Supraslab earthquake clusters above the subduction plate boundary offshore Sanriku, northeastern Japan: Seismogenesis in a graveyard of detached seamounts?

Thousands of offshore repeating earthquakes with low-angle thrust focal mechanisms occur along the subduction plate boundary of NE Japan. Double-difference relocation methods using P- and S-wave arrivals reveal clusters of events above these repeating events. To assure good depth control we restrict our study to events that are close to seismic stations. These supraslab earthquake clusters are regional features at depths of 25 to 50 km, and most of these clusters are below the depth of the forearc Moho, which we determined from converted waves. Seismicity over this depth range does not occur under the inland area of NE Japan except just below the vicinity of the arc volcanoes. Re-entrants in the inner trench slope indicate that repeated collisions of seamounts have occurred in the past. Our preliminary interpretation of supraslab clusters is that they represent seismicity in seamounts detached from the Pacific plate during slab descent, driven by the resistance of seamounts to subduction. Detachment during slab descent probably occurs on the sedimented and hydrothermally altered seafloor on which seamounts were originally constructed since these are known as zones of weakness during active island growth. High fluid pressure produced during dehydration of clay minerals and other low-temperature hydrous minerals could enable detachment at depths. Seamount crust is thus accreted to forearcs, possibly leading to a long-term component of near-coastal uplift. Supraslab earthquake clusters may be our most direct evidence of the fates of seamounts and suggest that tectonic underplating is actively occurring in this subduction system.