Depth dependent rupture properties in circum-Pacific subduction zones

Abstract

Depth dependence of the source rupture duration of interplate thrust earthquakes is examined for seven subduction zones around the Pacific to explore variations in faulting properties. Multi-station deconvolutions of teleseismic P waves for moderate size earthquakes yield estimates of the source time function and centroid depth for each event. Analysis of 17 to 75 earthquakes in each region reveals a consistent trend of decreasing source duration (inferred from the source time functions, after correction for differences in total energy release) with increasing depth. Rupture duration patterns vary somewhat between subduction zones as well as along strike within a given zone, and the data have large scatter, implying significant variation in rupture processes along the interplate megathrusts, but the depth dependence appears to be robust. The rupture duration variations prompt consideration of two end-member models: 1) depth-dependent rupture velocity is caused by variations of rigidity of materials in the fault zone, while static stress drop is constant, and 2) static stress drop varies with depth while material properties and rupture velocity are constant. For the first model, the volumetrically averaged rigidity of the fault zone must increase with depth in each region by a factor of 5 between depths of 5 to 20 km. If rupture velocity is constant, the stress drop must increase by an order of magnitude over the same depth range. This systematic variation in rupture behavior with depth may reflect spatial variations in the amount, compaction and porosity of sediment in the fault zone, topography on the subducting plate, phase transitions in the fault zone materials, thermal structure of the megathrust, and varying presence of fluids in the fault zone. Such physical variations appear to control the physics of rupture propagation, leading to intrinsic dependence of rupture velocity on materials and fluids within the fault zone.