A 2D lattice particle model is used to simulate the dynamic rupture process of a normal fault. The system equations for the particle motions are solved numerically by the finite-difference method, under a given block boundary condition. The flexibility of the implementation of a 2D lattice particle model to simulate an earthquake dynamic process was demonstrated in previous modeling of a shallow angle thrust fault (Shi et al., 1998). Numerical results indicate that the particle motions (displacement, velocity, and acceleration) along the fault are discontinuous both in the fault-parallel and fault-normal directions, with a localized slip rupture and localized fault separation. In the vicinity of the fault outcrop (the position at which the fault intersects with the free surface), the particle velocity and acceleration increase rapidly, both on the hanging wall and the footwall. The particle motions on the hanging wall are larger than those on the footwall. These motions are amplified as the fault scarp develops (rupture breaks out at the surface), with strong asymmetry between the hanging wall and the footwall. Along the free surface, as the distance from the fault outcrop increases, the particle velocity and acceleration decrease rapidly on the footwall and less rapidly on the hanging wall. The asymmetrical particle motion results from the geometrical effect of the dip of the fault, the free surface, and the dynamic source rupture. The combination of all of these effects causes a strong asymmetry in stress when the rupture pulse approaches the free surface. The dynamically propagating rupture is characterized by a ramp slip time function accompanied with fault opening. The slip pulse becomes sharper when the rupture approaches the free surface; consequentially, the hanging wall in the vicinity of the fault exhibits a large vibration, which generates a strong surface wave propagating along the free surface away from the fault scarp on the hanging-wall side. This result is similar but significantly different from the numerical simulation of a normal fault with a moving double-couple dislocation source (Benz and Smith, 1988). In addition, the numerical result is qualitatively in agreement with recent foam-rubber experiments (Brune and Anooshehpoor, 1999) and similar to results from a finite-element simulation (Oglesby et al., 1998; Oglesby, 1999). Comparing with a 2D strike-slip fault and thrust fault, the particle motions in the vicinity of the normal fault on the free surface are smaller for a same-size rupture pulse.

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