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Shattered Rock and Precarious Rock Evidence for Strong Asymmetry in Ground Motions during Thrust Faulting

Recent thrust faulting models, both physical and numerical, have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults. In this article, recent field evidence supporting this strong asymmetry is described. Earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1g and 100 cm/sec. New shattered rock evidence on the hanging wall of thrust faults in Southern California supports the occurrence of intense ground motions. On the other hand, the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in Southern California indicate relatively low ground motions, thus indicating strong asymmetry in ground motions near the fault trace for thrust faults. Some current ground-motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall. This could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the Los Angeles Basin and elsewhere in the world. The new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models. If such detachment occurs, it could also have important implications for understanding thrust fault rupture mechanics, for example, the paradox of large overthrusts and the heat flow paradox for thrusts.