On the Geological Environment of Tremor and Slow Earthquakes (Invited)

Tremor and slow earthquakes have been observed primarily near the seismic-aseismic transition on subduction interfaces, but also within accretionary prisms, and on the deep extension of the San Andreas fault. In subduction margins, tremor and slow earthquakes have been found to correlate with zones of inferred fluid overpressure, which may correlate with fluid release from the downing slab. Along the San Andreas fault, however, the metamorphic conditions are not compatible with a local fluid source. Considering the variety in depth, temperature, and tectonic setting of the locations where tremor and slow earthquakes have been observed, the physical process causing these slip styles must occur at a range of crustal conditions. Deformation within a fault-fracture mesh as magma moves through the crust has been postulated as a mechanism leading to volcanic tremor. An analogous mechanism for non-volcanic tremor would be the movement of water through the crust. Hydrothermal veins provide a record of fracturing and precipitation from an aqueous fluid within a fracture system. Slickenfibre-coated veins represent shear slip at low effective stress. The microstructure of slickenfibre shear veins suggest repeated, incremental slip at length-scales typically in the 10 - 100 micrometer range, comparable with that inferred for single events within the tremor signal. Moreover, slickenfibre shear veins form in a variety of conditions, which have in common the presence of weak, pre-existing planes, and relatively high local fluid pressure. Based on these similarities, slickenfibre veins may be a product of tectonic tremor and slow slip, and preserve a record of the processes behind tremor activity, if this hypothesis is correct, it implies that tremor and slow slip can occur wherever there are localized zones of shear displacement along pre-existing weak planes under low effective stress - meaning that slow earthquakes require lithological heterogeneity in addition to low effective stress.