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1985). 70 km southwest of the San Andreas fault; Gulf Coast average trend presented for reference. Fluid pressure and fault stability within seismogenic crust The seismogenic regime occupies the upper part of deforming continental crust and appears to represent the zone of unstable frictional sliding. Background microearthquake activity extends to depths of 10-15 km in regions of moderate to high heat flow (60-100 mW m 2), but deepens significantly in regions of rapid thrust convergence (> 10 mm a -1) such as the thrust-front of the Himalaya.

Potential for fault-valve behaviour (a) Impermeable barrier separating hydrostatic and suprahydrostatic fluid pressure regimes. (b) Breaching of barrier by fault rupture X-Y, leading to an upwards discharge of fluids. H. SIBSON assemblage of impermeable crystalline rocks. Development of such gradients requires the existence of localized or regionally extensive low permeability barriers. These may take various forms such as laterally extensive sequences of shales or evaporites, a crystalline rock assemblage, or a horizon of hydrothermal cementation and sealing that may be widespread or localized within the fault zone itself (Etheridge et al.

Earthquake damage to oil fields and to the Paloma cycling plant in the San Joaquin Valley. Californian Division of Mines Bulletin 171,221-226. JOHNSTON W. D. 1940. The gold quartz veins of Grass Valley, California. US Geological Survey Professional Paper, 194. KERRICm R. 1986, Fluid infiltration into fault zones: chemical, isotopic and mechanical effects. Pure and Applied Geophysics, 124,225-268 KNOPF, A. 1929. The Mother Lode system of California. US Geological Survey Professional Paper, 157. McCLAY, K.