Seismic activities and ocean-bottom hydrostatic pressures changes along the Nankai trough detected by the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET)

Abstract:

The Japan Agency for Marine-Earth Science and Technology installed permanent ocean bottom observation network called Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) off the Kii Peninsula, southwest of Japan, to monitor earthquakes and tsunamis. We detected the long-term hydrostatic pressure changes from the ocean-bottom pressure records, starting from March 2013, at several DONET stations. We detected these changes after removing the contributions of ocean mass variations and sensor drift from the records. In addition, we detected a decrease in the background seismicity of a nearby earthquake cluster that was synchronous with the hydrostatic pressure changes. We interpreted these observed hydrostatic pressure changes to reflect vertical deformation of the ocean floor, and we consider the cause of the seafloor crustal deformation to be a slow slip event (SSE) beneath the stations. Because the pressure changes were observed at stations with distances less than 20 km, we inferred that the SSE occurred in the shallow part of the sedimentary wedge, such as on a splay fault system. The synchronous observation of an SSE and a seismicity change suggests that both were triggered by a change in the regional stress that may be associated with stress accumulation and release processes occurring along the Nankai trough. We estimated the fault geometry of the SSE from the observed ocean-bottom displacements. The mechanism of synchronous occurrence of the SSE and seismicity change would be discussed in the presentation. To detect similar signals related to the occurrence of megathrust or other types of large earthquakes, seismic and crustal activities in this region should be carefully and continuously monitored by DONET observations.