U.S. Geological Survey
Open-File Report 00-219
Online version 1.0

Preliminary Potential-Field Constraints on the Geometry of the San Fernando Basin, Southern California

By

V.E. Langenheim, A. Griscom, R.C. Jachens, and T.G. Hildenbrand

Isostatic gravity map with 1994 (black circles) and 1971 (red circles) aftershock seismicity. Some aftershocks appear to be controlled by a northwest-trending structure coinciding with a gravity gradien. The blue "X" marks where the geophysical character fo the Verdugo fault changes along its strike. Faults are shown in magenta.

ABSTRACT

Gravity and magnetic data provide new insights on the structural underpinnings of the San Fernando Basin region, which may be important to ground motion models. 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 over 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 estimate is the result of high densities for the gravels of the Pliocene-Pleisocene Saugus Formation. The geometry of the southern margin of the deep basin is not well-constrained by the gravity data, but may dip to the south. Recently acquired seismic data along the LARSE (Los Angeles Regional Seismic Experiment) II profile may provide constraints to determine the location and attitude of the basin edge. Gravity and aeromagnetic models across the eastern margin of the San Fernando Valley indicate that the Verdugo fault may dip to the southwest along its southern extent and therefore have a normal fault geometry whereas it clearly has a thrust fault geometry along its northern strand.

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For questions about the content of this report, contact Vicki Langenheim

For more about gravity and magnetic data and methods, check out the Geophysical Unit Menlo Park website

For more about geologic mapping in Southern California, visit the SCAMP website

For more about earthquake studies in Southern California, visit the USGS Pasadena Office of Earthquake information

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URL of this page: http://geopubs.wr.usgs.gov/open-file/of00-219
Maintained by: Carolyn Donlin
Created: 6/21/00
Last modified: 6/21/00(cad)