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Magnitude 5.4 Chino Hills Earthquake, July 29, 2008

At 11:42 am on Tuesday, July 29, 2008, the greater Los Angeles metropolitan area experienced its largest earthquake in years as a magnitude 5.4 event occurred 8.5 miles beneath the Chino Hills. This earthquake was felt across much of southern California, and as far away as Las Vegas, Nevada.

Earthquake Preparedness Information

Because this earthquake was widely felt, but did little serious damage, it served as an excellent opportunity for southern Californians to test their readiness in the event of a major earthquake. In November, people all across the region will participate in The Great Southern California ShakeOut, the largest-ever earthquake drill in the United States. Please join us as we make history!

Question: The magnitude was once given as 5.8; why is it now only magnitude 5.4?

Answer: The magnitude was initially determined automatically by a computer. The first preliminary magnitude determination occurred roughly 80 seconds after the origin time of the earthquake, once 40 amplitude ratings had been gathered by seismic instruments around southern California. This initial estimate was ML5.6. The designation ML stands for local magnitude. It is roughly analogous to the original Richter magnitude scale, which was developed in the 1930s specifically for use in southern California.

At about 140 seconds after the origin time of the earthquake, a final local magnitude rating was automatically assigned by computer analysis: ML5.8. This was not the end of the process of magnitude determination, however.

Computers and humans continued to process the data to arrive at a complete moment tensor solution for the earthquake. This process, which takes about 5 minutes, calculates the likely orientation of the causative fault plane, and also the "seismic moment", or energy released by the earthquake. This allows for the determination of the size of the earthquake on the moment magnitude scale, denoted as MW, which for earthquakes of magnitude 5 or greater, gives a more accurate reflection of the true size of an earthquake than does local magnitude ML (because of the amount of data needed to compute moment magnitude, these determinations tend to become less reliable below magnitude 5; for different reasons, local magnitude tends to "saturate" above magnitude 6).

Since this earthquake met the minimum magnitude requirement, and since moment magnitude is the preferred measure within the seismological community, the size of the earthquake was then "revised" from local magnitude (ML) 5.8 to moment magnitude (MW) 5.4. This was not so much a reduction in size as a change of scale. Still, as the media does not generally report the type of magnitude scale being used (local, moment, or others), this sort of change often creates much confusion.

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