Mechanical modeling of time dependent deformation in the lower crust and its effect on earthquake recurrence

Time dependent deformation in the lower crust is an important component in the mechanics of stress transfer and strain accumulation during earthquake cycles in complex, tectonically active regions. Postseismically, time dependent relaxation in the lower crust can enhance the magnitude, duration, and spatial extent of areas of stress decrease following great earthquakes. Conversely, the coseismic fault is reloaded at rates that are initially much faster than otherwise expected. Thus, large earthquakes in complex, multiple fault systems generate transients in loading rate on all of the faults within that system. Chapter 2 investigates these time dependent postseismic stress perturbations following great strike-slip earthquakes. A number of different lower crustal structures, all proposed to exist in northern California, are considered and compared. The net result of these postseismic perturbations is that strain accumulation along active faults is a time dependent process and loading rates vary continuously throughout the earthquake cycle. This is exemplified by postseismic data following the 1906 San Francisco earthquake. To establish a more complete temporal and spatial record of post-1906 deformation, Chapter 3 combines re-analyzed historical triangulation data with more modern geodetic data. An effective relaxation time for postseismic deformation following the 1906 earthquake of 36 ± 16 years is inferred and afterslip at depth is characterized using simple elastic dislocation models. Chapter 4 presents a more complete analysis of the post-1906 deformation data using naturally time dependent mechanical models of postseismic deformation. We find that (1) discrete shear zones beneath each of the sub-parallel faults in northern California improves model misfits and (2) the effective relaxation time inferred from postseismic geodetic data is indeed effective, representing some net measure of deformation in a complex system. Finally, Chapter 5 investigates time dependent lower crustal deformation as a possible mechanism for the generation of earthquakes driven not by far-field plate boundary forces but instead by local or regional scale perturbations to the stress field. This approach is particularly intriguing as a possible explanation for the occurrence of repeated intraplate earthquakes in locations like the New Madrid seismic zone in the south-central United States.