Earthquake Swarm at Coso Junction, Eastern California, July 2001

Posted 24 July, 2001
Egill Hauksson, Kate Hutton, and David Oppenheimer

Summary
As of Tuesday, July 24, 9:00 am PST, Coso Junction, CA, there have been 25 earthquakes equal to or greater than M3.0 and 5 earthquakes equal to or greater than M4.0 . During this time there have also been thousands of earthquakes with magnitudes less than 3.0. These earthquakes have occurred in the area of the Coso Geothermal field, a region of known geothermal, tectonic and magmatic activity. The seismicity of these earthquakes demonstrates their source as crustal, not volcanic in origin. A similar swarm occurred in the beginning of 1992. The earthquake swarm activity in the Coso Range occurs within a right-stepping offset between the Airport Lake and Owens Valley faults in the Eastern California Shear Zone along the east side of the Sierra Nevada. North of the Coso Range, the major active fault along the eastern margin of the Sierra Nevada is the Owens Valley fault - the source of the M7.6, 1872 Owens Valley earthquake. At present, the Owens Valley fault slips ~6 mm/yr, comparable to the ~5-mm/yr rate for the Airport Lake fault south of the Coso Range. M3.0 and greater month-long earthquakes swarms occur almost every year.

The recent earthquake swarm near Coso Junction began last week and has so far culminated with the M4.9 event, 17 July 2001. Focal mechanisms of most of these earthquakes show right-lateral motion, striking N5° to N20°W with a small normal component. This swarm is located along the southern edge of a similar swarm that occurred in 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 earthquakes.

Earthquake swarms are common in the Coso Range and the area 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 step-over 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, the Owens 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 swarm activity in this region is quite common. The histogram below shows that magnitude 3 and greater earthquakes occur almost every year, and that notable swarms can persist for months. Many swarms contain an earthquake whose magnitude exceeds 4, but occasionally quakes as large at 5.8 have occurred.

This text will be updated as conditions warrant.

Topographic Map for this Area
There will be a red X at the earthquake epicenter.
(Note: Takes you off this site.)

Real-time Earthquake Map, Coso Junction
Map centered at 118-36
List of Earthquakes on this map

Coso: Permanent GPS Site Information
(Note: Takes you off this site.)

Steady-State Deformation of the Coso Range, East-Central California, Inferred from Satellite Radar Interferometry
Chuck Wicks, U.S. Geological Survey, Menlo Park, California
The southern Coso Range, in the northern Mojave Desert west of Death Valley, is subsiding, or sinking, at a nearly constant rate (from 1992 through 1997). We have measured this from information contained in images of the area taken by European Space Agency radar satellites. The area that is sinking is about 10km by 15 km in extent centered on the Coso geothermal field that is within a volcanic field that last erupted about 10,000 years ago