California Earthquakes This Week: North and South Not Related


Two moderate sized earthquakes have been felt in California this week: a magnitude 5.7 in northern California on May 23 and a magnitude 4.8 in southern California on May 30. These two earthquakes were not related. Both events were widely felt, but little damage resulted. The northern event (M5.7) was nearly ten times larger than the southern event (M4.8), but occurred in a remotely populated area of northern California. 

This morning, on May 30, 2013 at 7:38 am, a magnitude 4.8 struck near Isla Vista, CA, approximately 12 miles (20 km) from Santa Barbara. Unlike last week's earthquake in northern California, this event occurred in a densely populated area. The USGS ShakeMap (version 10) indicates maximum shaking intensity of VI (strong) on the Modified Mercalli (MMI) Scale. The USGS PAGER report states that only 2,000 people were exposed to this level of ground shaking. This event was felt throughout the Bakersfield-Los Angeles-San Luis Obispo area. Approximately 64, 000 people were exposed to moderate shaking (MMI V), 170,000 to light shaking (MMI IV) and over 1,200,000 to weak shaking (MMI II – III) according the USGS PAGER report.

Isla Vista Shakemap - May 23, 2013

Source: USGS

There are no reports of damage or injuries as a result of this morning’s earthquake and little or no damage would be expected from an earthquake of this magnitude. Several people have reported objects swaying and rattling in the immediate area around the epicenter.

Last week, on the evening of May 23, 2013 at 8:47 pm, an unrelated magnitude 5.7 earthquake struck near Greenville, CA, approximately 150 miles (240 km) from Sacramento. The USGS ShakeMap (version 25) indicates maximum shaking intensity of VIII (Severe).

Canyondam Shakemap - May 30, 2013

Source: USGS

Commonly, this level of severe shaking is associated with the potential for moderate to heavy damage; however, due to the remote location of the epicenter and the resistant building codes in California, this degree of damage was not expected. The USGS PAGER report estimates that no people were exposed to greater than MMI VI (strong shaking) and less than 10,000 people were exposed to moderate-to-strong (MMI V to VI) shaking. This event was felt over a wide distance. The USGS received nearly 10,000 felt reports in the past week, from as far away as the San Francisco Bay area and southern Oregon. This event is part of an earthquake swarm than has continued for several days. The M5.7 event is the largest in this swarm to occur to date.

 Northern California Shakemap - May 30, 2013


Had this morning's earthquake in southern California been the same magnitude as last week's earthquake in northern California, moderate damage could have occurred, especially to less-resistant structures.

California is one of the most seismically active areas in the world, with a risk of producing very large earthquakes. The southern California area experiences about 10,000 earthquakes every year, but most of them are so small that they are not felt. Only 15-20 events per year are greater than magnitude 4.0, like this morning’s event. Throughout history, several major damaging earthquakes have occurred throughout the state, but it has been many years since an event with significant losses. The events of the past week serve as a reminder of the earthquake risk in California.

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