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

分析了K-NET和KiK-NET台站的强震记录，研究了短周期震源谱振幅与地震矩的关系。本研究所涉及的地震是发生在鹿儿岛西北地区、鸟取西部地区、北方宫城地区和新泻中越地区。这些事件的矩震级为3.3 - 6.6。震中距台站小于60 km，为了消除波传播路径对台站的影响，计算了各组中最大事件与其它事件的横向分量速度图的谱比。然后对各台站的谱比进行平均，以消除震源机制差异的影响。因此，记录的谱比代表震源谱比，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性的预期。大地震比小地震更有效地辐射短周期地震能量，用经验绿色函数方法模拟未来大地震的强地面运动时，应适当修正对地震相似性的偏离。如果我们假定地震的相似性，合成的短周期地震波的大地震将被低估。提出了一个以目标大震与小震记录为经验绿色函数的地震矩差作为修正因子的方法。

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.