Effects of subsurface structures of source regions on long-period ground motions observed in the Tokyo Bay area, Japan

Abstract We compared the long-period ground motion observed in the Tokyo Bay area during two shallow M6.7 earthquakes that occurred in northern Nagano Prefecture, Japan, on March 12, 2011, and November 22, 2014. The magnitudes, focal depths, and source mechanisms of these events were almost identical, but their seismograms were quite different. Significant long-period later arrivals with a predominant period of 5 s were recognized in the velocity traces of the 2011 event, but there were no such remarkable later arrivals in the 2014 event traces. The ground motions at stations located outside the basin area were studied as incident waves to the Kanto Basin. A large wave packet with a predominant period of 5 s was recognized in the velocity traces of the 2011 event, but there was no significant wave packet in the 2014 event traces. Based on particle motion, this wave packet was hypothesized to be a Rayleigh wave. The source regions of the two events have quite different subsurface structures. The different characteristics in long-period ground motion in the Tokyo Bay area during the two events were due to different Rayleigh wave excitations in the source regions. Graphical abstract

The 2002 Seismic Refraction/Reflection Surveys in the Kozu-Matsuda Fault Zone and Ashigara Valley

We conducted seismic surveys in western Kanagawa prefecture including the Kozu-Matsuda fault zone and Ashigara valley, in February and March 2002. The reflection surveys were carried out using vibrator sources along 2 receiver lines. Line A is 15-km long and extends east to west across the Kozu-Matsuda fault zone and Ashigara valley. Line B is 10-km long and extends north to south in the valley. 1151 receivers were deployed at intervals of 25m for 210 vibration points. 3-D refraction surveys were also conducted with 4-dynamite shots at both sides of the lines A and B and repeated vibroseis sweeps were made at 3 points. In addition to the 1151 reflection receivers, 108 receivers were deployed along the 5 lines between the shot points, and 128 independent receivers were located over a wide area in the Ashigara valley. We overview the surveys and present datasets of seismograms and travel times