Differentiating frictionally locked asperities from kinematically coupled zones

Abstract

Seismogenic areas on plate-boundary faults resist slipping until earthquakes begin. The delay in slip relative to plate motion, termed slip deficit, represents plate coupling as an interseismic proxy of seismic potential. However, when a part of a frictional interface sticks together (locked), the unlocked sliding surroundings are braked and slowed (coupled), causing coupled zones always wider than the locked zones that rupture during earthquakes. This study investigates the frictional physics that locked and unlocked zones should observe, laying the foundation for inferring frictionally locked segments, known as asperities in fault mechanics. Various friction laws are shown to have a unified representation of locking. (I) Locking means the pre-yield phase, where the fault interface does not slip, and unlocking means the post-yield phase, where stress on the interface equals strength. (II) For intersesismic periods, while locking still denotes constant slip, unlocking signifies quasi-steady creeping of constant stress. Locking inversion, a variant of conventional coupling inversion that incorporates this unified frictional physics, estimates the distribution of locking, determining slip and stress distributions consequently. We solve the locking inversion by a method that distributes circular asperities on unlocked interfaces. By applying this method to on-/off-shore GNSS data in southwestern Japan, we detect five primary locked segments along the Nankai subduction zone. Those segments accord with slip zones of historical megathrust earthquakes correlated with seafloor basins. Estimated locked zones avoid the occurrence zones of deep slow earthquakes, reproducing the hypothesis that the areas hosting slow earthquakes are normally, in interseismic timescales, coupled but unlocked.