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The Equivalent Strength of Geometrical Barriers to Earthquakes

Stefan Nielsen, & Leon Knopoff

Published 1998, SCEC Contribution #409

We present a quantitative framework for evaluating the influence of non-planar fault geometry on repeated seismic ruptures. We model quasistatic ruptures on a non-planar fault trace imbedded in a two-dimensional elastic medium under in-plane strain. Because of the presence of fault segments that are not parallel to the regional shear stress (i.e. bends), the apparent strength at a given point on the fault is not fixed, but fluctuates with normal stress. Compressional features behave as increasingly strong barriers to fracture unless the stored normal stress is released in order to unlock the fault. Since slip on the fault itself cannot get rid of the normal stress, this is achieved through the action of off-fault morphological features such as secondary faulting, folding and vertical motions, that we introduce parametrically in the form of an aseismic relaxation. The apparent strength of a fault bend will stabilize in a narrow interval of values after repeated ruptures, characterized by a non-dimensional “hardness” parameter, whereby the relaxation rate is scaled by the tectonic loading rate.

On a fault structure having several small, widely separated bends, three families of events can be identified whose frequency and magnitude depend on the hardness (relaxation) parameter and the geometry: small events that cluster in the tension zones of the bends, intermediate size ruptures involving a single interbend segment, and large ruptures that break through bends and link on or more interbend segments. Large multi-segment events are most likely to occur for low values of the hardness, i.e., fast relaxation and slow loading rate.

Regions with compressional features act as barriers that stop most ruptures; stress is stored at these sites until they themselves break and initiate motion on the smoother, long reaches of the fault.

Nielsen, S., & Knopoff, L. (1998). The Equivalent Strength of Geometrical Barriers to Earthquakes. Journal of Geophysical Research, 103(B5), 9953-9965.