Method

Local geologic and ground conditions play important roles in characterizing and estimating the site amplification capability of strong ground motion and geotechnical hazards. Accurate evaluation of site amplification characteristics requires detailed soil profile information such as data on shear-wave velocity structures. However, high-densed boring or PS-logging data throughout Japan has not been available. It has been noted that approximate estimation of site amplification is possible with the average shear-wave velocity of ground in the upper 30 meters, Vs30. For a wider area, mapping Vs30s can be estimated from surface geology and/or geomorphologic conditions digitized in Japan Engineering Geomorphologic Classification Map (Fig. 1). The combined usage of the GIS-based amplification factor derived from Vs30 map and observed seismic records during an earthquake can realize seamless and detailed estimation of strong ground motion map.

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Fig. 1 Japan Engineering Geomophologic Classification and Average Shear-wave Velocity (Vs30) Maps

Data flow and spatial interpolation procedure are shown in Fig. 2. First, downloading the observation records at seismic stations from the portal site of National Research Institute for Earth Science and Disaster Prevention (NIED), intensity values (peak ground velocity (PGV) and instrumental seismic intensity) at the stations are calculated. Since the earthquake motion on the ground surface is affected by the amplification characteristics of surface layers, the interpolation should be carried out at the base rock (outcrop) level. Next, the amplification factor is used to convert the intensity values recorded on the ground surface to those at the base rock. After completing the spatial interpolation by IDW (Inverse Distance Weighted) for QuickMap or by Simple Kriging with considering attenuation characteristics from the seismic source for QuakeMap, finally, the spatial distribution of the ground motion intensity values on the ground surface are obtained by multiplying the corresponding GIS-based amplification factor for each grid cell.

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Fig. 2 System Overview and Calculation Procedure

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