Seismogenic zone structure along the Middle America subduction zone, Costa Rica

Most large (MW > 7.0) underthrusting earthquakes nucleate along a shallow region of unstable frictional stability on or near the subducting plate interface termed the seismogenic zone. The studies presented here investigate along-strike spatial and temporal variability in microseismicity and seismic velocity and provide spatial constraints on the updip and downdip limits of microseismicity within the Middle America subduction offshore western Costa Rica. All chapters utilize data recorded by the Costa Rica Seismogenic Zone Experiment (CRSEIZE), a collaborative seismic and geodetic study undertaken from September 1999--June 2001 to better understand subduction zone behavior near the Osa and Nicoya Peninsulas, Costa Rica. Chapter 1 serves as a broad introduction to the thesis while Chapter 2 provides an overview of Costa Rica seismicity, the CRSEIZE experiment setup, data processing, and data quality. Chapter 3 discusses simultaneous inversion for 1D P- and S-wave velocity models, station corrections, and hypocenter parameters for both the Nicoya and Osa experiments and presents a refined location for the continental Moho in northern Costa Rica. Chapter 4 presents absolute and relative relocations of ∼300 aftershocks of the 1999 Quepos, Costa Rica, underthrusting earthquake and analyzes seismogenic zone structure offshore central Costa Rica during a period of increased seismicity rate. Subduction of highly disrupted seafloor north of the Osa Peninsula has established a set of conditions that presently limit the seismogenic zone to be between 10--35 km below sea level, 30--95 km from the trench axis. Chapter 5 presents high resolution earthquake locations and P-wave and P-wave/S-wave 3D velocity models for the locked Nicoya Peninsula segment of the Middle America subduction zone calculated using an iterative, damped least squares local tomography method. In the southern Nicoya Peninsula, microseismicity along the plate interface extends from 12--26 km depth, 73--100 km from the trench axis, while in the northern Nicoya Peninsula where hydrothermally cooled oceanic crust subducts, interplate seismicity extends from 17--28 km depth, 75--87 km from the trench axis. Seismic velocity results suggest 10--30% serpentinization of the mantle wedge along the Nicoya Peninsula with the continental Moho occurring at 30--34 km depth. Chapter 6 correlates changes in noise levels of the CRSEIZE ocean bottom seismometers and local and regional seismicity patterns to fluid flow rate excursions measured by collocated fluid flow meters. Combined, the studies provide the highest resolution images of seismicity and seismic velocity along the Middle America seismogenic zone to date and lend further insight into the thermal, mechanical, hydrological, and compositional interactions potentially responsible for controlling shallow subduction zone seismicity.