Risk of Long-period Ground Motion due to the Deep Basement Structure of the Kanto Basin

In the Kanto basin, long-period (T=6-8 s) ground motions develop during large (M>7) earthquakes due to the resonance of surface waves with thick (>3000 m) sedimentary layers covering rigid bedrock. Such long-period ground motions can cause significant damage to modern large-scale, long natural-period structures such as skyscrapers, oil storage tanks, and long bridges. Therefore, it is important that residents of modern Tokyo metropolitan area cities be forewarned about the potential for such disasters. In the present paper, the development of large and long-duration, long-period ground motions in central Tokyo is investigated by analyzing waveform data from recent large earthquakes obtained from dense seismic networks extending over the Kanto basin. It is demonstrated that the interaction of surface waves with the three-dimensional sedimentary structure beneath the Kanto region causes peculiar and directional dependencies related to the amplification strength of the long-period ground motions. For example, the long-period ground motions resulting from earthquakes occurring in northern Japan cannot develop efficiently in central Tokyo. This might be one reason for the relatively weak long-period ground motions observed in central Tokyo during the 2011 Off Tohoku M9.0 earthquake compared to those of other M7-8 class events, such as the 1944 Tonankai (M7.9) and the 2003 Off Tokachi (M8.0) earthquakes. The results of computer simulations using detailed subsurface structures and source-slip models for an anticipated Nankai Trough M8.7 earthquake indicates that the strength of long-period ground motions in central Tokyo can be expected to be at least double than those observed during the 2011 Off Tohoku earthquake.