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根据巧家、石棉的小地震的观测资料,指出P波初动半周期在震级比较小时几乎是恒定的,在震级比较大时随震级的增大而增大;并指出P波初动振幅的对数也随震级的增大而增大。以圆盘形均匀位错面作为中、小地震震源的理论模式,计算了它所辐射的地震波远场位移,从而导出了体波初动半周期及振幅与震源尺度及波速等物理量的定量关系,解释了P波初动半周期及振幅与震级之间的经验关系。考虑到波在介质中的衰减和频散、地表面的影响以及地震仪器的频率特性,通过褶积方法合成了上述位错源产生的理论地震图,提出了直接由实际地震图上的初动半周期及振幅测定震源尺度、地震矩、应力降和错距等震源参数以及介质的晶质因数Q的方法.运用上述方法,测定了巧家、石棉两地区介质的品质因数Q和小震的震源参数.这两个地区介质的品质因数Q分别为620和560.石棉地区小地震的应力降大约在2—30巴之间,巧家地区小地震的应力降比较接近,平均约1.4巴.将这个结果和1962年3月19日新丰江地震与1975年2月4日海城地震的前、主震的应力降作对比,我们看到,巧家地区小震的应力降的特征与上述两次大地震的前震的应力降的特征是类似的,因此不能排斥巧家地区的小震是一个较大地震的前震的可能性.以测得的小震应力降的平均值(约1.4巴)作为这个可能发生的较大地震应力降的下限估计值,从主震震级和主震应力降的经验关系可以推知,其震级的下限是5.2级.
According to the observation data of Qiaojia and asbestos small earthquakes, it is pointed out that the initial half-period of P wave is almost constant at relatively small magnitude and increases with the magnitude when the magnitude is relatively large. It is also pointed out that the initial amplitude of P wave The logarithm also increases with magnitude. Taking the disc-shaped uniform dislocation surface as the theoretical mode of moderate and small earthquakes, the far-field displacement of the seismic wave radiated by it is calculated, and the quantitative relationship between the initial half-period of body wave and the physical quantities such as the source scale and the wave velocity is deduced , Explaining the first half cycle of P wave and the empirical relationship between amplitude and magnitude. Taking into account the attenuation and dispersion of waves in the medium, the influence of the ground surface and the frequency characteristics of seismic instruments, the theoretical seismograms generated by the above-mentioned dislocation sources are synthesized by the convolution method. Half cycle and amplitude, source parameters such as seismic source scale, seismic moment, stress drop and offset distance, and the crystal quality factor Q. Using the above method, the quality factor Q and medium earthquakes of small earthquakes The source parameters are Q and Q. The quality factors Q in these two regions are respectively 620 and 560. The stress drop of small earthquakes in asbestos region is about 2-30 bar, and the stress drop of small earthquakes in Qiaojia region is relatively close, with an average of 1.4 bar. Comparing this result with the stress drop of the former and main shocks of March 19, 1962, Xinfengjiang and Haicheng, February 4, 1975, we can see that the characteristics of the stress drop of small earthquakes in Qiaojia are similar to The characteristics of the stress drop of the foreshocks of the two major earthquakes are similar and therefore can not exclude the possibility that the small earthquakes in the Qiaojia area are the precursors of a larger earthquake.According to the average value of the measured small earthquakes drop As this may happen The lower bound of the large earthquake stress drop is estimated from the empirical relationship between the magnitude of the main shock and the stress drop of the main shock. The lower limit of the magnitude is 5.2.