Application of numerical techniques to faulting and flexure of the lithosphere.

Abstract

This research investigates problems of faulting and flexure of the lithosphere using the finite element technique. I examined two aspects of faulting, the nucleation and growth of dip slip faults in a stable craton in Chapter 2, and the rupture of the Loma Prieta earthquake in Chapter 3. Linear elastic fracture mechanics and the shear fracture energy criteria are used in conjunction with the finite element method to evaluate the stability of fault rupture. I investigated fault nucleation and growth under conditions of variable dip and shear fracture energy. Strain required for fault growth varied with respect to shear fracture energy gradient and rupture direction but not with dip. Rupture upward required more than twice the initial strain to cause fault growth, as compared to rupture downward, and would result in very large earthquakes. Rupture downward in models associated with the higher G(c) gradient resulted in stable growth of the fault. I suggest that fault nucleation and growth is a stable process which would preferentially occur by rupturing downward under stable conditions. In Chapter 3, I model the rupture of the Loma Prieta Fault, a moderately dipping reverse fault which is subparallel to the San Andreas Fault. Rupture initiated at 18 km and propagated updip in an oblique slip motion to 8-5 km depth. A similar technique as outlined above is used to investigate controls on the rupture propagation and the genetic relationship of the Loma Prieta and San Andreas faults. Low stress in the upper crust caused by a low strength material or the presence of other faults is responsible for stopping rupture. An evaluation of the stress at the intersection of the two faults indicates that the Loma Prieta Fault in the upper crust is still closer to the point of failure than the San Andreas Fault. Thus, rupture of the Loma Prieta did not significantly increase the seismic hazard of the San Andreas in that region. In Chapter 4, I analyze the problem of buckling a faulted elastic plate. I studied the effect of changes in fault depth, spacing and dip on the buckling stress, wavelength and effective Young's modulus. Modeling results show that as the fault depth increases both the buckling stress and wavelength decrease. Similarly, the effective Young's modulus is a decreasing function of depth. As fault spacing was increased, the effect of the faults on the buckling stress and wavelength was minimized. Fault dip appears to have no significant effect on buckling response. The buckling response of a thick, faulted lithosphere is not equivalent to that of a thinner elastic plate.