Effects of seamounts on an interplate earthquake at the Suruga trough, Japan

Detailed magnetic surveys carried out in the Suruga trough area, where the Philippine Sea plate subducts beneath Japan, revealed some local magnetic anomalies on the overriding plate side. We first show that these anomalies reflect seamounts sitting on and subducting with the Philippine Sea plate, since no magnetic sources corresponding to the anomalies are identified in the seafloor topography. An important issue to be addressed is how these seamounts affect the generation of a great interplate earthquake at the Suruga trough (often referred to as the Tokai earthquake). So we made a numerical simulation of earthquake generation, applying the rate- and state-variable friction law to the plate interface. Here we incorporated the seamounts in the simulation as an additional normal stress exerted locally to the interface. We then found how the seamounts affect the generation of an interplate earthquake depends on whether or not they are located in the seismogenic zone. The most notable effect is that the amount of slip immediately before the main shock, often called a pre-slip, tends to be enhanced if they are located inside the zone.