Seismicity Maps of the Santa Rosa 1° x 2° Quadrangle, California for the Period 1969–1995

This map set depicts several aspects of recorded seismicity that occurred from 1967 through 1995 within the Santa Rosa, California quadrangle. Map A on sheet 1 and the related cross sections shown on sheets 2 and 3 illustrate the locations, depths, and magnitudes of earthquakes. On maps A (sheet 1) and D (sheet 3) we also plot representative focal mechanisms to indicate the fault orientation and direction of motion on many faults. On map B (sheet 2) and on the cross sections of sheets 2 and 3 we identify the most significant temporal clusters of earthquakes to illustrate the time-dependent properties of earthquake occurrence. Maps C (sheet 3) and D (sheet 3) are enlarged views of the seismicity at The Geysers geothermal area. Map C shows the time-dependent relation between the inception of geothermal power production and the occurrence of earthquake activity.

In the following discussion we make a subtle inference about the relation of seismicity to faults mapped at the surface. Where concentrations of earthquakes locate beneath mapped faults, we infer that the seismicity defines the subsurface orientation of these faults. Even though some concentrations of earthquakes do not underlie mapped faults, we also infer that these concentrations represent active, but as yet unnamed faults. Earthquakes also occur as isolated events beneath mapped faults and throughout the region.