Re-examination of possible great interplate earthquake scenarios in the Nankai Trough, southwest Japan, based on recent findings and numerical simulations

Great earthquakes have occurred repeatedly along the Nankai Trough in southwestern Japan with recurrence intervals of 100-200 years. The magnitudes of Nankai earthquakes have historically varied, with many studies to date asserting that an event's magnitude is controlled only by the number of broken segments arranged along the trough. Recent evidence suggests that large tsunami with recurrence intervals of several hundred years have occurred along the Nankai Trough. Therefore, it is now essential to reconsider the scenario of the Nankai earthquakes, including the possibility of seismic slip on the shallow subduction interface. Recently, model calculations of interplate seismic cycles have been conducted using rate- and state-dependent friction. In these calculations, assuming heterogeneous distribution of fracture energy in the seismogenic zone and its shallower extension, both massive earthquakes with slips up to the trough and ordinary earthquakes in the seismogenic zone can occur. Here, we apply a heterogeneous distribution of fracture energy in the shallow plate interface of the Nankai Trough. In the model, we represent the difference in fracture energy by the difference in the characteristic slip distance L To model the conventional seismogenic zone as a zone of low fracture energy, we set L to be 5 cm at depths of 10-20 km. Conversely, the shallow plate boundary near the trough is modeled as a zone of high fracture energy by setting L to 5 m. As a result, two types of earthquake with different moment magnitude occur alternately. The recurrence interval of the larger type is about 370 years, which is comparable to the recurrence of larger tsunami deduced from geological findings. Large coseismic slip extends to the trough during the larger types. In contrast, the smaller one, whose seismic slip is distributed along the seismogenic zone, occurs after similar to 200 years of the larger one. (C) 2013 Elsevier B.V. All rights reserved.