Geophysical Setting of the San Fernando Basin, Southern California

Abstract Gravity and magnetic data provide new insights on the structural underpinnings of the San Fernando Basin region. Gravity data indicate that a deep basin (>5 km) underlies the northern part of the San Fernando Valley. This deep basin is required to explain the lowest gravity values in the region of the Mission Hills thrust fault. Gravity modeling, constrained by well data and density information, shows that the basin may reach a thickness of 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. This deep estimate results from the realization that gravels of the Pliocene-Pleisocene Saugus Formation are denser than typical sedimentary rocks of this age and is supported by recent velocity data from the Los Angeles Regional Seismic Experiment (LARSE) II cross-profiles. The geometry of the southern margin of the deep basin is not well-constrained by the gravity data, but may dip to the south. Gravity and aeromagnetic models across the eastern margin of the San Fernando Valley indicate that the geometry of the Verdugo fault changes from northwest to southeast. The southeastern part of the fault apparently dips to the southwest, which would suggest a normal fault. An alternative explanation for the apparent normal geometry may be that the fault zone consists of a series of thrust faults that step to the southwest from the mountain front. The northwestern strand clearly dips to the northeast, indicative of a thrust fault geometry.