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地震矩和相应的矩震级Mw一般由远场体波频谱得出。近场记录,特别是大地震的近场记录,一般不用来获得地震矩和相应的矩震级Mw,原因在于不同类型的波同时到达,无法定义地震矩与频谱之间的简单关系。我们给出了一种新方法,可以通过近场记录的位移谱测定矩震级Mw。把实际记录到的低频谱振幅与用Mw来衡量运动破裂模型算出的合成谱振幅进行了比较。在不同的断层方位上计算出了震中距在1～100km范围内的合成记录和平均值。频谱的初始部分受到加速度记录中基线变化的影响,用各台站的截止频率,通过高通滤波器可以自动识别和消除这种影响。合成谱值、震中距和只经过一次计算并存入表格中的滤波器是矩震级的函数。实际记录的谱振幅被简单地插入合成数据的表格中,这样可以快速测定Mw。用震级在3.9～7.7之间的22个浅源地震对这种方法进行了验证。我们给出,地震发震时刻后80s的信号窗对所考虑的整个震级范围提供了可信的Mw值。也可以用更短的窗长,但会低估大地震的矩震级Mw。这种方法非常适用于近实时的快速测定Mw。
The seismic moment and the corresponding moment magnitude Mw are generally derived from the far-field bulk wave spectrum. Near-field records, especially near-field recording of large earthquakes, are generally not used to obtain seismic moment and corresponding moment magnitude Mw because different types of waves arrive at the same time and can not define a simple relationship between seismic moment and frequency spectrum. We present a new method to determine the moment magnitude Mw by the displacement spectrum of near-field recording. The actually recorded low-frequency amplitudes were compared with the amplitude of the resultant spectrum calculated from the model of the rupture using the Mw. Synthetic records and averages of epicentral distances within the range of 1 to 100 km were calculated for different fault orientations. The initial part of the spectrum is affected by changes in the baseline in the acceleration record, which is automatically identified and eliminated by the high-pass filter using the cut-off frequency of each station. The resultant spectral values, epicentral distances, and filters that are calculated once and stored in the table are a function of moment magnitude. The actual recorded spectral amplitudes are simply inserted into the table of synthetic data, which allows for rapid determination of Mw. This method was validated by 22 shallow earthquakes with magnitudes between 3.9 and 7.7. We show that the signal window 80s after the earthquake onset provides a credible Mw value for the entire magnitude range considered. Shorter window lengths can also be used, but the moment magnitude Mw of large earthquakes can be underestimated. This method is very suitable for near real-time rapid determination of Mw.