Fluid pressure transients on seismogenic normal faults

Abstract

Fluid inclusions in hydrothermally altered footwall rocks of the Dixie Valley fault, Nevada, and the Wasatch fault, Utah, indicate that pore fluid pressure fluctuated. Minimum entrapment pressures for fluid inclusions consisting of H 2O-CO 2-NaCl ranged from 295 MPa to 60 MPa in the temperature range 350° to 170 ° C on the Wasatch fault, and from 158 to 35 MPa in the temperature range 350° to 200 ° C on the Dixie Valley fault. Scatter in the pressure estimates at constant temperature is interpreted as paleo-fluid pressure transients at depths of up to 11 km on the Wasatch fault and 3 to 5 km on the Dixie Valley fault. Observed pressure transients range from 5 MPa, within the limits of error in pressure determination, to 120 MPa on the Wasatch fault and 7 to 126 MPa on the Dixie Valley fault. The pressure transients are greatest on both faults in the temperature range 270 ° to 310 ° C. The fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures. High fluid pressures may initiate rupture, then dilatancy, pore-pressure reduction, and dilatant hardening may arrest the rupture. However, decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90% which reduces the dilatant hardening effect and may permit ruptures to propagate.