Non-Double-Couple Earthquakes 2. Observations

Most studies assume that earthquakes have double-couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many well-recorded earthquakes have radiation patterns that depart radically from this model, indicating fundamentally different source processes. Seismic waves excited by advective processes, such as landslides and volcanic eruptions, are consistent with net forces rather than DCs. Some volcanic earthquakes also have single-force mechanisms, probably because of advection of magmatic fluids. Other volcanic earthquakes have mechanisms close to compensated linear vector dipoles and may be caused by magmatic intrusions. Shallow earthquakes in volcanic or geothermal areas and mines often have mechanisms with isotropic components, indicating volume changes of either explosive or implosive polarity. Such mechanisms are consistent with failure involving both shear and tensile faulting, which may be facilitated by high-pressure, high-temperature fluids. In mines, tunnels are cavities that may close. Deep-focus earthquakes occur within zones of polymorphic phase transformations in the upper mantle at depths where stick-slip instability cannot occur. Their mechanisms tend to be deviatoric (volume conserving), but non-DC, and their source processes are poorly understood. Automatic global moment tensor services routinely report statistically significant non-DC components for large earthquakes, but detailed reexamination of individual events is required to confirm such results.