GALWAY LAKE FAULT

A previously unmapped active fault in the Mojave Desert

by

DAVID J. BEEBY add ROBERT L. HILL

Geologists

California Division of Mines and Geology

At 6:28 p.m., 31 May 1975, a 5.2 magnitude earthquake occurred on the Mojave Desert in the Galway Lake area of San Bernardino County (figure 1). In response to a report from Dr. Clarence Allen of the Seismological Laboratory at the California Institute of Technology that surface rupture had been observed by U.S. Geological Survey geologists Gary Fuis and John Coakley, the California Division of Mines and Geology sent a team of geologists to the area to observe and document surface disturbance. Field investigations were made by Los Angeles District staff members Bob Hill, Dave Beeby, Jim Kahle, Russ Miller, Drew Smith, and Mike Manson.

Even though the Galway Lake area is generally uninhabited, data from this part of California are especially valuable because they are very scarce; only one previous event (the Manix Earthquake of 1947) is known to have caused surface rupture in the Mojave province in historic time. Because electricity generating facilities may be constructed in the Mojave Desert in the near future, geologically safe locations are an absolute necessity. This article gives a brief summary of our work on the surface effects of the earthquake associated with the previously unmapped fault. A more detailed report on this work and on work still in progress will be published at a later time.

Observed surface effects of the 31 May 1975 earthquake were of two types, ground rupture and ground shaking. Ground cracks were arranged in a left-stepping en echelon pattern indicative of right lateral strike-slip movement. This was confirmed by detailed examination of individual cracks which showed right lateral offsets ranging between 2 and 15 millimeters. Surface rupture, beginning about l kilometer north of Galway Lake and extending south toward Emerson Lake, was mapped for a distance of 6.8 kilometers in a zone up to 100 meters wide. It is proposed that this zone of rupture be named the Galway Lake fault.

Ground rupture caused by the Galway Lake earthquake followed the trace of a pre-existing unmapped fault through the study area. Eroded older scarps, sheared basement highs and aligned dikes were common in the central portion of the new rupture zone. Shallow exploration pits dug across the cracks showed caliche filled older cracks and roots concentrated in the cracked zone. These features indicate that previous Quaternary ground rupture had occurred on the Galway Lake fault. Reconnaissance on the nearby Emerson fault showed no evidence of surface rupture or movement caused there by the 31 May earthquake.

Ground shaking effects, including cracking, overturned boulders, disrupted surfaces, rolled rocks, and collapsed animal burrows, were common in the central section of the Galway Lake rupture zone.

The surface effects, especially ground cracking, were badly degraded due to heavy winds and drifting sand during the course of study. Fortunately, quantitative measurements had already been made, but the ephemeral nature of these surface effects emphasizes the need for a prompt response to any moderate earthquake, even in remote and uninhabited regions. In future work of this type, immediate response should continue to be given high priority so that data valuable to understanding and mitigating the earthquake hazards of California will not go unrecorded and will not be lost.