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提出一种高频震级标度(m): m=2log■_(kf)+3式中,■_(kf)是在震源或距断层10km处加速度傅里叶振幅谱的高频水平,单位是cm/s(平均的或其中任意一个水平分量)。可以从仪器资料或地震的有感面积来测定m。在北美东部和加州,我们规定对应于“平均”应力降的地震,m=M(矩震级)。如果M也是已知的,则m提供应力降的测量。观测的m与M之间的关系表明,对于北美东部地区地震,平均应力降大约是150bar;对于加州地区地震,平均应力降大约是70bar。北美东部地区应力降的变化又比加州地区的大得多。所提出标度的主要理由是它可用来解释有仪器记录的以前的大地震,而这些地震对北美东部地震危险性估计是十分重要的。对于这样的地震,m可以比M或m_N(Nuttli震级)的测定更为可靠,并且形成测定高频地面运动的更好基础。当将m和M作为一对使用时,则可提供为覆盖整个工程频带的地面运动的一个好的指标。如果能够给出一个地震的两种震级,那么,就可以用一个地面运动模型,如随机模型,获得可靠的反应谱和地面运动峰值。
(M): m = 2log ■ _ (kf) +3 where, ■ _ (kf) is the high-frequency level of the acceleration Fourier spectrum at 10 km from the source or from the fault, in units of Is cm / s (average or any of the horizontal components). M can be determined from the area of the instrument or the area of the earthquakes. In eastern North America and California, we specify an earthquake corresponding to the “average” stress drop, m = M (moment magnitude). If M is also known, then m provides a measure of the stress drop. The observed relationship between m and M shows that the average stress drop for earthquakes in the eastern North America is about 150 bar; for the California earthquake, the average stress drop is about 70 bar. The stress drop in eastern North America is much larger than in California. The main reason for the proposed scale is that it can be used to explain previous large earthquakes recorded by instruments that are important for seismic risk estimation in the eastern part of North America. For such earthquakes, m can be more reliable than the determination of M or m_N (Nuttli magnitude) and form a better basis for the determination of high frequency ground motion. When m and M are used as a pair, then a good indicator of ground motion covering the entire engineering frequency band can be provided. If two magnitudes of an earthquake can be given, then a ground motion model, such as a stochastic model, can be used to obtain a reliable response spectrum and peak ground motion.