Comparison of the coseismic rupture with the aftershock distribution in the Hyuga‐nada Earthquakes of 1996

Abstract

On October 19, 1996, a large underthrusting earthquake (Ms = 6.7), the Hyuga‐nada, Japan, earthquake occurred along the southern end of Nankai trough. About two months later, a second large earthquake (Ms = 6.7) occurred in the adjacent region. We study the source process of the two large earthquakes in the Hyuga‐nada region and compare the coseismic rupture area with aftershock distribution. The main source parameters obtained for the first mainshock are: (strike, dip, rake)= (210°, 12°, 81°); the seismic moment Mo = 2.3×10 19 Nm (Mw = 6.8); the rupture area S = 20 × 15 km², and the source duration T = 17 s. For the second main‐shock, (strike, dip, rake) = (210°, 12°, 87°); the seismic moment Mo = 1.5×10 19 Nm (Mw = 6.7); the rupture area S = 18 × 18 km², and the source duration T = 15 s. The coseismic rupture areas do not overlap the aftershock area, while the aftershock areas of the two main‐shocks mutually overlap. This implies that the common aftershock region takes a role of barriers to dynamic rupture. It is also seen that the aftershock area expanded during the first one day.