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

Seismicity is irregularly distributed in the central Coast Ranges of Califo rnia, with several large areas having virtually no seismic activity since 1 968. Aseismic areas correlate with zones of abnormally high pore fluid pres sures (AHPs) > 130% of hydrostatic. Level line surveys suggest that aseismi c areas have experienced significant historical uplift. Moreover, these ase ismic areas show a remarkable correlation with perennial saline springs tha t represent tectonically expelled formation fluid, as indicated by their pe rennial flow, delta(18)O and delta D enrichment to +6.3% and -13% respectiv ely, elevated [B] to 331 ppm, and chemistry dominated by Na and Cl. Aseismi c areas are dominantly located at the center of structural blocks that are under compression and are bounded by major faults and seismicity belts. Vol umetric strain within these blocks creates pockets of overpressured formati on fluid that greatly reduce seismic activity while promoting aseismic upli ft. Seismic cross sections reveal three-dimensional pockets of aseismicity that are floored and surrounded by scattered hypocenters. We suggest that o verpressured formation fluids originate at depths of 1 to 7 km within these pockets, on the basis of Na-K-Ca geothermometry estimates that are consist ent with stratigraphic information and available drill hole data. These ove rpressured fluid pockets may also influence seismicity on major faults, suc h as the upper 3 km of the active Greenville fault, which is relatively ase ismic where it separates two overpressured pockets. Decreasing flow rates o bserved at some saline springs may represent a local transition to a period of increased seismic activity. While many studies have linked generation o f earthquakes to man-made fluid overpressures, our data show that the domin ant, long-term effect of AHPs is to reduce seismicity and promote uplift.