Earthquake Moment-Area Scaling Relations and the Effect of Fault Heterogeneity on Slow to Fast Earthquake Slip

Citation

Luo, Yingdi (2018) Earthquake Moment-Area Scaling Relations and the Effect of Fault Heterogeneity on Slow to Fast Earthquake Slip. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9SQ8XMV. https://resolver.caltech.edu/CaltechTHESIS:09062017-132833234

Abstract

Earthquake moment-area scaling relations play a key role in both earthquake physics studies and earthquake hazard assessment. A three-stage moment-area relation, based on advances in earthquake source inversion, is currently in use in Japan. The second stage has a scaling exponent outside the range of commonly accepted models of small and very large earthquakes. We develop theoretical insight on the mechanical origin of this second-stage scaling. We utilize an analytical dislocation model, a numerical crack model and multi-cycle rate-and-state simulations of strike-slip faults with heterogeneous friction properties. We find that the second stage in earthquake moment-area scaling results from a combination of surface rupture effects, comprising an effective rupture elongation along-dip due to a mirror effect and systematic changes of the shape factor relating slip to stress drop. Based on this physical insight, we propose a simplified formula to account for these effects in moment-area scaling relations.

Geological, seismological, geodetic and experimental studies provide evidence of the heterogeneous structure of natural faults. To advance our understanding of the mechanical role of fault heterogeneity on the diversity of earthquake slip behavior, we conduct a theoretical and computational study of heterogeneous fault models. We consider faults with a mixture of frictionally stable and unstable materials and spatial contrasts of fault zone pore fluid pressure, akin to hydraulically sealed brittle asperities embedded in a ductile fault zone matrix. We first study faults with a regular alternation of materials, using linear stability analysis and quasi-dynamic rate-and-state simulations. We find transitions in fault behavior from fast to slow earthquakes to steady slip, and determine how these transitions depend on the composition and strength contrast of the material mixture. Based on these results, we develop rate-and-state models with stochastic distributions of brittle asperities in a ductile matrix to study slow slip and tremor phenomena. We focus on the hierarchical patterns of tremor migration observed in subduction zones, which feature distinct tremor propagation speeds in different directions. Our models are in quantitative agreement with observations of episodic slow slip and tremor events in Cascadia. We discovered that, in contrast to a common view, slow slip might well be a result of tremor activity rather than its cause. The collective interaction of asperities with a broad range of material properties, mediated by creep, is a novel and robust mechanism for the generation of slow slip events. We find that the hierarchical patterns of tremor migration and the nucleation locations of tremor swarms provide constraints on fault rheology. Our study also shows that, despite multiple asperity interactions, there is a close relation between tremor rate and the underlying slip rate which supports an approach to constrain slow slip rate via observed tremor rates.

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