We have determined precise hypocentral locations for over 3,600 aftershocks that occurred in the Yuha Desert region following the 4 April 2010 M_{w 7.2 El Mayor-Cucapah (EMC) earthquake until 14 June 2010 through a series of absolute and relative relocation procedures with algorithms including hpoinverse, velest and hypoDD. Location errors were reduced to ~20 m horizontally and ~80 m vertically. The locations reveal a complex pattern of faulting with en echelon fault segments trending in toward the northwest, approximately parallel to the North American-Pacific plate boundary and en echelon, conjugate features trending to the northeast. The relocated seismicity is highly correlated with the mapped faults showing triggered surface slip in response to the EMC mainshock. Aftershocks are located between depths of 2 km and 11 km, consistent with previous studies of seismogenic thickness in the region. Three-dimensional analysis reveals individual and intersecting fault planes between 5 km and 10 km in the along-strike and along-dip directions. These fault planes remain distinct structures at depth, indicative of conjugate faulting, and do not appear to coalesce onto a through-going fault segment. Measurements of surface displacement along the Laguna Salada-West and East (LS-W and LS-E, respectively) branches reveal no triggered surface creep on either branch following the EMC mainshock. Approximately 2 mm of triggered surface creep was observed on the LS-W in response to the 14 June 2010 M5.7 Ocotillo earthquake, however no creep was measured along the LS-E. Relocated seismicity shows that aftershock activity occurs along the LS-E, however not along the LS-W in the time period between these earthquakes. The area east of the LS-W was seismically active following the EMC mainshock, however; no systematic migration patterns were observed. Aftershock activity abruptly shuts off in this area following the Ocotillo earthquake. The majority of seismicity in the two-year period following the Ocotillo earthquake suggesting a complex spatial and temporal distribution of aftershock activity. The conjugate pattern of faulting, combination of seismicity and triggered surface creep and non-uniform aftershock migration patterns suggest that strain in the Yuha Desert is being accommodated in a complex ma}

A number of outstanding questions in earthquake physics revolve around under-

standing the relationships among local-scale stress changes, fault interactions (i.e. how stresses are transferred) and earthquake response to stress changes. Here, I employ seismological observations and numerical simulation tools to investigate how stress changes from a mainshock, or by fluid injection, can either aid or hinder further earthquake activity. Chapter 2.2 couples Coulomb stress change models with rate- and state-dependent friction to model the time-dependent evolution of complex aftershock activity following the 2010 El Mayor-Cucapah earthquake. Part III focuses on numerical simulations of earthquake sequences with the multi-cycle earthquake simulator, RSQSim. I use RSQSim in two applications; 1) multi-cycle simulation of processes that controlling earthquake rupture along parallel, but discontinuous, offset faults (Chapter 3), and 2) investigation of relationships between injection of fluids into the subsurface and the characteristics of the resulting induced seismicity (Chapter 4).

Results presented in Chapter 2.2 demonstrate that both increases and decreases in seismicity rate are correlated with regions of positive and negative Coulomb stress change, respectively. We show that the stress shadow effect can be delayed in time when two faulting populations are active within the same region. In Chapter 3, we show that the pre-rupture stress distribution on faults governs the location of rupture re-nucleation on the receiver fault strand. Additionally, through analysis of long-term multi-cycle simulations, we find that ruptures can jump larger offsets more frequently when source and receiver fault ruptures are delayed in time. Results presented in Chapter 4 demonstrate that induced earthquake sequences are sensitive to the constitutive parameters, a and b, of the rate-state formulation. Finally, we find the rate of induced earthquakes decreases for increasing values of hydraulic diffusivity.