Long-period Ground Motions Recorded in the Tokyo Metropolitan Area during the 16 July, 2007 M6.8 Off Niigata-ken Chuetsu, Japan Earthquake

Abstract

Large and prolonged shaking with long-period ground motions having periods of about 7 sec were observed in central Tokyo during the Off Niigata-ken Chuetsu, Japan, M6.8 earthquake on 16 July, 2007. The observed ground motions from a dense nationwide strong motion network (KNET and KiK-net) demonstrate clearly that the long-period ground motions consist of Rayleigh waves, which developed at the northern edge of the Kanto Basin and were induced by conversion from the S waves radiating from the earthquake source. The amplitude and the duration of the long-period surface waves were enhanced dramatically as they propagated in the Kanto Basin, which has a thick cover of sedimentary rocks overlaying rigid bedrock. Observed ground motions of long-period signals at the center of Tokyo from the 2007 Off Niigata-ken Chuetsu event correlated well with observations from the Chuetsu earthquake on 23 Oct. 2004 (M 6.8). By analyzing waveform data from the main shock and aftershocks of the 2007 Off Niigata-ken Chuetsu earthquake and the Chuetsu earthquake in 2004, it is found that the long-period surface wave having a dominant period of about 7 sec at the center of Tokyo is developed efficiently by a large earthquake with a magnitude greater than about M6.5-7, but it is not developed by small earthquakes of less than about M6.5.