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GMEG - Geophysics Unit of Menlo Park, CA- (GUMP)


Geophysics Unit of Menlo Park, CA (GUMP)

U.S. Geological Survey - Western Region - Geology and Geophysics


Geophysical Studies of the San Francisco
Bay Area


INTRODUCTION--What does gravity tell us?

The gravity field, reduced to isostatic anomalies, tells us about density variations of the Earth's upper and middle crust. In the San Francisco Bay area, gravity highs (warm colors on the gravity map below) tend to occur over outcrops of the Franciscan Complex (dark green pattern on the geologic map) and over outcrops of plutonic rocks (red and blue on the geologic map). Gravity lows occur over Cenozoic sedimentary rocks (yellow and brown).

Gravity gradients commonly occur where faults have juxtaposed rocks of different densities. However, faults are not always marked by gravity gradients, as rocks of similar densities may be also juxtaposed along a fault.

Color gravity map

Color contour map of isostatic residual gravity map. Red colors, gravity highs; blue colors, gravity lows; black lines, mapped faults.


See the map: Roberts, C.W., and Jachens, R.C., 1993, Isostatic residual gravity map of the San Francisco Bay area, California: U.S. Geological Survey Geophysical Investigations Map GP-1006. Available from the USGS--Information Services, Box 25286, Bldg. 810, Denver Federal Center, Denver, CO 80225 303 236-4210.

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