Microearthquake study of the Elsinore fault zone, Southern California

Microearthquakes along the Elsinore fault zone, southern California, were monitored during the summer and fall of 1972. Four arrays of at least five portable, high-gain, seismographs were operated for about 12 days each from the vicinity of Corona to just north of the Mexican border. Over 5,000 hr of noisefree records were accumulated and analyzed. The recorded rates of seismic activity show a marked increase going from north to south along the fault— 0.5 events per day in the vicinity of Lake Elsinore to 3.7 events per day in the south near Monument Peak. Fifty-three events located, assuming a four-layer crustal-velocity model, show considerable scatter along the fault and are generally very shallow, averaging 3.3 km below sea level. A signal duration (D) versus magnitude (M) relationship was found: M = −1.9+2.0 log D. First motions of the located earthquakes indicate a complex pattern of faulting along the Elsinore fault zone. In comparison to the San Jacinto Fault to the east, the Elsinore Fault shows very little strike-slip displacement and is a seismically quiet area except for a localized area of east-west faulting in the far south near Vallecito Mountain.