The environment surrounding the subduction zone plate interface

The interface between a subducting and overriding plate usually exhibits low seismic velocities within a thin (<6 km thick) layer. The hydrologic and petrologic conditions surrounding this layer control the behavior of the plate interface fault, which shows a wide range of rupture behavior from the megathrust through the down-dip transition to tremor and slow-slip. Many analyses of the properties of this plate interface low-velocity layer (LVL) use receiver functions (RFs), which sample sharp seismic velocity gradients and depend primarily on the time separation between RF phases from the top and bottom of the layer, to provide diagnostic estimates of thickness and Vp/Vs. Previous hypotheses for plate interface rupture behavior have invoked high pore-fluid pressure to explain inferences of apparently high Vp/Vs (exceeding 2.2) along the seismogenic zone and the adjacent down-dip slow-slip region determined from RF analyses. However, new higher-resolution analyses of scattered teleseismic P-wave coda that sample this region in two different subduction zones reveal Vp/Vs within the range of normal lithologies (1.6-2.0) and remove the observational requirement for a thick region of high pore-fluid pressure. These results agree with recent laboratory analyses of the properties of exhumed megathrust rocks. Instead, the mechanical properties of a thick damage zone surrounding the interface or entrained sediments explain both the scattered-wave observations and observed fault rupture behavior in the seismogenic zone and deeper. Pore pressure could play a role, but it may operate more locally or intermittently than conventionally thought. Additionally, from this analysis we find that the frequency content of the scattered phases generated at the delimiting boundaries of the LVL are limited and do not provide enough resolution to include direct (up-going P-to-S converted waves) conversions in LVL RF analyses without biasing the results to high values of Vp/Vs and thickness.