| In Chapter 1, two possible triggering mechanisms are examined for the phenomenon of "triggered aseismic fault slip". We evaluate the possible triggering role of static stress changes by examining observations made after 10 large earthquakes in California. Three discrepancies or failures with this model exist, which implies that static stress triggering either is not or is not the sole mechanism causing the observed triggered slip. We then use a spring-slider system as a very simplified fault model to study its slip behavior and the impact of transient (dynamic) loading on it. We show that a two-state-variable rate- and state-dependent frictional law can generate creep events. Certain types of transient loads can cause a large time advance of (or trigger) the next creep event.; In Chapter 2, we examine 16 moderate-size earthquakes (3.8 ≤ mb(Lg) ≤ 5.2) that occurred since 1990 in eastern North America (ENA). Important source parameters, such as focal mechanism, focal depth and seismic moment, are determined using a regional waveform inversion technique in which the best fitting double-couple mechanism is obtained through a grid search over strike, dip and rake angles. Our results are compatible with 10 previously well-determined focal mechanisms in the region. We further examine the distribution of P/T axes, the geographic variation in focal depths and apparent discrepancies between Mw and mb (Lg) magnitudes.; In Chapter 3, changes in the frequency of moderate-size events (M ≥ 4.0) before and after the June 28, 1992 Landers earthquake are investigated, and their implications are discussed in the context of Coulomb Failure Stress (CFS) evolution since 1812 in Southern California. For the larger circular regions with radii of 140 to 160 km, the rate and slope of the frequency-magnitude distribution for moderate-size events just before the main shock appear to be anomalous compared to those for either the preceding or subsequent periods. The most pronounced changes, however, do not appear to be related to stress buildup to the Landers sequence itself. We also derive an "Index Value" to better quantify differences in the frequency of occurrence of moderate-size events as a function of time. |
