GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 9, PAGES 1033-1036, MAY 1, 1995 Overlapping fault planes of the 1971 San Fernando and 1994 Northridge, California earthquakes Jim Mori and David J. Wald U.S. Geological Survey, Pad.leto, California Robert L. Wesson U.S. Geological Survey, Reston, Virginia Abstract. Aftershocks of the 1971 San Fernando and 1994 Northridge earthquakes were relocated using a three- dimensional velocitymodel that was derivedfrom inverting P- wave travel time data. The hypocenters show clear orientations of the dipping fault planes. The SanFernando aftershocks form a planeextending from a depthof 15 km to the surface, dipping toward the northeast at about 40ø. The Northridgeaftershocks delineate a fault extending from a depth of 18 km up to about 5 km, dipping toward the southwest at about40 ø. In the region the aftershocks overlap in map view, the San Fernando plane cuts off the Northridge planeat a depth of 5 to 8 km, preventing it from reaching the surface. The similar but oppositely dipping fault planes suggest a pair of conjugate planes reflecting a horizontal northeast-southwest compression. Introduction The January 17, 1994 Northridge (Mw 6.7) and the February 9, 1971 San Fernando earthquakes (Mw 6.7) occurred in close proximity along the southern edge of the westernTransverse Ranges in southern California. The two eventshave similar thrust-faulting focal mechanisms (i.e. Wesson et al., 1971, Whitcombet al., 1973, Scientists of the U.S. Geological Survey and SouthernCalifornia EarthquakeCenter, 1994.), but the inferred faultplanes of thetwo events dip in opposite directions. The locations of the Northridgeaftershocks, as determined by the Southern California Seismic Network (SCSN), outline the orientation of a southwest dipping fault plane. However, in previous studies, the 1971 aftershocks did not clearlyshow the orientation of the SanFernando fault plane(Wesson et al., 1971, Whitcombet al., 1973, Allen et al., 1975). This papercombines improved aftershock locations from the San Fernando earthquake with the well-located aftershocks of the Northridge earthquake to infer the geometry of the rupture planes and investigate the relation between these two adjacent earthquakes. Three-Dimensional Velocity Structure In order to obtain better aftershock locations, we determined a three-dimensional (3-D) velocity structure using P-wave arrival data from earthquakes located in and near the San Fernando Valley. We combined data from 94 San Fernando This paper is notsubject to U.S. copyright. Pub- lished in 1994 by theAmerican Geophysical Union. Papernumber95GL00712 aftershocks, 119 earthquakes occurring from 1983 to 1993, and 343 aftershocks from the 1994 Northridge earthquake. The 1971 data were recorded on 21 temporary stations deployed close to the aftershock area (Wesson et al., 1971) and 10 stations of the seismic network operated by the California Institute of Technology from February to April, 1971. The 1983 to 1993 data include arrivals from 82 SCSN stations within 100 km. The Northridge events were also recordedby the same SCSN stations,plus 7 stationsthat were quickly deployedin and around the San Fernando valley followingthe January 17 main- shock. Figure 1 shows the epicenters of the 556 earthquakes used for the inversion and the locations of the stations in the region of the SanFernando Valley. Using the phase data, we solved for a velocity structure, station corrections, and earthquake locations with a computer program developedby Thurber (1983) and Eberhart-Phillips (1993). First, we calculated a one-dimensional (I-D) velocity structure from the data. Then we used the 1-D structure as a starting model for determining a 3-D structure. The 3-D structure was parameterizedby a grid (7x6) of node points spaced horizontally at 7 km intervals and at 8 depths of 0, 1, 3, 5, 10, 15, 25, and 32 km. The final model reduced the model variance by 70% compared to the 1-D model, and the average rms error for the earthquake locations improved from 0.19 to 0.10 sec. Two of the better resolved depth slices at 5 and 10 km of the resultant velocity structureare shown in Figure 2. The model shows lower velocitiesin the region of the Santa Clarita Valley north of the San Fernando Valley (dottedline). This is consistent with the large thickness of sediments in this region described by Yeats et al. (1994). The low velocities contrast with the higher velocities in the Santa Susana Mountains which separate the two valleys. There is alsoa suggestion of lower velocities underthe SanFernando valley at 5 and 10 km depths. Aftershock Locations We usedthe 3-D velocitystructure and station corrections to relocate 1066 aftershocks from the San Fernando event that occurred during February through April, 1971 plus 2572 aftershocks from the Northridge events that occurred in January and February,1994. All the phase data are available from the Southern California Earthquake Center Data Center. The resultant hypocenters for events thatwererecorded on 6 or more stations and had rms errors of 0.3 sec or less are shown in Figure 3. The locations of the 3-D model show a much more fo- cused pattern in the aftershocks compared to previouslocations done with a 1-D structure, especially in theregion wherethe two aftershock distributions intersect. 1033