Azimuthal dependence on amplifications for long-period ground motions propagating in the Kanto Basin, Japan

In the Kanto Basin, Japan, it has been reported that the dominant periods of long-period ground motions vary depending on the areas where earthquakes occurred. This suggests that the characteristics of long-period ground motions are closely related to their propagation paths in the sedimentary layers. We, firstly, investigated the azimuthal dependence of amplifications for long-period ground motions based on the spectral ratios of ground motions on the surface to those on the basement, using both the observation data and the simulation data in the Kanto Basin. In both the data for periods of more than 3 s, the seismic stations where the dominant periods vary for individual earthquakes are concentrated in the central part of the Tokyo Metropolitan Area. The azimuthal dependence on amplifications for long-period ground motions suggests that the wave propagation features of long-period ground motions are related to the laterally complex three-dimensional subsurface structures along the propagation paths between stations and epicenters. We, secondly, investigated the amplification characteristics for body and surface waves, to evaluate their contributions to earthquake ground motions for periods of more than 1 s observed in the basin. It is found that body and surface waves tend to dominate at periods of less than 2 s and more than 3 s, respectively. We, finally, understood the amplification characteristics of body and surface waves including the dominant periods and those amplification factors, to appropriately predict broadband earthquake ground motions. Highlights The characteristics of long-period ground motions including the dominant periods and those amplification factors were quantitatively investigated based on the spectral ratio by seismic data observed on the surface and the bottoms of the deep boreholes. The azimuthal dependence of amplifications for long-period ground motions propagating in the Kanto Basin was demonstrated by analyzing both the observation data and the simulation data. The dominant periods of the spectral ratios vary for individual earthquakes in the central part of the Tokyo Metropolitan Area. This suggests that the wave propagation features of the long-period ground motions depend on the laterally complex subsurface structures in the Kanto Basin along the propagation paths between stations and epicenters. The amplification characteristics for body and surface waves for periods of more than 1 s observed were investigated to evalua