Particle motions in a foam rubber model of shallow-angle thrust faulting show many features different from those commonly assumed in dislocation models of subduction thrusts (Brune, 1996). As a complement to a physical foam rubber experiment, we have carried out dynamic simulation using a 2D lattice numerical model. The model is constructed as a triangular block sliding over a rectangular elastic block. It consists of a 2D set of particles interacting with each other by nonlinear Hooke's forces and obeying Newton's equations of motion. Rough surfaces are introduced on the contact plane of the two blocks to simulate realistic friction.
The numerical simulations demonstrate several key features of thrust faulting, including stick slip, fault opening, and strong breakout and shaking at the hanging-wall toe (the wedge-shaped tip of the outcropping hanging-wall block) of the thrust fault, all consistent with the foam rubber experiments. The stick-slip motion shows an approximate time and slip repeatable behavior. Each slip event is characterized by a self-healing pulse associated with fault opening and frictional locking. The rupture pulse propagates steadily along the fault. When rupture reaches the toe of the fault outcrop, the hanging wall breaks away from the foot wall and creates a large opening vibration of the hanging wall. The peak acceleration observed at the hanging-wall toe of the fault outcrop is about 3 to 4 times the peak acceleration in the center part of the fault, and about 2 to 3 times the motion of the foot wall. Such a large increase in peak acceleration at the toe is caused mainly by multiply reflecting stress waves trapped in the wedge-shaped hanging wall of the fault. The strong asymmetry of the particle velocity between the hanging wall and foot wall is an important feature of the results consistent with the foam rubber model but different from standard dislocation models. This dynamic result illustrates the important effects of thrust faulting geometry on fault slip and ground motion.