Particle motions in a physical model of shallow angle thrust faulting

Abstract

A physical model of thrust faulting has been constructed out of elastic foam rubber. The thrust wedge has an angle of 25°., and is forced from the back by a steel-hydraulic system of effectively infinite rigidity, analogous to the force exerted by tectonic plates. The observed particle motions show many features different from those commonly assumed in dislocation models of subduction thrusts. Interface waves associated with fault opening propagate along the thrust plane (from the back) and temporarily decouple the overlying hanging wall plate from the foot wall. Because of the geometry of the reflecting fault boundary, and the free surface (also reflecting), energy is trapped in the wedge and, as a consequence, the particle motions and energy in the hanging wall are much greater than in the foot wall. The interface wave and the energy trapped in the upper plate propagate up the wedge and break out at the toe of the thrust with a spectacular increase in motions. If this model is analogous to the real earth, it suggests that some common assumptions in dislocation modeling may be incorrect. The model may explain apparent missing energy radiated to teleseismic distances, the anomalously low number of empirical Green’s functions needed to model teleseismicp waves, and the evidence of intense shaking on the hanging wall toe of some thrust fault earthquakes. The results support the suggestion that interface waves may explain the lack of high frictional heat generation from thrust faults, and may explain the ‘paradox of large overthrusts’. The results also suggest that the seismic hazard of great subduction thrust earthquakes and continental shallow angle thrust faults might in some cases be seriously underestimated.