Abnormally high fluid pressures in the region of the Coalinga earthquake sequence and their significance

Abstract

Abnormally high fluid pressures (AHP's) permitted or aided thrusting of buried tectonic wedges of Franciscan assemblage eastward beneath coeval Great Valley sequence onto mafic basement and thus contributed to the 1983 Coalinga earthquake sequence. Much of the southwestern San Joaquin Valley and adjoining parts of the Diablo Range are underlain by rocks with pressure/depth (P/D) ratios near or greater than 0.5 psi/ft (the hydrostatic gradient for oil-field waters is about 0.47 psi/ft). Plots of P/D ratio versus depth for more than 300 wells show an average ratio of about 0.6 psi/ft above 14,000-ft depth, a lower bound of 0.47 psi/ft, and an upper bound greater than 0.9 psi/ft; ratios greater than 0.60 psi/ft are interpreted to be abnormally high. Several likely sources of AHP's are recognized, but their relative contributions cannot be determined. Chief of these sources are diagenetic-metamorphic generation of fluids in the Great Valley sequence and Franciscan rocks, compaction disequilibrium and aquathermal pressuring in Tertiary strata, and horizontal tectonic compression. Yerkes, Levine, and Wentworth infer the presence of near-lithostatic fluid pressures in the Great Valley sequence and Franciscan rocks below the Coalinga anticline on the basis of (1) a mapped seismic low-velocity zone; (2) active generation of metamorphicmore » fluids in the Great Valley sequence and Franciscan rocks, which pond below relatively impermeable layers and thus increase fluid pressure; and (3) apparent southward and westward propagation of the 1983 main-shock rupture, which indicates rupture on a gently southwest dipping surface and implies an inverse relation between depth and crustal strength.« less