Tectonic Summary

A M5.1 earthquake occurred at 9:09PM on March 28, 2014, located 1 mile easy of La Habra, CA, or 4 miles north of Fullerton, CA. The event was felt widely throughout Orange, Los Angeles, Ventura, Riverside, and San Bernardino counties. It was preceded by two foreshocks, the larger of M3.6 at 8:03pm. The demonstration earthquake early warning system provided 4 second warning in Pasadena.

There have been 23 aftershocks as of 10:00PM on March 28, the largest of which was a M3.6 at 9:30PM, and was felt locally near the epicenter. 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. The CIIM map is shown below.

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 M5.9 1987 Oct. 1 Whittier Narrows earthquake.

Previously, the M5.4 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.

My cousin was at Disneyland/world when it hit. (? not sure which is which. LOL!) As she said, they "were shaken not stirred." They were in the middle of a ride, and everyone was scared but there was no panic that she could see. Rides and events were cancelled, and she said the park staff was great at reassuring and guiding everyone.

It has been 20 years since Southern California experienced a major earthquake, a powerful 6.7-magnitude temblor that rolled through Northridge, killing 57 people. But this stretch of seismic calm, though welcome in obvious ways, has undermined efforts to force Los Angeles to deal with what officials describe as potentially lethal deficiencies in earthquake preparation.

That may be changing. Since two back-to-back earthquakes Friday evening — a relatively small one with a magnitude of 3.6, followed by a long and rolling 5.1 quake — Los Angeles has been shaken by nearly 175 smaller aftershocks.
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Mr. Garcetti said he was preparing a plan to deal with what Ms. Jones, a familiar figure in California known as the Earthquake Lady, identified as the three most serious shortcomings in the city’s earthquake preparation. The first challenge is to fix unreinforced buildings that could collapse in a big quake, an issue that promises to stir debates about acceptable risk and who should pay for that very expensive endeavor.

The other two are to plan for catastrophic collapses in water supply and basic communication, as many of the aqueducts and Internet cables that supply Los Angeles cross the San Andreas Fault.