Subsurface-Soil Thickness in Ikonobe Area, Yokohama, Evaluated from Microtremor Observation

Abstract

High-densed microtremor measurements are made in Ikonobe area, Yokohama, in which a differential settlement between buildings occurred in 2014 and wooden houses were severely damaged during the 1923 Kanto earthquake. An empirical relationship between the predominant period of H/V spectrum (Tp) and subsurface-soil thickness (D) is obtained from microtremor measurements conducted at bore-hole sites. The estimated D is averagely 10-15m with the maximum depth of 20 m in the northern edge of the lowland along the northern plateau. The differential settlement spot in 2014 is located in the drastically D-changed area. Strong-motion spectral ratio (As) is estimated from the peak value of H/V spectrum (Ap). It is found that As median of Kouchi and Kawamukoh hamlets on Holocene deposits with wooden-house collapsed ratio over 30% is larger than 5 whereas that of Yabune hamlet mainly on Pleistocene deposits with the collapsed ratio lower than 10% is smaller than 3 suggesting that ground-motion amplification of soft subsurface soil deposits worsen the housing damage. However, the cause of the especially-high damage ratio of Kouchi hamlet cannot be explained in terms of ground-motion amplification.