The forearc seismic belt: A fluid pathway constraining down-dip megathrust earthquake rupture.

A portion of fluids subducted in an oceanic plate is thought to return to Earth's surface and has the potential to influence faulting during that migration. We used machine learning to detect and locate earthquakes in the northeast Japan forearc using data from a cabled ocean-bottom seismic network. That seismicity shows vertical distributions extending from the subducted plate to near the sea floor. The "forearc seismic belt" corresponds to the seismicity assumed to result from fluid supply from the underlying plate to the forearc and may also act to constrain the down-dip limit of megathrust rupture. The zone of high seismicity continues inland under metropolitan Tokyo, aligning with the Boso slow slip events and the eastern edge of the1923 Kanto earthquake, supporting forecasts of shallow damaging earthquakes. Editor's summary: Subsurface fluids are known to trigger earthquakes through engineering and in nature. For the latter, the effect of fluids on seismicity above subduction zones is of particular interest for populated areas where megathrust earthquakes are known to occur, such as Japan. A machine-learning analysis of earthquake data from ocean-bottom cables led Suzuki et al. to define a belt of relatively shallow seismic activity caused by fluids coming off the subducted Pacific plate. This belt includes the area beneath Tokyo and helps to explain the region's high seismic activity relative to the rest of Japan. —Angela Hessler [ABSTRACT FROM AUTHOR]