Constraints on Crustal Stress from Coseismic Slip Models and Focal Mechanisms.

Abstract

Constraining crustal stress that leads to earthquakes is an active area of research with profound implications on understanding the forces that deform the surface of the earth and generate slip on faults. Surface deformation related to strain accumulation on faults prior to, during and following earthquakes are recorded geodetically (InSAR and GPS). These data are used to infer fault geometries and models of coseismic slip of an earthquake. Seismic energy radiated during earthquakes are used to produce focal mechanisms, which are geometric representations of faults, and provide insight on stress changes due to earthquakes. However, earthquakes are the response to stress accumulation on faults, but direct measurements of accumulated stress are difficult. In this dissertation, I develop, test, and apply a Bayesian Monte Carlo (BMC) estimation technique to infer crustal stress from both focal mechanisms and coseismic slip models, the latter of which has never been done prior to the work I present here.

I apply the BMC method to investigate stresses leading to the 2008 Wenchuan, China, earthquake, and to the 1999 İzmit and Düzce, Turkey, earthquakes. I use various coseismic slip models from all three events, aftershock focal mechanisms of the Wenchuan earthquake, and seismicity recorded in the Sea of Marmara, adjacent to the İzmit earthquake. I find that a homogeneous stress is statistically consistent with slip during the Wenchuan earthquake, and that heterogeneous stresses along the trace of the mainshock, previously argued for based on aftershock focal mechanisms, may simply reflect ambiguities in the interpretation of stress from focal mechanisms. Coseismic slip models from the İzmit and Düzce earthquakes are also consistent with a homogeneous stress along all fault segments that slipped in those earthquakes, particularly if the coefficient of fault friction is about 0.2 or less. In the Sea of Marmara, inferred stresses from focal mechanisms indicate that stress differs from the eastern to the western segments of the Main Marmara fault. Additionally, results indicate a potential stress rotation along the western segment between about 1999 and 2003, towards a transform stress regime similar to the stress leading to the İzmit and Düzce earthquakes.