Geological and seismic evidence of a new branch of the Agua Blanca Fault

Abstract

The Agua Blanca fault is a major, trans‐peninsular, right‐lateral fault located in northern Baja California. Its WNW‐ESE orientation is markedly different from the general trend of the San Andreas‐Gulf of California fault system. From a geological point of view, the Agua Blanca fault is considered active, but there has been little significant seismic activity directly associated with it. On October 9, 1981, the onset of an earthquake swarm was detected at the Ensenada seismic station (ENX), with a S‐P time average of 2 sec. Shortly after this, an array of up to seven portable seismic stations was installed by CICESE around Todos Santos Bay. More than 180 events were recorded during the following 10 days, after which the local seismicity decreased. A second swarm consisting of 100 microearthquakes was detected between November 28 and December 5, 1981. The epicentral locations of those events recorded at four or more local stations all lie inside Todos Santos Bay. The composite fault plane solution for both swarms indicates a right‐lateral strike‐slip vertical fault with a strike of N52°W. Geological observations allow us to conclude that the seismic activity reported here is associated with the Agua Blanca fault zone.