On the influence of fault bends on the growth of sub-Rayleigh and intersonic dynamic shear ruptures

Abstract

Earthquake ruptures are modeled as dynamically propagating shear cracks with the aim of gaining insight into the physical mechanisms governing their arrest or, otherwise, the often-observed variations in rupture speeds. Fault bends have been proposed as being the main cause for these variations. Following this line of reasoning, the existence of deviations from fault planarity is chosen as the main focus of this study. Asymmetric impact is used to generate shear loading and to propagate dynamic mode-II cracks along the bonded interfaces of two otherwise identical homogeneous constituents. Secondary paths inclined at various angles are also introduced to represent fault bends or kinks. The experiments show that certain fault bend inclinations are favored as alternate paths for rupture continuation, whereas others suppress further motion of the incoming rupture. The asymptotic elastodynamic stress fields at the tip of the growing rupture are used to develop two criteria (one energetic and one stress based) for rupture propagation or arrest at the kinked interfaces. These criteria correlate very well with the experimental results. Since most field evidence suggests that the average rupture speeds during crustal earthquakes are sub-Rayleigh, this work first focuses on incoming rupture speeds that are just below the Rayleigh wave speed. Reports of intersonic crustal fault rupture speeds having surfaced recently, experiments and analyses are also performed within that speed regime.

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