LANDWARD VERGENCE IN ACCRETIONARY WEDGES: EVIDENCE FOR PAST SEAFLOOR RUPTURES?

The very large slip up to the subduction front of the 2011 Tohoku-Oki earthquake has challenged our classical view of the megathrust undergoing only aseismic slip at shallow depth. Landward vergence in accretionary wedges is rare and has been described at very few places: along the Cascadia subduction zone and along Sumatra where the 2004 Mw 9.1 Sumatra-Andaman event and the 2011 tsunami earthquake are suspected to have reached the seafloor. Using mechanical analysis, we here show that a very low effective friction along the megathrust is required to form landward-vergent frontal thrusts. This very low effective friction is most likely due to a high pore fluid pressure that could either be a long-term property or due to dynamic effects such as thermal pressurization. However, we here show that landward vergence can only appear if the prism is far from the compressional critical state allowed by sudden and successive decreases of the effective friction. We thus favor a dynamic origin. Landward vergence would then be indicative of occasional or systematic propagation of earthquakes to the trench. As a consequence, the presence of landward vergence of thrusts in Cascadia might indicate future frontal ruptures of the shallowest portion of the megathrust. The vergence of thrusts could thus be used to improve the seismic and tsunamigenic risk assessment.