Thermal Models of the Sumatra Subduction Zone: Implications for the Megathrust Seismogenic Zone

We have developed 2-D finite element models for the thermal regime across the Sumatra subduction zone and temperatures on the subduction thrust fault. The main objective is to examine thermal controls of the downdip extent of great earthquake rupture. The rupture updip and downdip limits are important both for tsunami generation and earthquake shaking. However, another important application is the dehydration and upward fluid expulsion from the downgoing slab with increasing temperature and pressure. Two recent well-studied M9 great earthquakes occurred in 2004 and 2005, the Sumatra-Andaman and the Nias earthquakes, respectively. Finite element thermal models on three 2-D profiles are developed, allowing for smoothly varying subduction dip, thermal properties of the rock and sediment units, frictional heating, the thermal regime of the incoming plate, etc. The common updip thermal limit to seismic behaviour of 100-150°C occurs directly at the trench. The common downdip limit of 350-450°C occurs globally at depths of 30 to 50 km, 200 to 220 km inland from the trench. An alternate downdip seismogenic limit, the intersection of the subduction thrust with the aseismic forearc mantle, is poorly constrained but is estimated to be at a depth of ~30 km. For a profile 50 km north of Sumatra 350 and 450°C on the thrust are well below the forearc mantle intersection and the latter is inferred to apply. For two profiles through Sumatra, 350°C occurs on the thrust near the forearc Moho with the transition zone to 450°C extending to greater depths. Either the thermal or forearc Moho limits therefore may apply for this region. The maximum downdip limit of rupture in the two great earthquakes is not well determined, but most estimates are at a depth of ~30 km, which supports the idea that the intersection of the downgoing plate with the aseismic forearc Moho is governing the downdip limit of the seismogenic zone in northern Sumatra, ~ 160-200 km landward from the trench. The updip limits of rupture are predicted to be at the trench in the two northern profiles and about 15 to 20 km inland from the trench in the southernmost profile. These values fit well with results of tsunami models.