15. Anomalous Earthquake Ruptures at Shallow Depths on Subduction Zone Megathrusts

Abstract

Interplate thrust faults in subduction zones host the largest earthquakes and majority of seismic energy release on Earth. The seismically coupled portion of these megathrusts typically extends 100 ± 50 km across the depth range from 5 – 10 to 25 – 55 km, with convergent motions between underthrusting and overriding plates being accommodated by a mixture of earthquake slip, postseismic deformation, and interseismic creep. There are varying proportions of seismic and aseismic slip distributed with depth and varying seismic coupling coefficients in different subduction zones. Rupture of the shallow portion of megathrusts is the focus of this review; both large and small thrust earthquakes in the upper 10 km of the seismogenic zone sometimes display unusual seismic energy release attributes, with low rupture velocity and/or low stress drop. The largest of these events can be exceptionally tsunamigenic, with seismic slip extending updip of the apparent seismic front defined by smaller events. The transition from aseismic stable sliding on the shallow megathrust contact with any sedimentary wedge to the unstable slip regime of the seismically coupled zone at greater depth is variable from region to region and may involve zones of conditional stability that display bimodal deformation behavior as strain rates fluctuate. Systematic variations of rupture process with depth across the upper 10 km of the seismogenic regime have been observed, suggesting depth dependence of mechanical properties of the megathrust that may provide key information about sediment induration, hydrothermal processes, and frictional behavior of the megathrust. The acute seismic faulting heterogeneity observed in the shallow megathrust complicates generalization about the environment based on isolated spatial sampling of its properties, but there is great need for in situ ground truth determinations of structures and properties in the shallow megathrust. Anomalous Earthquake Ruptures at Shallow Depths on Subduction Zone Megathrusts Thorne Lay and Susan Bilek Anomalous Earthquake Ruptures at Shallow Depths on Subduction Zone 477