[S26-1] Exploring connections between heat flow and tectonics I

[S26-1-03] Two-dimensional thermal modeling associated with subduction of the Philippine Sea plate beneath southern Kyushu, Japan

^○Nobuaki Suenaga¹, Shoichi Yoshioka^{1, 2}, Takumi Matsumoto³ (1.Research Center for Urban Safety and Security, Kobe University, Jpan, 2.Department of Planetology, Graduate School of Science, Kobe University, Japan, 3.National Research Institute for Earth Science and Disaster Resilience)

There is a non-volcanic region sandwiched between the Aso and Kirishima volcanic zones in southern Kyushu, Japan. Several characteristic seismic events were identified around there; Postseismic slips were found associated with the two Hyuga-nada earthquakes (M6.6), which were interplate earthquakes that occurred on October 19 and December 3, 1996. Tectonic tremors were observed around the mantle wedge corner beneath the Pacific coast of Miyazaki prefecture. To investigate the cause of these events, we performed 2-D box-type time-dependent thermal modeling in southern Kyushu. We set a profile to pass through the regions in the plate convergence direction where tectonic tremors and postseismic slips were identified. To constrain calculated temperature structure, we used surface heat flow data. In our model, we considered the subduction history of the Philippine Sea (PHS) plate, and changed the age of an ocean floor and subduction velocity along the profile at each time step. To explain the observed low heat flow above the mantle wedge corner, we introduced a low viscosity layer at the plate boundary. It became difficult for mantle flow to intrude into the mantle wedge corner, by incorporating the low viscosity layer, and surface heat flow above there became lower than that without it. As a result, the temperature range of the upper surface of the subducting PHS plate where postseismic slip associated with the 1996 Hyuga-nada earthquake on December 3 occurred became approximately 300 degree. Interplate temperature where tectonic tremors occurred beneath Miyazaki prefecture ranges from 400 to 500 degree. We also estimated the dehydration process along the profile and used the phase diagram of hydrous MORB in the oceanic crust. As a result, blueshist transformed into lawsonite blueshist in the postseismic slip region, and lawsonite blueshist transformed into lawsonite eclogite in the active region of tectonic tremors.