Borrego Mountain Earthquake of April 8, 1968

by T. Leslie Youd, (A.M.ASCE), Res. Civ. Engr.; U.S. Geol. Survey, Menlo Park, CA,
Robert O. Castle, Geologist; U.S. Geol. Survey, Menlo Park, CA,

Serial Information: Journal of the Soil Mechanics and Foundations Division, 1970, Vol. 96, Issue 4, Pg. 1201-1219

Document Type: Journal Paper


The strongest earthquake (M6.5) to strike California in more than 15 years hit the western edge of the Imperial Valley near Borrego Mountain at 6:29 p.m. (PST) April 8, 1968. The location of the shock in a largely undeveloped area was apparently responsible for the generally light structural damage and absence of serious injury. However, a joint California Institute of Technology—U.S. Geological Survey team observed a number of ground effects that have important engineering implications. Tectonic ruptures along a 20-mile segment of the Coyote Creek fault were especially significant because of the unusual width and complexity of the fractured zone. The locations of individual fractures could have been closely predicted over no more than half their extent. Ground effects characteristics of Modified-Mercalli intensities of VIII-IX were observed at distances of up to 30 miles from the ruptured fault and, as such, roughly matched those indicated for the epicentral area. It is inferred that intensity was governed as much by ground conditions as proximity to the epicenter.

Subject Headings: Mountains | Earthquakes | Geological faults | Cracking | Damage (structural) | Accidents | Joints | Geological surveys | California | United States

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