Ground motion characteristics and the depth of the S-wave velocity discontinuity in the post-LGM incised-valley fills beneath the Tokyo Bay area, central Japan, based on microtremor surveys

Abstract

The average S-wave velocity and peak frequency of the microtremor H/V spectrum, or the resonance frequency of the ground, were estimated at 16 sites in the Tokyo Lowland along the coast of inner part of Tokyo Bay, central Japan. The results indicate that the resonance frequencies become lower with an increase in thickness of the post-Last Glacial Maximum (LGM) deposits. The post-LGM deposits are divided into the lower Nanagochi Formation and the upper Yurakucho Formation in the Tokyo Lowland. The depth of the S-wave velocity discontinuity, which determines the resonance frequency of the ground, was calculated by applying the quarter-wavelength law. At 7 of the 13 microtremor observation points examined (i.e., 3 points on a buried flat surface are excluded from the examination), the S-wave velocity discontinuity is located within alternating layers of sand and mud in the Nanagochi Formation, even though the boundary between the Nanagochi and Yurakucho formations has conven-tionally been considered the most significant boundary in terms of physical properties in the post-LGM deposits. This discrepancy is perhaps caused by the strong influence of either the lithologic bound aries in the alternatin layers of sand and mud in the upper part of the Nanagochi Formation, or the more conspicuous boundary at the top of the basal gravels of the Nanagochi Formation, or both. The variations in the peak frequency of the H/V spectrum are caused by the heterogeneity of meandering river and estuary deposits in the Formation.