INVESTIGATIONS OF FLUID FLOW AND HEAT TRANSPORT RELATED TO THE STRENGTH OF THE SAN ANDREAS FAULT

Open Access
Author:
Fulton, Patrick Michael
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 31, 2008
Committee Members:
  • Demian Saffer, Committee Chair
  • Chris J Marone, Committee Member
  • Kamini Singha, Committee Member
  • Derek Elsworth, Committee Member
  • Barbara A Bekins, Committee Member
Keywords:
  • fault strength
  • overpressure
  • fluid pressure
  • heat flow
  • San Andreas Fault
  • fluid flow
  • heat trnasport
Abstract:
The shear strength of faults is an important factor in earthquake hazard assessment, and in understanding the earthquake process and the forces that drive tectonic deformation. However, on the basis of both geomechanical and thermal observations, many plate boundary faults, including the San Andreas Fault (SAF) in California, have been interpreted to slip at shear stresses considerably less than predicted by laboratory-derived friction laws and for hydrostatic fluid pressures. An understanding of whether plate-boundary faults truly are “weak” and the potential causes for such weakness are thus key unknowns in the physics of faulting. In the first section of this thesis, I evaluate whether thermal and hydrologic effects might disturb heat flow data which are used to interpret the strength of the SAF. Using numerical models of coupled fluid flow and heat transport, and by comparing model results with observational constraints, I show that redistribution of heat by groundwater flow is an unlikely explanation for the lack of a near fault increase in heat flow that would be associated with frictional heat generation on a strong fault (i.e. one that supports large shear stresses). I also show that the effects of topographic and subsurface refraction may account for previously unexplained spatial scatter in heat flow data around the fault, but even with these effects the data are most consistent with little or no frictional heat generation. In the second section of this thesis, I evaluate hypotheses invoking regional sources of fluid resulting from metamorphic dehydration reactions within the crust or upper mantle as mechanisms that generate large fluid overpressures within the fault zone required to explain the apparent weakness of the SAF. I calculate reasonable fluid source terms for both crustal and mantle dehydration following the creation of the SAF. I show that crustal dehydration sources are too small and short-lived to generate large overpressures, but it is plausible that a source of mantle-derived water contributes toward generating large overpressures. The results also illustrate how different fault zone hydraulic architectures affect overpressure generation, and highlight the role of dipping country rock permeability anisotropy in fluid pressure localization.