- © 2008 by the Seismological Society of America
The 29 July 2008 Mw 5.4 Chino Hills earthquake was the largest event to occur within the greater Los Angeles metropolitan region since the Mw 6.7 1994 Northridge earthquake. The earthquake was widely felt in a metropolitan region with a population of more than 10 million people and was recorded by hundreds of broadband and strong-motion instruments. In this report we present preliminary analysis of the event and discuss its significance within the seismotectonic framework of the northern Los Angeles basin as revealed by previous moderate earthquakes.
The Chino Hills mainshock-aftershock sequence began at a depth of about 15 km in the east Los Angeles area at 11:42 am (PST) (Figure 1). The epicenter is between two mapped faults: the Whittier fault to the west and the Chino Hills fault to the east. The focal mechanism indicates a mixture of strike-slip and thrust faulting on a west-southwest or a west-northwest striking nodal plane. The mainshock was followed by only two aftershocks with M >3; M 3.8 at 11:52 am (PST) and M 3.6 at 13:40 (PST). In the first two hours, 37 smaller aftershocks were also recorded in the magnitude range of 1.3 to 2.8. By 14 August the Southern California Seismic Network (SCSN), a joint project of the California Institute of Technology (Caltech) and the U.S. Geological Survey (USGS) had recorded ∼ 150 aftershocks of M ≥ 1.0. The mainshock was not preceded by foreshock activity.
During the 2008 Chino Hills sequence, the SCSN automatically processed real-time waveform data from 370 stations across southern California. The first location and ML magnitude estimate of 5.6 were released ∼ 80 s after the origin time. An updated location and final ML 5.8 were released after ∼ 140 s. The automatic moment tensor and the Mw estimate of 5.4 were available ∼ …