PRELIMINARY VELOCITY MODEL OF THE UPPER PART OF THE SAN FERNANDO VALLEY SEDIMENTARY BASIN

In the second phase of the Los Angeles Region Seismic Experiment (Oct. 1999, LARSE II), three auxiliary seismic profiles were recorded in the San Fernando Valley (SFV, lines 3-5). These lines, 11-22 km long, recorded 2-4 shotpoints each. The lines were designed to constrain the basin geometry and seismic velocities of the San Fernando Valley in an effort to identify areas of potentially high seismic hazard.

Line 3, in the southern part of the SFV, strikes northwest-southeast through the Sepulveda flood control basin. Line 4, in the northern part of the SFV, strikes east-west from Hansen flood control basin to a point north of Chatsworth reservoir, crossing the Verdugo fault, the Northridge Hills blind thrust fault, and the Chatsworth reservoir fault. Line 5 extends northwestward from the east end of line 4 into the Santa Susana Mts, crossing the gravity minimum of the SFV (-90 mgal).

The tomographic inversion model for line 3 has useful ray coverage to 1.3-km depth and shows slightly different velocity structures northwest and southeast of an apparent discontinuity in the western Sepulveda flood control basin. To the northwest, velocities reach only 3.75 km/s at 1.3-km depth; whereas to the southeast, they reach 4.75 km/s. The higher velocities in the southeast may represent older (Cretaceous?) sedimentary rocks or crystalline basement rocks.

The velocity model for line 4 has adequate ray coverage to 5-km depth. Differing velocity structures are seen in the west, middle, and east parts of the model. The boundary between the structures in the west and middle parts may represent the Chatsworth reservoir fault. A horst-like body in the east part appears to be bounded on its west side by the Verdugo fault.