Overlapping fault planes of the 1971 San Fernando and 1994 Northridge, California earthquakes

Aftershocks of the 1971 San Fernando and 1994 Northridge earthquakes were relocated using a three‐dimensional velocity model that was derived from inverting P‐wave travel time data. The hypocenters show clear orientations of the dipping fault planes. The San Fernando aftershocks form a plane extending from a depth of 15 km to the surface, dipping toward the northeast at about 40°. The Northridge aftershocks delineate a fault extending from a depth of 18 km up to about 5 km, dipping toward the southwest at about 40°. In the region the aftershocks overlap in map view, the San Fernando plane cuts off the Northridge plane at 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.