Non-Double-Couple Earthquake Mechanisms

Historically, most quantitative seismological analysis has been based on the assumption that earthquakes are caused by shear faulting, for which the equivalent force system in an isotropic medium is a pair of force couples with no net torque (a “double couple”, or DC). Observations of increasing quality and coverage, however, now resolve departures from the DC model for many earthquakes, and find some earthquakes, especially in volcanic and geothermal areas, that have strongly non-DC mechanisms [Julian et al., 1998; Miller et al., 1998]. Study of non-DC earthquakes can advance our understanding of a variety of phenomena, including tectonic faulting, volcanic and geothermal processes, glacial icequakes, and mine collapses. This presentation summarizes the current state of observations of non-DC earthquakes and analyzes many possible physical non-DC processes. Contrary to long-standing assumption, sources within the Earth can sometimes have net force and torque components, described by first-rank and asymmetric second-rank moment tensors, which must be included in analyses of landslides and some volcanic phenomena. Non-DC processes that lead to conventional (symmetric second-rank