Variations in gravity caused by changes in rock density, San Pablo Bay

This map illustrates variations in gravity caused by differences in rock density under San Pablo Bay. Dense metamorphic rocks west of the fault create higher gravity values (from 3.6 to -11.3 mGal; pink, red, orange, yellow); whereas sedimentary rocks to the east yield lower gravity values (from -12.2 to -37.9 mGal; green, blue, purple). A sharp change in gravity values (dashed white line, overlying a shift from green to blue) lies close to locations where chirp seismic-reflection profiles revealed the Hayward fault (plus signs).