 8:03 pm and a magnitude 2.1 at 8:16 pm.
There were 38 aftershocks as of 10:45 pm, the largest of which was a magnitude 3.6 at 9:30pm, and was felt locally near the epicenter.
The USGS said the aftershock sequence may continue for several days to weeks, but will likely decay in frequency and magnitude as time goes by.
The maximum observed instrumental intensity was VII, recorded in the LA Habra and Brea areas, although the ShakeMap shows a wide area of maximum intensity of VI. The maximum reported intensity for the Community Internet Intensity Map (Did You Feel It?) was reported at VI in the epicentral area.
This sequence could be associated with the Puente Hills thrust (PHT). The PHT is a blind thrust fault that extends from this region to the north and west towards the City of Los Angeles. It caused the magnitude 5.9 in the 1987 Oct. 1 Whittier Narrows earthquake.
Previously, the magnitude 5.4 in the 2008 Chino Hills earthquake occurred in this region. It caused somewhat stronger shaking in Orange County and across the Los Angeles Basin.
The moment tensor shows oblique faulting, with a north dipping plane that approximately aligns with the Puente Hills thrust.
The demonstration earthquake early warning system provided four second warning in Pasadena.