Validation of spatial autocorrelation (SPAC) method with L-shape array in Jyoso City, Japan

Abstract

This study is aimed to validate the efficiency of an L-shape array for SPAC method using microtremor in estimating the shear wave velocity (V,) structure. The experiment for validation was conducted in the Toyota Community Baseball Ground, Jyoso City, Ibaraki Prefecture, Japan in March 2009 with an equilateral triangle array with side length of 40 m and in June 2010 with an equilateral triangle array with side length of50 m, together with an L-shape array of the similar size. Multichannel Analysis of Surface Waves (MASW) was also performed simultaneously in June 20I 0. In the same lot PS-logging data are available from nearby LBRH IO station of the K.IK-NET (NTED, Japan) that shows soft sediment of about 20 m thick with V, of 110 m/s in the geological column of the site.

The comparison of the determined phase velocity and that calculated from PS logging data shows close matching of two sets of curves separately. One is between PS logging and the triangle array (40 m), and the other is between the triangle array (50 m) and the L-shape array (50 m). Former two are of the almost same place whereas other two arrays are deployed about 200 m away from the other set. Some discrepancy between two sets is shown. This seems due to lateral variation of underground velocity structure which is consistent with the result of MASW.

Based on the results of analysis one can say that a L-shape array can be applied to estimate shear wave velocity for shallow depth so it can be layout in urban areas to determine phase velocity information from microtremors. Therefore, it may be feasible to apply it in the Kathmandu Valley, Nepal that is based upon the soft soil with high possibility of liquefaction or earthquake hazard.