A Year of Strain Measurements in Southern California

Abstract

STRAIN measurements have been obtained nearly continuously for over a year from a surface laser interferometric strain-meter¹ deployed at Piñon Flat Geophysical Observatory. This site(33.49° N, 116.46° W) is located in the wedge of granite that forms the San Jacinto Mountains, between the San Andreas fault and an active branch, the San Jacinto fault, to the south of their confluence. The area lies at the southern end of the trace of the 1857 Fort Tejon earthquake(magnitude 8.5) and was possibly loaded with 10 m of potential displacement by that event. To the south, in the Imperial Valley, a roughly continuous fault movement occurs accompanied by moderate seismicity, whereas to the north no recent motion has been detected along some 350 km of the San Andreas fault which is aseismic at present.