Dissertation/Thesis Abstract

Heterogeneity of earthquake stress drops, focal mechanisms and active fault zones
by Bailey, Iain W., Ph.D., University of Southern California, 2009, 142; 3368693
Abstract (Summary)

Faults and fault networks are heterogeneous structures where elastic strain from ongoing tectonic motions is released in earthquakes. Three studies presented in this thesis aim to illustrate the importance of fault heterogeneity using numerical models and quantify fault heterogeneity using earthquake data. In the first part, a model fault consisting of a grid of discrete slip patches surrounded by a 3-D elastic half-space is used to investigate the statistics of earthquake stress drops. Predictions for stress drops on a homogeneous fault with properties constrained by experiments are an order of magnitude larger than typical observations from seismic data. In the model, heterogeneous frictional properties result in an evolving distribution of spatially heterogeneous stress. Since stress must exceed the fault strength only in the hypocenter region, lower average stress at times of rupture lead to stress drops consistently lower than predictions for a homogeneous fault. The second part of the thesis concerns quantifying the geometry of earthquake faulting for different spatial scales and magnitude ranges in a plate boundary region. We compute and compare potency tensor summations of subsets of a catalog containing ∼ 170,00 focal mechanisms for 0 < ML</italic> ≤ 5 earthquakes from southern California. Results indicate that earthquake deformation is influenced by scales related to plate tectonic motions (∼ 700 km), large scale fault zones (∼ 50–250 km) and smaller geometric complexities (∼ 5–50 km). However, individual fault zone length-scales have distinct characteristics, indicating departure from self-similarity despite the lack of a single characteristic length-scale. In the third part of the thesis higher quality focal mechanisms are used to compare levels of heterogeneity between nine strike-slip fault zones in California. Focal mechanism heterogeneity is quantified using two measures that represent the scatter and asymmetry in faulting orientations, both computed from normalized potency tensor summations. The scatter correlates with complexity of the fault surface traces, and asymmetry correlates with the dominant fault orientation relative to plate motion directions. These observations indicate that focal mechanism heterogeneity relates both the long term evolutionary properties of the fault zone and the ability of the fault to efficiently release plate tectonic strains.

Indexing (document details)
Advisor: Ben-Zion, Yehuda
Commitee: Becker, Thorsten W., Nakano, Aiichiro, Platt, John P., Sammis, Charles G.
School: University of Southern California
Department: Geological Sciences
School Location: United States -- California
Source: DAI-B 70/08, Dissertation Abstracts International
Subjects: Geophysics
Keywords: Dynamics and mechanics of faulting, Earthquake focal mechanisms, Earthquake source observations, Fault complexity, Seismicity and tectonics
Publication Number: 3368693
ISBN: 9781109294811