STRUCTURE OF THE SAN FERNANDO BASIN, CALIFORNIA, BASED ON ANALYSIS OF GRAVITY AND MAGNETIC DATA

Gravity and magnetic data provide new insights on the structural underpinnings of the San Fernando Basin region, which may be important to tectonic and ground shaking models. The San Fernando basin is at the eastern end of a large, east-west-trending gravity low over the Ventura basin. The 40-mGal gravity low over the San Fernando Valley suggests that the underlying basin is asymmetric and that a deep basin underlies the northern part of the San Fernando Valley. Gravity modeling, constrained by well data, shows that the floor of the basin may be as deep as 8 km, coinciding with the upper termination of the 1994 Northridge earthquake mainshock rupture. The basin is deeper than previous estimates by 2 to 4 km; our estimate results from high densities for the Pliocene-Pleistocene Saugus Formation and is confirmed by recent velocity results from Los Angeles Regional Seismic Experiment (LARSE). The geometry of the southern margin of the deep basin is not well-constrained by the gravity data, but the margin may dip to the south.

Gravity and magnetic data clearly image other major basin-bounding structures. Magnetic data suggest a major magnetic terrane boundary at or near the Verdugo fault, the eastern margin of the basin. The Verdugo fault may dip to the southwest along its southern part and therefore have a normal fault geometry (or a stack of several thrust faults stepping away from the mountain front). The northern segment of the Verdugo fault produces gravity and magnetic anomalies clearly indicative of a northeast-dipping thrust fault geometry. Gravity data suggest that the western margin of the basin is linear and strikes about N45E. The northeast-trending gravity gradient follows part of the 1971 San Fernando aftershock distribution called the Chatsworth trend. The aftershocks of the 1994 Northridge earthquake also appear to follow this northeast-trending gravity gradient as it bends into alignment with the east-west-trending gravity gradients associated with the Ventura basin margins.