THERMAL CONSTRAINTS ON THE SEISMOGENIC PORTION OF THE SOUTHWESTERN JAPAN SUBDUCTION THRUST

For coastal cities an important factor in earthquake hazard from subdu ction zone earthquakes is the landward extent of the seismogenic porti on of the subduction thrust fault. In this study we test the hypothesi s that the maximum downdip extent is defined by a critical temperature . We have developed a transient thermal model for the Nankai subductio n zone of southwest Japan to allow comparison of the thermally estimat ed downdip extent of the seismogenic zone with that from (1) seismicit y and tsunami data for two great subduction earthquakes, (2) the cosei smic faulting extent of these events estimated from geodetic deformati on data, and (3) the interseismic locked zone determined from intersei smic geodetic data. The Nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatur es. It has earthquake, tsunami, and geodetic data that constrain the c oseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and st oring interseismic elastic strain. On the Nankaido margin off Shikoku Island, the thermal model indicates that a temperature of 350 degrees C (taken to be the limit for seismic initiation from laboratory and fi eld data) is reached on the subduction thrust fault 150 km from the tr ench. A transition zone into which rupture may extend with decreasing offset (taken to be 450 degrees C) extends an additional 45 km downdip . The thermal model results are in excellent agreement with the maximu m downdip extent of coseismic displacement for the 1946 Nankaido M(s) = 8.2 s earthquake off Shikoku Island and with the downdip extent of t he present locked zone. In the region of the 1944 Tonankai M(s) = 8.2 earthquake to the northeast, the subduction angle is s much steeper an d the thermal models indicate a narrower downdip seismogenic extent. T he seismogenic-locked zone from earthquake and geodetic data is also n arrower. Thus our analysis of the southwest Japan margin indicates tha t all three constraints on the downdip extent of the seismogenic zone, thermal, coseismic and interseismic geodetic data, are in general agr eement. The study also supports the hypothesis that the seismogenic po rtion of 3 subduction thrust faults is limited primarily by temperatur e. The thermal control implies that subduction thrust faults with shal low dip have wider seismogenic zones compared to those with steep dip. The subducting plate age and thus heat flow, and the thickness of the insulating sediments on the incoming plate, are also very important t o the thermal regime and thus to the seismogenic width. The relation o f the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as Cascadia where there have been no great historical eve nts.