OVERLAPPING FAULT PLANES OF THE 1971 SAN-FERNANDO AND 1994 NORTHRIDGE, CALIFORNIA EARTHQUAKES

Aftershocks of the 1971 San Fernando and 1994 Northridge earthquakes w ere relocated using three-dimensional velocity model that was derived from inverting P-wave travel time data. The hypocenters show clear ori entations of the dipping fault planes. The San Fernando aftershocks fo rm a plane extending from a depth of 15 km to the surface, dipping tow ard the northeast at about 40 degrees. The Northridge aftershocks deli neate a fault extending from a depth of 18 km up to about 5 km, dippin g toward the southwest at about 40 degrees. In the region the aftersho cks overlap in map view, the San Fernando plane cuts off the Northridg e plane at a depth of 5 to 8 km, preventing it from reaching the surfa ce. The similar but oppositely dipping fault planes suggest a pair of conjugate planes reflecting a horizontal northeast-southwest compressi on.