How Faults Are Loaded: The Influence of Tectonic Environment

The crust is everywhere subjected to stresses generated by tectonic plate motions. Local and regional sources of stress, resulting from fluid effects, thermal effects, and various gravitational loads, also exist. Further, the earth's crust is laterally inhomogeneous. Variations in crustal thickness and zones of weakness resulting from prior tectonic episodes are present. In combination, these factors lead to distinct differences in the nature of stress accumulation within discrete plate boundaries, diffuse plate boundaries, and intraplate seismic zones. To address these differences, I examine the mechanics of stress localization in each region and the influence of these processes on earthquake recurrence rate and time-varying interseismic deformation rate. At plate boundaries, relatively thin crust experiences sharp gradients in tectonic velocity. Since all plate boundary fault systems merge with adjacent systems, over geologic time-scales significant total offsets can accumulate. Frequent, large earthquakes directly attributable to relative plate motions are the result. Near plate boundaries, interseismic deformation rates can be significantly different from average plate velocities. Thus plate-boundary processes are relatively easy to monitor geodetically. Conversely, in intraplate regions with thick, cold crust, velocity gradients are nominal and tectonic stresses are transmitted with little chance of brittle failure. Individual fault systems are isolated within otherwise rigid tectonic plates. This minimizes cumulative fault offsets as well as differences between observed interseismic and long-term plate velocities. Relative interseismic motions across the fault may be extremely small and difficult to observe geodetically. Distance from the plate boundary, relative strength, anomaly size, and local tectonics all help to determine which faults fail. Zones of diffuse deformation can result from large areas that support only slightly less stress than typical intraplate regions. By virtue of their size, plate motion derived stresses deform these areas more readily than their surroundings. Superposed local and regional sources of stress can also drive the plate to failure. Finally, isolated, anomalously weak areas can concentrate stress and, when subjected to transient stress perturbations, may produce geologically short-lived spurts of seismic activity.