Geological constraints on the mechanisms of slow earthquakes

The recognition of slow earthquakes in geodetic and seismological data has transformed the understanding of how plate motions are accommodated at major plate boundaries. Slow earthquakes, which slip more slowly than regular earthquakes but faster than plate motion velocities, occur in a range of tectonic and metamorphic settings. They exhibit spatiotemporal associations with large seismic events that indicate a causal relation between modes of slip at different slip rates. Defining the physical controls on slow earthquakes is, therefore, critical for understanding fault and shear zone mechanics. In this Review, we synthesize geological observations of a suite of ancient structures that were active in tectonic settings comparable to where slow earthquakes are observed today. At inferred slow earthquake regions, a range of grain-scale deformation mechanisms accommodated slip at low effective stresses. Material heterogeneity and the geometric complexity of structures that formed at different inferred strain rates are common to faults and shear zones in multiple tectonic environments, and might represent key limiting factors of slow earthquake slip rates. Further geological work is needed to resolve how the spectrum of slow earthquake slip rates can arise from different grain-scale deformation mechanisms and whether there is one universal rate-limiting mechanism that defines slow earthquake slip. The discovery of slow earthquakes transformed the understanding of how tectonic plate motions are accommodated; however, the mechanics of slow earthquakes remain enigmatic. This Review synthesizes geological observations of ancient deformation structures that likely hosted slow earthquakes and discusses the insights into the mechanics of fault slip. Key pointsThe global distribution and pressure-temperature range of seismologically observed slow earthquake hypocentres implies that no single mineral phase, lithology or metamorphic reaction controls slow earthquake slip.There is no universal deformation structure or deformation mechanism that is currently a clear indicator of slow earthquakes in the rock record. Multiple different mechanisms or combinations of mechanisms can produce the same macroscopic behaviours.Geological evidence from slow earthquake source regions suggests that material heterogeneity, geometric complexity and deformation at low differential stress are common to slow earthquake sources.A seismologically observed low-frequency earthquake source could consist of multiple anastomosing faults, shear bands and/or vein networks (potentially including synchronous slip across multiple subparallel surfaces), rather than a single planar fault surface.Geodetically observed slow slip events can be accommodated by ductile shear zones, which are commonly identified in many exhumed fault zones.

Automated summary

The recognition of slow earthquakes has transformed the understanding of plate motion mechanisms, but the mechanics behind slow earthquakes remain enigmatic. Geoscientists have synthesized geological observations of ancient deformation structures to gain insights into fault slip mechanics.