Preliminary Report on the 1995 Ridgecrest Earthquake Sequence in Eastern California

Egill Hauksson, Kate Hutton, Hiroo Kanamori, Lucile M. Jones, James Mori, Susan E. Hough, & Glenn Roquemore

Published 1995, SCEC Contribution #226

The Ridgecrest earthquake sequence began on 17 August 1995 with a ML 5.4 earthquake. As of October 3, 1995, the Southern California Seismic Network (SCSN) had recorded over 4,500 events in the sequence, with eight events of M ≥ 4.0. These earthquakes are occurring along the eastern edge of the Indian Wells Valley along a small stretch of the thoroughgoing Eastern California Shear Zone (ECSZ). Previous large events within the ECSZ include the 1992 (MW7.3) Landers earthquake sequence and the 1872 (M 7.6) Owens Valley earthquake. The only large earthquake to occur near Indian Wells Valley, was the 1946 Walker Pass (M 6.0) earthquake on an unknown fault in the Sierra Nevada mountains to the west. The ECSZ transfers some of the relative motion between the North America and Pacific Plates away from the San Andreas fault to the western Great Basin of the Basin and Range province.

The Indian Wells Valley is flanked by the Coso Range to the north, the Argus Range to the east and the Sierra Nevada to the west. The valley floor is cross-cut by a northerlytrending mosaic of fault segments that merge towards the north with the frontal fault of the Sierra Nevada or the rupture zone of the 1872 earthquake. In the south, this mosaic of segments diffuses into a broad zone of faulting that disappears before it is cut off by the west-striking Garlock fault. The mosaic of fault segments consists of north-to-northwest striking as well as a lesser number of northeast-striking faults, most of short length, of less than a kilometer up to 10 km length (Figure 1).

During the last three decades the seismicity of this region has been characterized by swarms of earthquakes, some of which have lasted more than 12 months. These swarms typically have thousands of small earthquakes, with the largest earthquakes in the magnitude range of 4 to 5. The swarms tend to migrate in space. For instance, the fourteen-month-long swarm in 1980—1981 migrated from north to south over a distance of 12 km, with temporal bursts in activity. The largest earthquake to occur in the valley itself was a ML 4.9 event in April, 1982. It caused some ground cracking (Roquemore and Zellmer, 1983) on two short fault segments.

Hauksson, E., Hutton, K., Kanamori, H., Jones, L. M., Mori, J., Hough, S. E., & Roquemore, G. (1995). Preliminary Report on the 1995 Ridgecrest Earthquake Sequence in Eastern California. Seismological Research Letters, 66(6), 54-60. doi: 10.1785/gssrl.66.6.54 .