Experimental Study of the Role of Pore Fluid Pressure in Earthquake Nucleation

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EAR-0230170Terry E. TullisABSTRACT We will conduct a systematic study to understand the relationship between stress change and pore pressure change in experimental fault zones. It is important to understand the mechanical contributions of pore fluid pressure change during earthquake nucleation and how pore fluid pressure change may trigger seismicity. The geophysical purpose of this research is determine to what degree changing pore fluid pressure plays a mechanical role in earthquake occurrence and under what conditions. Geologic observations show that seismic faulting occurs in the presence of crustal fluids. However rarely is fluid behavior accounted for in models of seismicity, earthquake nucleation and triggered seismicity. It is most often assumed that shear induced dilatancy during seismic slip will decrease the pore pressure, tend to stabilize the slip, and discourage earthquake occurrence. However, recent laboratory observations suggest that for slip localized within thick fault zones, elastic dilation or compaction of the fault zone with stress change is of the opposite sense and greatly exceeds shear induced dilatancy. Thus, common assumptions about the role of pore fluid pressure in earthquake source mechanics require renewed scrutiny. No laboratory experiments where stick-slip sliding occurs have been conducted in the presence of pore fluids, no laboratory measurements of the poroelastic properties of fault zones have been made, and no poroelastic measurements have been made on unfaulted (country) rock at the significant differential stresses appropriate for seismic faulting. Therefore we will conduct two series of experiments; the first designed to measure the poroelastic properties of fault zones near the failure stress, the second to determine and compare the magnitudes of shear induced dilatancy (inelastic strain) and elastic strain during stick-slip in thick gouge layers. We will develop constitutive equations for seismic fault slip based on the experimental observations and extrapolate the results to crustal conditions.