THE 1994 NORTHRIDGE EARTHQUAKE SEQUENCE IN CALIFORNIA - SEISMOLOGICAL AND TECTONIC ASPECTS

The M(W) 6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north-northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km depth in the lower crust. It: had a thrust faulting focal mechanism with a rake of 100 degrees on a fault plane dipping 35 degrees to the south-southwest and striking N75 degrees W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges, The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970, The mainshock was followed by an energetic aftershock sequence, Eight aftershocks of M greater than or equal to 5.0 and 48 aftershocks of 4 less than or equal to M < 5 occurred between January 17 and September 30, 1994. The aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illuminating a previously unmapped thrust ramp, extending from 7-10 km depth into the lower crust to a depth of 23 km. No flattening of the aftershock distribution is observed near its bottom. At shallow depths, above 7-10 km, the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults. The dip of the ramp increases from east to west. The west side of the aftershock zone is characterized by a dense, steeply dipping, and north-northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Canyon! lateral ramp, along the ease side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of;the aftershocks were dominated by thrust faulting in the large aftershocks, with some strike-slip and normal faulting in the smaller aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north-south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of she faults and the depth of faulting. The 1971 north-northeast trend of left-lateral faulting (Chatsworth trend) was not activated in 1994.