Geological constraints on the mechanisms of slow earthquakes

Abstract:

The global distribution, wide depth and temperature range, and variety of tectonic settings where slow earthquakes are observed suggest they are a general, commonly occurring manifestation of active faulting. The similarity in seismic signature of low frequency earthquakes at different depths presents a challenge to understanding the mechanics at the source of slow earthquakes, because the grain-scale deformation mechanisms that accommodate displacement should be different at different P-T conditions. Using geological observations of ancient, exhumed structures that were active in tectonic settings where slow earthquakes are observed today, we have compiled a list of characteristics of a generic slow earthquake source. Comparison of deformation structures that formed across metamorphic environments shows that no single mineral phase, lithology, or metamorphic reaction controls slow earthquake slip. Overall, the geological evidence suggests that the slow earthquake source can be described as a mixed-mode shear zone, within which the interplay between distributed and localized deformation is important. Multiple different mechanisms or combinations of mechanisms, including frictional and plastic deformation, produce the same macroscopic behaviors. This suggests a lack of dependence on a particular rheologic model, and implies that slow earthquake phenomena arise from some combination of loading and local geologic conditions, which can develop and generate similar seismological signals in a large variety of settings.