Relationship between seismicity and subsurface fluids, central Coast Ranges, California

Seismicity is irregularly distributed in the central Coast Ranges of California, with several large areas having virtually no seismic activity since 1968. Aseismic areas correlate with zones of abnormally high pore fluid pressures (AHPs) >130% of hydrostatic. Level line surveys suggest that aseismic areas have experienced significant historical uplift. Moreover, these aseismic areas show a remarkable correlation with perennial saline springs that represent technically expelled formation fluid, as indicated by their perennial flow, δ 18 O and δD enrichment to +6.3‰ and −13‰ respectively, elevated [B] to 331 ppm, and chemistry dominated by Na and Cl. Aseismic areas are dominantly located at the center of structural blocks that are under compression and are bounded by major faults and seismicity belts. Volumetric strain within these blocks creates pockets of overpressured formation fluid that greatly reduce seismic activity while promoting aseismic uplift. Seismic cross sections reveal three‐dimensional pockets of aseismicity that are floored and surrounded by scattered hypocenters. We suggest that overpressured formation fluids originate at depths of 1 to 7 km within these pockets, on the basis of Na‐K‐Ca geothermometry estimates that are consistent with stratigraphic information and available drill hole data. These overpressured fluid pockets may also influence seismicity on major faults, such as the upper 3 km of the active Greenville fault, which is relatively aseismic where it separates two overpressured pockets. Decreasing flow rates observed at some saline springs may represent a local transition to a period of increased seismic activity. While many studies have linked generation of earthquakes to man‐made fluid overpressures, our data show that the dominant, long‐term effect of AHPs is to reduce seismicity and promote uplift.