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

分析了K-NET和KiK-net台站强震记录，探讨了短周期震源谱幅值与地震矩的关系。本研究处理的地震是发生在鹿儿岛西北部、鸟取县西部、宫城县北部和新泻中越地区的地震。这些地震的矩震级在3.3到6.6之间。震中距离监测站不到60公里。为了消除波传播路径对台站的影响，在每个台站计算每组中最大事件与其他事件的横向分量加速度图的谱比。然后对各站的光谱比进行平均，以消除震源机制差异的影响。因此，记录的谱比代表源谱比。地震的相似性要求短周期谱比与地震矩比的立方根成正比。然而，观测到的短周期谱比通常大于相似度的期望。大地震比小地震更有效地辐射短周期地震能量。在用经验格林函数法模拟未来大地震的强地震动时，应适当地修正与地震相似度的偏差。如果我们假定地震的相似性，那么大地震产生的短周期地震波就会被低估。根据目标大地震和小地震的地震矩差，提出了一种校正因子，小地震记录作为经验格林函数。

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.