Scaling of source spectra based on radiated energy and its application to strong motion prediction

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Strong motion records at K-NET and KiK-net stations are analyzed to examine relationship between short-period source spectral amplitude and seismic moment. Earthquakes dealt with in this study are those occurred in Northwestern Kagoshima region, Western Tottori region, Northern Miyagi region, and Niigata Chuetsu region. Moment magnitude of these events are from 3.3 to 6.6. Epicentral distance to the stations is less than 60 km.Spectral ratio between transverse component aaelerograms of the largest event and each of the other event in each group are calculated at each station in order to remove the effects of wave propagation path to the stations. Then the spectral ratios at all stations are averaged to remove the effect of difference in focal mechanism. Thus, the spectral ratio of the records represents the source spectral ratio.The similarity of earthquake expects that the short-period spectral ratio is proportional to the cubic root of the ratio of the seismic moment. However, the observed short-period spectral ratio is generally larger than the expectation from the similarity. Larger earthquakes radiate short-period seismic energy more effectively than smaller ones.Appropriate correction for the deviation from the similarity of earthquake should be made when we aim at simulation of strong ground motion for future large earthquakes by the empirical Green's function method. If we assume the similarity of earthquake, the resultant short period seismic wave for large earthquakes would be underestimated. A correction factor is proposed in terms of the difference in seismic moment between a target large earthquake and a small earthquake whose records are adopted as empirical Green's functions.