High‐resolution imaging of fault zone structures

Abstract

Damaged fault zone (FZ) rocks with high crack density are expected to produce several indicative wave propagation signals. These include scattering, anisotropy, nonlinearity, and FZ guided head and trapped waves. These signals can be used to obtain high‐resolution imaging of the FZ structures and to track possible temporal evolution of FZ properties. Results associated with systematic analysis of such signals recorded at several large strike‐slip FZs can be summarized as follows: The observed FZ trapped waves are generated by relatively shallow structures that extend only over the top ∼3−−4km of the crust. The shallow trapping structure in the North Anatolian fault (NAF) is surrounded by broader anisotropic and scattering zones that are also confined primarily to the top 3 km. Systematic analyses of anisotropy and scattering around the NAF do not show precursory temporal evolution of properties. However, the scattering results show clear co‐seismic changes and post‐seismic logarithmic recovery. These effects are likely to reflect mostly changes in properties of the shallow crust in response to strong shakings of nearby major earthquakes. Analyses of FZ head waves along the San Andreas Fault point to the existence of material interfaces that extend to the bottom of the seismogenic zone.