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Seismicity of the Garlock fault, California

SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91125

Abstract

The Garlock fault is a 265-km-long left-slip fault striking northeastward from the San Andreas fault in southern California. Relocations of earthquakes that occurred from 1932 to 1981 on and near this major fault were made using the master-event technique. The spatial distribution of seismicity along the fault is different west and east of its midpoint near Rand, where the largest en-echelon offset and a marked change in strike occur. These two segments also display distinct geologic features and different seismic and aseismic behavior. The 150-km-long segment west of Rand has shown continuous low seismic activity during the past 50 yr, well-documented aseismic creep, and has a relatively complex fault trace. In contrast, the 155-km-long segment east of Rand has very few small earthquakes, no demonstrable creep, and a simpler fault trace. P-wave first-motion studies substantiate predominantly left-slip motion along the Garlock fault. Overall energy release during this 50-yr period gives a seismic moment rate of 2.75 x 10²¹ dyne-cm/yr, much lower than that inferred from Holocene geologic offsets, thus indicating that the Garlock fault currently represents a temporal seismic gap, and that the potential exists for large earthquakes. If behavior of the Garlock fault is similar to that of the San Andreas, the western segment of the fault can be compared with the central creeping segment of the San Andreas, and the eastern segment with those segments broken by the 1857 and 1906 earthquakes; thus larger events might be expected on the eastern segment than on the western segment, or, if the entire fault breaks during a single event, larger displacements toward the east.