Abstract

An earthquake of magnitude 5.2 occurred on May 31, 1975, near Galway Lake in the Mojave Desert, San Bernardino County, California. Preliminary information from the Seismological Laboratory at the California Institute of Technology placed the hypo-center at a depth of 5.8 km with the epicenter at lat 34°31.4′N and long 116°29.3′W. This event was accompanied by surface rupture in a zone 6.8 km long and as much as 100 m wide, trending N25°W to due north along an existing but unmapped fault. Field evidence and aftershock data by others indicate that the fault is vertical. Left-stepping en echelon fractures indicate predominant right-lateral displacement. Individual fractures are vertical and have right slip generally from 0.2 to 0.5 cm up to a maximum of 1.5 cm. Shallow pits dug across surface faults in Quaternary alluvium revealed plant roots concentrated in fractures indicating that surface rupture occurred along these fractures during at least one previous event. These and other features combined with topographic evidence confirm previous Quaternary displacement and may indicate at least one previous event accompanied by surface rutpure in Holocene time. This rupture zone has been named the Galway Lake fault zone.

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