Link Between Geometrical and Physical Property Changes Along Nankai Trough With Slow Earthquake Activity Revealed by Dense Reflection Survey

We examined the possible factors affecting the spatial distribution of very low frequency earthquakes and tremors in the shallow megathrust of Nankai Trough (<30 km) using a dense network of prestack depth migrated profiles at the frontal wedge. Geometrical parameters examined were decollement roughness, taper angle, and underthrust thickness. Physical properties such as effective basal friction (mu(b)) and pore pressure ratio (lambda*) were calculated from the taper angle and p-wave velocity. Regions of low lambda* (0.39 +/- 0.08) and smooth decollement showed no slow earthquake activity. In contrast, high activity of slow earthquakes was observed in areas with a rough decollement due to the presence of subducted seamounts or bathymetric highs. The low taper angle (3.8 degrees) off Muroto where slow earthquakes also occur translates to a wide zone of low mu(b) (0.21 +/- 0.06) and high lambda* (0.66 +/- 0.06). However, our results also show that slow earthquakes don't always occur in areas with high lambda*.