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Brietzke, Gilbert Björn (2009): Dynamic Earthquake Ruptures in the Presence of Material Discontinuities. Dissertation, LMU München: Faculty of Geosciences
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Abstract

A general feature of tectonic faults is the juxtaposition of materials with dissimilar elastic properties in a variety of contexts and scales. Normal and reverse faults offset vertical stratifications, large strike-slip faults displace different crustal blocks, oceanic and continental crusts at subduction interfaces, and oceanic transforms juxtapose rocks of different ages. Bimaterial interfaces associated with rock damage are present with various degrees of sharpness in typical fault zone structures, and failure along a bimaterial interface can be effective even on microscopic scale of grain boundaries. A first order representation of a geological fault for seismic events is a frictional interface embedded in an elastic body. This study focusses on dynamic effects in the presence of material discontinuities altering dynamics of failure and dynamic rupture propagation on frictional interfaces. When the medium surrounding a fault is heterogeneous, the symmetry of stress is broken up and perturbations of normal stress introduces additional instability potentially generating additional propagation modes of rupture. This study presents three specific numerical investigations of the aforementioned rupture phenomena associated with material contrasts at the fault. A first numerical study (a) investigates 2-D in-plane ruptures in a model consisting of two different half-spaces separated by a low-velocity layer and possible simultaneous slip along multiple faults. This study shows that bimaterial frictional interfaces are attractive trajectories of rupture propagation, and ruptures tend to migrate to material interfaces and becoming self-sustained slip pulses for wide ranges of conditions. In a second numerical study (b), the propagation of a purely material contrast driven rupture mode, that is associated with the so-calledWeertman or Adams-instable pulse, is shown to exist also in the general 3-D case, where there is a mixing of in-plane and anti-plane modes, the bimaterial mechanism acting in the in-plane direction only. Finally, in a further numerical investigation (c) it is demonstrated, that the rupture dynamics and ground motion can be significantly influenced by bimaterial mechanisms of rupture propagation for ranges of parameters. The model studied here comprises heterogeneous initial shear stress on a slip-weakening frictional interfaces separating two dissimilar elastic bodies, a free surface. The discussion focusses on the diversity of existing rupture propagation modes and ground motion. The investigated models and obtained results are motivated and discussed in the context of complementary numerical investigations, theoretical studies of stability analysis, seismological vii observations of earthquakes and aftershock sequences, geological observations of fault zone structures, tomographic studies, and geodetic observations.