A study on the predominant period of long-period ground motions in the Kanto Basin, Japan

Abstract

We analyzed seismic waveforms of moderate to large shallow earthquakes recorded in the Kanto Basin, Japan, to clarify the characteristics of long-period ground motions in deep sedimentary basins. Our analysis revealed a tendency of long-period ground motions to develop in limited periods up to around 7 s. In addition, we found a specific relation between the predominant period of long-period ground motions and the depth of bedrock. The predominant period tends to increase in proportion to bedrock depth until it reaches approximately 7 s; however, it becomes almost constant for bedrock depths greater than 2 km. These observations, in conjunction with numerical experiments performed in this study using realistic sedimentary basin structure models, indicate that the characteristic of long-period ground motions, excited by the basin-induced fundamental Love waves, is controlled strongly by the shallow sedimentary structure, if the bedrock depth exceeds approximately 2 km. This result might also be important in understanding and evaluating the characteristics of long-period ground motions in other deep sedimentary basins of the world for moderate to large shallow earthquakes.