FLUID-PRESSURE INDUCED SEISMICITY AT REGIONAL SCALES

The role of high fluid pressure as a seismogenic agent has been the su bject of intense study (Hubert and Rubey, 1959; Hanshaw and Bredehoeft , 1968; Healy and Rubey, 1968; Simpson, 1976; Walder and Nur, 1984; Si bson, 1990). Of particular interest is the so-called fault-valve mecha nism (Sibson, 1976; Sibson, 1990) a hypothesis whereby fluid pressure rises (as a result of tectonic compression and pore volume reduction) until crustal failure occurs, triggering seismic activity and upward f luid discharge. Sealing and healing of the rock matrix (Richter and Si mmons, 1977; Sprunt and Nur, 1979; Angevine et al, 1982) following cos eismic stress drop facilitates reaccumulation of fluid pressure, initi ating another loading cycle. The fault-valve mechanism is entertained as a plausible explanation for present-day seismic activity in the wes tern Transverse Ranges of California. We provide a quantitative test o f the fault-valve hypothesis that uses geologic data and rates of acti ve tectonics for a cross-section through an active fold-and-thrust bel t on the flank of a developing mountain range. Rates of fluid pressure buildup and average recurrence times of large earthquakes in the fold -and-thrust belt are estimated to be on the order of 10(4) Pa/yr and h undreds of years, respectively.