Overpressured Underthrust Sediment in the Nankai Trough Forearc Inferred From Transdimensional Inversion of High-Frequency Teleseismic Waveforms

Active-source seismic surveys have resolved the fine-scale P-wave velocity (V_p) of the subsurface structure in subduction forearcs. In contrast, the S-wave velocity (V_s) structure is poorly resolved despite its usefulness in understanding rock properties. This study estimates V_p and V_s structures of the Nankai Trough forearc, by applying transdimensional inversion to high-frequency teleseismic waveforms. As a result, a thin (∼1 km) low-velocity zone (LVZ) is evident at ∼6 km depth beneath the sea level, which is located ~3 km seaward from the outer ridge. Based on its high V_p/V_s ratio (∼2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities and that accompanied strain release helps impede coseismic rupture propagation further updip.