Earthquakes and folding, Coalinga, California

Abstract

The magnitude 6.6 Coalinga earthquake of May 2, 1983, and most of the other earthquakes of magnitude 4 and greater near that town are tentatively attributed here to flexural slip on the flanks of growing folds. This explanation apparently conflicts with the 100-yr-old theory that all tectonic earthquakes are caused by slip on faults. Perhaps the Coalinga events, in an area where no active faults had been mapped, will serve to suggest another possible mechanism for the generation of some earthquakes. Here the strongly folded and long Coalinga anticline and syncline in a thick stratigraphic section seem to require a mechanism of bedding-plane slip. The magnitude 2.5 Lompoc earthquake of April 7, 1981, can also be attributed to flexural slip on the limb of a fold. Flexural slip (bedding-plane slip), usually unseen and unmappable, is clearly revealed in several areas in southern California. Many small earthquakes may be produced in these and other areas where direct evidence of active flexural slip by folding is unavailable. If the May 2 Coalinga earthquake was caused by folding, unusual geologic conditions may be responsible for its high intensity.