Western USA

Earthquake 29 July 2008 Los Angeles 

Information from the USGS web site

This event occurred at 11.42 am local time in the East Los Angeles area, at a depth of approx 15 km. It was felt at a maximum intensity of MM VI in the epicentral area, and was felt as far away as Las Vegas. The largest immediate aftershoock was Mag 3.8 at 11.52, and another of Mag 3.6 at 1.41 pm. 37 smaller aftershocks were also recorded in the first 2 hours after the main event

shake map

Historical seismicity

IMPORTANT EARTHQUAKES OF WEST COAST U.S.A.

The map below shows some of the important earthquakes of the west coast of the U.S.A.

Table of events plotted above

Plot of events for 12 months to 10 Sep 2005 ( map by IRIS)

TECTONIC SUMMARY

The boundary between the Pacific Plate and the North American plate is indicated on the map above, and runs approximately diagonally from the south east to the north west of the map. The San Andreas Fault is on the lower half.

The Pacific Plate is moving to the northwest at about 48 mm per year. Most of this (34 mm) is contained in motion on the San Andreas Fault. Approximately 11 mm per year is supplied by intra-plate motion east of the San Andreas Fault, in the Basin and range Province. The other 6 mm per year is assumed to occur to the west of the San Andreas Fault.

In the northern half of the map, sea-floor spreading occurs at the Gorda rise and the Juan de Fuca Ridge to its north. To the west of the Juan de Fuca Ridge is the Juan de Fuca Plate, which is being subducted under the western margin of the North American Plate, and results in volcanism, as is seen in the Mt. St. Helens Volcano, in the Cascade Range. As with the Juan de Fuca Ridge, the Gorda Rise is not particularly seismically active, but the transform faults at its northern and southern ends, the Blanco Fault and the Mendocino Fracture Zone, are.

CENTRAL AND EASTERN U.S.A

East of the Rocky Mountains, the most seismically active region in the USA by far is the New Madrid Seismic Zone. Although they are usually small, two to three earthquakes are located in this zone each week.

This zone was the source of probably the most important sequence of intra-plate earthquakes in the world in modern times. Between December 1811 and February 1812, at least 6 earthquakes of magnitude over 7 occurred, and two earthquakes of magnitude more than 8. At least 18 events from the region were felt on the eastern seaboard of the USA. Since 1812, there have been two events of magnitude greater than 6 in the region (in 1843, mag ~6.3, and 1895, mag ~6.6)

In contrast to the Southwest seismic zone of Western Australia, the New Madrid Seismic Zone has a unique shape, well defined by plots of earthquake hypocentres. It is approximately 150 km long, and 15 km wide. Zones of recent ( Quaternary) faulting and deformation have recently been identified to the north of the NMSZ.

Below - recent seismicity of the New Madrid Seismic Zone, central USA, ( figure from SIUC website)