Wave Field Estimated by Borehole Recordings in the Reclaimed Zone of Tokyo Bay

Abstract

We estimate the wave field in the reclaimed zone of Tokyo Bay by analyzing the three components of surface and downhole strong-motion accelerograms recorded at two borehole stations during five medium earthquakes. The target period range is between 0.1 and 2.0 sec. First, wave types of observed seismic motions are identified by calculating theoretical amplification of body waves and surface waves and cross-correlation functions between the surface and downhole recordings. Then, the horizontal components of the surface and downhole recordings are separated into S -wave and surface-wave accelerograms. Finally, 2D wave-propagation simulations for basin structures are performed to interpret the recordings. The conclusions are as follows. (1) Main seismic motions are composed of not only S waves, but also Love waves. Surface waves are found to be more dominant than S waves in the period range of more than 1.0 sec. (2) Love waves are more heavily amplified than S waves at the theoretical predominant periods of ground in the soft surficial deposits. (3) The large surface-wave amplification is excited by the surficial deposits, together with a deep basin. (4) In the surficial deposits, quality factors are found to be about 10 around the theoretical predominant periods of ground. (5) The wave-propagation simulations suggest strong heterogeneities near the ground surface and can explain observed seismic motions qualitatively.