Characteristics of Near-Fault Ground Motions by Dynamic Thrust Faulting: Two-Dimensional Lattice Particle Approaches

Two-dimensional lattice particle models have been used to simulate dynamic thrust faulting and its effects on near-fault ground motions. The lattice particle modeling approach has been demonstrated as an efficient way to model the dynamic rupture phenomena observed from a foam rubber experiment on dipping faults. We constructed a 42 dipping fault model to simulate near-fault ground motions under different site conditions in which different combinations of fault geometry (blind fault underlying a sedimentary layer and outcropping thrust models) and near-surface sedimentary layers were investigated. In particular, dynamic behaviors between a blind fault with an overlying sedimentary layer and an outcropping fault without a sedimentary layer were compared. In this simulation, a dynamic slip pulse accompanied by fault separation was initiated at the deepest part of the fault and propagated updip along the fault under a subshear rupture velocity. A strong asymmetrical ground-motion pattern on the hanging wall and footwall, caused by nearsource rupture effects, was observed. Rupture directivity played an important role in determining the size and distribution of peak ground velocities and accelerations on the hanging wall and footwall. In the case of an outcropping thrust without a sedimentary layer, the hanging wall underwent a stronger ground motion, caused by the near-surface breakout phase, as the rupture reached the free surface. In the case of a blind thrust overlying a sedimentary layer, the peak ground particle velocity and acceleration could be much larger on the footwall. This is a result of the amplification effect of trapped long-period seismic energy in the sedimentary layer. The seismic energy emitted from the rupture-stopping phase was incident to the sedimentary layer and radiated under the rupture directivity effect. The radiated long-period seismic energy was trapped in the sedimentary layer, propagating away from the fault trace toward the footwall side. The numerical results show that, for an initial slip of 5 to 5.5 m, the horizontal peak ground velocity and acceleration could reach about 1.5 m/sec and 2g, respectively, on the footwall for a blind thrust with a sedimentary layer. With the same initial slip level, the peak ground velocity and acceleration were about 1. 1 m/sec and 1.5g, respectively, on the hanging wall for an outcropping thrust without a sedimentary layer. These results can partially explain the field observations of precarious rock distributions and overturned transformers in the vicinity of the White Wolf Fault during the 1952 Ms 7.6 Kern County earthquake. Furthermore, the current simulation can be used for near-fault strong-motion prediction for large thrust faults in the Los Angeles Basin or similar tectonic settings around the world.