Earth Planets Space, Vol. 56 (No. 1), pp. 3-14, 2004
Takao Kagawa1, Kojiro Irikura2, and Paul G. Somerville3
1Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka 550-0012, Japan
2Disaster Prevention Research Institute, Kyoto University, Japan
3URS Corporation, USA
(Received July 22, 2003; Revised December 19, 2003; Accepted December 26, 2003)
We have studied differences in ground motion and fault rupture characteristics between surface rupture and buried rupture earthquakes. We found that the ground motion generated by buried rupture in the period range around 1 second is on average 1.5 times larger than the average empirical relationship. In contrast, ground motion from earthquakes that rupture the surface is 1.5 times smaller in the same period range. This phenomenon is considered to be caused by differences in fault rupture process between the two types of earthquakes. To examine possible reasons of the above effect we analyzed source slip distribution data derived from waveform inversions, and divided them into two groups: surface rupture and buried rupture earthquakes. It was found that the large slip areas (asperities) of surface rupture earthquakes are concentrated in the depth range shallower than about 5 km. In contrast, large slip areas of buried rupture earthquakes are spread over the depth deeper than 5 km. We also found that the total rupture area of buried rupture earthquakes is 1.5 times smaller than that of surface rupture earthquakes having the same seismic moment, and that deep asperities have about 3 times larger effective stress drops and 2 times higher slip velocities than shallow asperities. These observations are verified by numerical simulations using stochastic Green's function method.
Key words: Strong ground motion, surface rupture fault, buried rupture fault, rupture process, asperity, stress drop, slip velocity.