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

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.[graphic not available: see fulltext]