Found this interesting Coso info on the Trinet site....

Michael

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11:53 PDT, Wednesday, July 18

Submitted by Egill Hauksson, Kate Hutton

The recent earthquake swarm near Coso Junction began last week and has so far culminated with a M4.9 event, 17 July 2001. Focal mechanisms of most of these earthquakes show right-lateral motion, striking N5 to N20W (DEG), with a small normal component. This swarm is located along the southern edge of a similar swarm that occurred in the first half of 1992 and lasted (off and on) for several months. These events are not volcanic in nature, because they have normal frequency content as expected for crustal tectonic earthquakes.

Earthquake swarms are common in the Coso Range and the erea to the south, extending into Indian Wells Valley, because of the extensional tectonics and presence of a magma chamber beneath the Coso Geothermal field. The extension in the Coso Range and to the south is driven by a releasing stepover between the right-lateral Airport Lake and Owens Valley faults along the east side of the Sierra Nevada. The footwall of this system, the Coso Range, is attached to the Sierra Nevada, and is effectively being pulled to the northwest from beneath Wildhorse Mesa, which is moving as part of the Walker Lane belt. The brittle upper crustal extension may be accommodated at depth by ductile stretching and emplacement of igneous bodies, the
presence of which have been inferred in the middle to upper crust from analysis of seismic waves. Late Cenozoic intrusions are interpreted to be the source of heat for the Coso geothermal field.

North of the Coso Range, the major active fault along the eastern margin of the Sierra Nevada is the Owens Valley fault that was the source of the M7.6 1872 Owens Valley earthquake. Surface rupture associated with the 1872 earthquake extended as far south as the western margin of Owens Lake, within 10 km of the Coso Range. Coseismic surface displacement during the 1872 event was predominantly dextral. Currently, theOwens Valley fault slips about 6 mm/yr, which is comparable to the 5 mm/yr rate estimated for the Airport Lake fault south of the Coso Range. These relations thus suggest that earthquake swarms on Quaternary faults in the Coso Range transfer dextral slip along the eastern margin of the Sierra Nevada northward from Indian Wells Valley to the southern Owens Valley.

• Earthquake Commentary for July, 2001