STRUCTURES POSSIBLY RELATED TO THE 1971 SAN FERNANDO AND 1987 WHITTIER NARROWS EARTHQUAKES BASED ON THE ANALYSIS OF MAGNETIC AND GRAVITY DATA

A potential-field study of blind thrust faults in the Los Angeles area leads to relationships between interpreted structural boundaries and hypocenters related to the San Fernando (Mw 6.7) and Whittier Narrows (Mw 6.0) earthquakes. The simultaneous inversion of magnetic and gravity data clearly defines a north-dipping boundary that correlates with an aftershock hypocenteral pattern of the San Fernando earthquake at depths greater than about 4 km. This northwest-trending boundary lying roughly 5 km south of the San Gabriel fault may represent the juxtaposition of magnetic, dense granitic or gneissic diorite on the north and less dense and magnetic granite or schist on the south. The magnetic and gravity boundary extends west beneath sedimentary rocks as far as the aftershock seismicity pattern, and to the east, it appears to follow the Sierra Madre fault. Our preliminary conclusions are that this interpreted structural boundary represents the western extension of the Sierra Madre fault and, in 1971, it experienced fault slip related to the San Fernando earthquake.

Two major structural boundaries are identified in the region of the 1987 Whittier Narrows earthquake. One boundary separating a major density contrast follows the Whittier fault zone northwest across the area of the 1987 epicenter to within 4 km of the Santa Monica-Hollywood-Raymond Hill (SMR) fault zone. The second boundary, based on the analysis of magnetic lineaments, trends east-west and lies south of the east-northeast-trending SMR fault zone. It separates zones of magnetic lineaments and faults primarily trending east-west on the north from those trending northwest on the south. Modeling of magnetic sources lying along and north of this boundary suggests that it dips north about 70 degress, intersecting the two Whittier Narrows main shocks at 12 and 15 km. We do not suggest that this east-west boundary defines a single fault plane, but it may be an upper-crustal manifestation of a deeper blind thrust reactivated during the Whittier Narrows earthquake.