Abstract
In order to clarify the factors of strong ground motions at the KiK-net Mashiki station which generated a large acceleration of more than 1G caused by the largest foreshock (Mj6.5) of the 2016 Kumamoto earthquake, we performed ground identification analysis and one-dimensional wave propagation analysis using the vertical array observation records and analyzed the strong motions in detail. In addition, we estimated the bedrock (S wave velocity 2700m/s) ground motions of the largest foreshock and evaluated their acceleration levels. As a result, the ground motions have a significantly larger amplification at higher frequencies than about 2 - 3Hz. And the effect of large site amplification of the ground shallower than the seismic bedrock of approximately depth of 250m has been shown as one of the factors of strong ground motions. On the other hand, seeing that a short-period spectral level of earthquake has an average value of inland crustal earthquake, and the estimated bedrock spectra can be mostly described from the theoretical bedrock spectra at the seismic bedrock considering the seismic source and propagation characteristics based on the omega-square model of earthquake, we showed the largest foreshock is not an abnormal earthquake from the viewpoint of earthquake magnitude and ground motion level. However, because the bedrock spectra has a slightly higher value than the theoretical bedrock spectra at approximately 0.4 - 4Hz, near-fault rupture directivity effect due to the fault rupture propagation has been shown in the strong ground motions at the KiK-net Mashiki station. It has been found that the large acceleration amplitude in the ground surface of the KiK-net Mashiki station by the largest foreshock was caused by large site amplification of shallow ground and near-fault rupture directivity effect.
