Theoretical Research:
Studies of fault mechanics often require one to focus on the behavior of individual faults; however, in nature, faults in nature are often influenced by neighboring faults to a comparable degree as they are by the regional state of stress. As a result, it is important to quantify how systems of faults and fractures interact. Thus far, my approach to this subject has addressed two aspects of fault and fracture interaction:
I am interested in understanding how fault structure, ranging from micro- to macroscopic scales, dictates the coupled hydro-thermal-mechanical behavior of faults. This study has involved a multidisciplinary approach in which I have combined field and microscopic observations with Finite Element Method (FEM) models to simulate the coupled processes of fluid flow and heat transfer along faults during seismic slip. The models focus on faults of the Mt. Abbot Quadrangle in the central Sierra Nevada. This is essentially a rock physics problem, where we numerically estimate the effect of damage-related fractures on relevant rock physical parameters such as the elastic modulus, Kd, and the effective permeability, kd of the fractured fault damage zone and compare these to the laboratory-measured values (Ke and ke respectively) of the undamaged elastic bedrock outside of the fault zone (Figure 3). The elastic modeling has been done in collaboration with Pablo Sanz (Civil Engineering, Stanford/Exxon-Mobil Upstream Research Company). The elastic and hydraulic material contrasts across faults have important implications for the balance of pore fluid pressure, temperature, and frictional strength of faults, as well as earthquake rupture directivity.
Numerical triaxial loading experiments testing the “effective” elastic modulus of a fault zone with mapped faults and fractures represented as frictional discontinuities. If all faults are allowed to slip and open, the effective Young’s modulus of the fault zone is reduced by more than 50%, even without accounting for microscopic deformation.
For my M.S. research, I integrated GPS measurements of horizontal surface velocities in the Los Angeles Basin, three-dimensional fault geometries interpreted from subsurface data (seismic, borehole, trenching), and numerical Boundary Element Method mechanical models to study the modern day mechanical interaction of active faults in the region. In one study(Griffith and Cooke, Bull. Seismol. Soc. Am., 2005; http://www.scec.org/research/050823basinmodel.html), I applied different horizontal velocities as boundary conditions on a fault model (Figure 4) to compare geodetic models for strain accommodation to fault slip rates from paleoseismic estimates. We found that contraction in the LA Basin is most likely accommodated by a combination of vertical thickening and horizontal expansion, and suggested that the seismic potential of five faults in the LA Basin might currently be underestimated. In another study, we used GPS velocities and a proxy for mechanical efficiency (strain energy density) to distinguish between proposed geometric configurations for a blind thrust fault responsible for the 1987 ML 5.9 Whittier Narrows earthquake (Griffith and Cooke, Bull. Seismol. Soc. Am, 2004).
Different geodetic models of tectonic strain in the Los Angeles Basin (left) ranging from “Escape Tectonics” (Walls et al., 1998) to “Vertical Thickening” (Argus et al., 1999) are differentiated based on their effects on modeled fault slip rates (right). For more information, see Griffith and Cooke, 2005.