The role of high fluid pressure as a seismogenic agent has been the subject of intense study (Hubert and Rubey, 1959; Hanshaw and Bredehoeft, 1968; Healy and Rubey, 1968; Simpson, 1976; Walder and Nur, 1984; Sibson, 1990). Of particular interest is the so-called fault-valve mechanism (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 fluid discharge. Sealing and healing of the rock matrix (Richter and Simmons, 1977; Sprunt and Nur, 1979; Angevine et al., 1982) following coseismic stress drop facilitates reaccumulation of fluid pressure, initiating another loading cycle. The fault-valve mechanism is entertained as a plausible explanation for present-day seismic activity in the western Transverse Ranges of California. We provide a quantitative test of the fault-valve hypothesis that uses geologic data and rates of active tectonics for a cross-section through an active fold-and-thrust belt 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 104Pa/yr and hundreds of years, respectively. ¿ American Geophysical Union 1993