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
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.
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