不同标度震级关系和台基影响问题探讨

利用近年中国数字地震台网观测报告中的中国及邻区地震目录资料,取能够同时计算面波震级MS和近震震级ML的地震,拟合出2种震级之间的关系,发现与过去使用30余年的换算公式存在系统差别。考虑现代化地震数字观测系统有动态范围大、频带宽、台站多、覆盖区域广、测定精度高的特点,由此拟合的关系应当更可靠。考虑不同误差因素统计面波震级MS和近震震级ML的关系表明,2种震级之间换算和不换算的差别并不大,因此建议MS和ML震级之间不换算为宜。分析地震台站的场地响应发现:一些基岩台的放大因子在1附近,场地响应不随频率变化;处于沉积土层的台站,放大因子在低频段大于1,高频段小于1;还有一些台站在某个频段范围大于1,或者小于1。另外,沉积土层台站的单台震级和台网平均震级差也随周期增大,呈现由负到正的增长趋势。看来并不存在固定的台站校正系数,因此不能把台站校正作为提高震级测定精度的方法。

Discussion on Relation Between Different Earthquake Magnitude Scales and Effect of Seismic Station Site on Magnitude Estimation

Based on the earthquake catalog reported by the Chinese digital seismic network in recent years,we select the earthquakes with both of surface wave magnitude and local magnitude and fitting them into the relation between two magnitudes. The systematic difference is found from the formula that has been used for 30 years. Because of large dynamic range and wide frequency range of current digital observation system,in addition to larger number of stations and earthquakes used than before,the relation obtained seems more reliable. Our calculation shows that there is no significant difference before and after magnitude conversion,the magnitude conversion is suggested to be given up. The site response of a station consists of the amplification at different frequencies. The amplifications are about 1 and change little with frequency at stations located on basement rock,the amplifications are greater than 1 on low frequency range and less than 1 on high frequency range at stations located on sediment layer,the difference between magnitude from single station located on sediment layer and the average magnitude from whole network increases from negative to positive with period. It seems that there is no fixed station correction factor and station correction method does not work for improvement of magnitude accuracy.