Non-double-couple earthquakes.

Non-double-couple (“non-DC”) earthquake mechanisms differ from what is expected for pure shear faulting in a homogeneous, isotropic, elastic medium. Until the mid-1980s, the DC assumption underlay nearly all seismological analysis, and was highly successful in advancing our understanding of tectonic processes and of seismology in general. In recent years, though, many earthquakes have been found that do not fit the DC model. Earthquakes that depart strongly from DC theory range in size over many orders of magnitude and occur in many environments, but are particularly common in volcanic and geothermal areas. Moreover, minor departures from the DC model are detected increasingly frequently in studies using high-quality data. These observations probably reflect departures from idealized models, caused by effects such as rock anisotropy or fault curvature. At the same time, it has become clear that industrial activities such as oil and gas production and storage, hydrofracturing ..