RESPONSE OF PORE PRESSURE AND TEMPERATURE FIELDS TO SEISMIC EVENTS

It has been observed that pore pressure and temperature fields show fluctuations in response to seismic events. These observed fluctuations indicate coupling between seismically induced stress-strain fields and subsurface hydrodynamics. The processes involved in the coupling system include pore pressure changes from volumetric strains, subsequent transient fluid and heat flow resulting from the pore pressure change, and permeability alteration due to deformation. To better understand the seismically induced pore pressure and temperature fluctuations, we study the transient processes coupling deformation, fluid flow, and heat transport during and after seismic activities. We first use an earthquake strain model to obtain the stress-strain field due to a fault dislocation. We then explore the linkage coupling stress and strain with pore pressure. Finally, we examine the transient processes involving pore pressure diffusion and heat transport. The application of our study to observed post-seismic pore pressure data suggests that the pore pressure diffusion time is shorter than conventional estimates, which are based on a diffusivity and a length scale. We find that the diffusion time is predominately a function of the diffusivity of the system, while the length scale influences the magnitude of the initial pore pressure. A diffusion time based on the diffusivity and a length may be misleading because significant localized flow occurs in complex three-dimensional systems. Post-seismic heat transport caused by enhanced fault permeability and fluid flow activity contributes to the observed temperature fluctuations. Close examinations of post-seismic behavior of pore pressure and temperature fields reveal insights into the nature and extent of the complex coupled processes as well as hydrologic and thermal properties of the geologic media. The theoretical basis of this work is developed assuming a single episode of dislocation. However, the methodology and the results can be readily applied to studying pore pressure conditions after multi-faulting events by simple superposition.