Guided Waves Generated by Sources Outside a Low Velocity Fault Zone Layer

In the last decade fault zone (FZ) guided head and trapped waves have been observed along several earthquake faults with receivers close to the FZ. There is hope that analysis of such phases may be used to provide a higher resolution imaging of FZ structure at depth than is possible with standard ray tomographic methods. Such imaging can provide important input for mechanical models of faults before, during, and after earthquake ruptures. In addition, low-velocity FZ layers can produce considerable amplification of ground motion that may increase the shaking hazard in populated areas close to faults. Several previous studies argued that FZ guided waves are only generated when the sources are close to or inside the low-velocity FZ layer. It is thought that only a small percentage of earthquakes in an active fault occurs inside this region. Here we discuss numerical simulations of 3D wave propagation which show that sources at considerable distance from the FZ are able to generate high amplitude trapped waves. We focus on a structure consisting of a shallow low velocity FZ layer (e.g., depth < 5 km) in a half space, and perform an extensive parameter study quantifying the amplification of ground motion for sources outside and below the FZ layer. In general, the trapping efficiency strongly depends on the source mechanism and orientation of the source with respect to the FZ layer. The generation of guided waves by sources in a volume outside the FZ has implications for evaluation of seismic hazard close to faults. These effects also have important implications for imaging structural properties at depth using FZ guided waves.