Subduction Faults as We See Them in the 21st Century

Abstract:

Major advances in geophysical monitoring and field and laboratory studies have reshaped our views of subduction faults over the past 15 years. The beginning of the 21^st century saw the discovery of Episodic Tremor and Slip, followed by the discovery of opposing motion of coastal and inland GPS sites decades after the giant megathrust earthquakes of Chile (1960) and Alaska (1964). The burst of great earthquakes since 2004 caused tragic losses, but the resultant massive observational data greatly improved our knowledge. Today, we know that all subduction faults are extremely weak, usually represented by apparent friction coefficients lower than 0.05. Smooth faults that have produced giant earthquakes are the weakest. Geometrical irregularities such as subducting seamounts give rise to stronger faults, but these faults creep. Rupture-zone average stress drops in great earthquakes are as small as 2 – 5 MPa but are still a significant fraction of the fault strength. Therefore, it takes time to rebuild fault stress to the level of failure, consistent with great earthquakes having long recurrence intervals. The process of stress rebuilding is strongly affected by the viscoelastic mantle rheology. Because of viscoelastic stress relaxation, most of the forearc area continues to move seaward following a great earthquake but gradually reverses direction to move landward. Advanced monitoring in the new century has revealed a wide range of slip behaviors of the shallowest part the megathrust, such as huge trench-breaching coseismic slip, slip that generates large tsunamis but not strong shaking, postseismic creep, and episodic slow slip. Rapidly expanding efforts of seafloor geodesy and seismology and ocean drilling allow us to study these phenomena in close range. The deeper part of subduction faults where the slab is in contact with the serpentinized upper-plate mantle is understood to be lined with weak hydrous minerals such as talc that cause slab-mantle decoupling but retard seismic slip. With increasing depth, frictional slip gives way to viscous shear. At a depth around 70-80 km, the subduction fault is terminated. Below this depth, the mantle material travels with the slab to form mantle-wedge corner flow, as inferred from forearc heat flow observations and temperature conditions for arc volcanism.