利用数字地震记录的,振幅比资料测定小震震源机制解的可靠性分析

RELIABILITY ANALYSIS OF FOCAL MECHANISM SOLUTIONS OF MICRO-EARTHQUAKES DETERMINED FROM AMPLITUDE RATIO OF  AND  RECORDED BY DIGITAL SEISMOGRAPH

震源机制解的可靠性越来越受到人们的重视 ,已有的检验工作证实中国早期或者手工作图测定的震源机制解可靠性极差。经比较 ,由远震波形反演的矩张量解 ,同一地震不同作者的结果之间差异甚小 ,可靠性较高 ;而基于P波初动方向测定震源机制解的格点尝试法所给出的解空间比其他方法要好。对 2 0 0 3年 4月发生在首都圈数字化地震台网范围内的 2次 4级地震进行了分析。取用不同的资料 ,由格点尝试法和 -P ,-S垂直向振幅比方法分别测定了震源机制解 ,二者基本一致 ,并且和过去当地发生的 5级以上地震的哈佛大学CMT解接近。这既说明了地震活动的继承性 ,也证实了震源机制解并非随机分布。由于中小地震发生的随机性 ,用统计的方法对用振幅比资料测定的震源机制解进行分析 ,结果是客观。

With the extensive application of focal mechanism solutions of micro-earthquakes,more and more at-tentions have been paid to the reliability of focal mechanism solution itself.The application of focal mecha-nism solution has been a significant progress in seismology,and it has played a non-fungible role in the recognition of seismic structure and seismogenesis.However,from the very beginning of the method devel-opment,the error has existed in the solution.In recent years,many methods and techniques for deriving focal mechanism solutions have been developed.Therefore,the analyses on the accuracy and reliability of these methods should also be made. The Capital-circle Digital Seismic Network was put into operation in2002.In the network,107seis-mographs may simultaneously record and transmit seismic data,and among them some are broadband seis-mograph with large dynamic range and some are short-period seismographs with greatly improved dynamic range.About half of the stations in the network had recorded clearly the two M=4earthquakes occurred successively in Ninghe,Tangshan seismic area in April,2003,providing an opportunity to make the relia-bility analysis. For the M4.1earthquake occurred on April23,2003,48distinct and identifiable initial motion sig-nals were selected from the P-wave initial motions recorded by stations with epicentral distance of less than320km among the107stations.For the data recorded at12stations with epicentral distance of less than43km,the emulation was made to obtain the record of displacement,and then the maximum amplitudes of the vertical-P and-S were read.The focal mechanism solution was then calculated using the grid-point trial method and the amplitude ratio method,respectively.The results indicate that this earthquake can be assigned to the strike-slip type with an upright nodal plane.The distribution of P-wave initial motion sig-nals on the spherical surface used by the grid-point trial method is rather homogeneous and the nodal line can be better controlled,but the initial motion signals are a little bit deviated from the center of the net.The inconsistent ratio of the average focal mechanism solutions is0.208,the minimum inconsistent ratio is0.188and the cut-off solutions are60.The discrete area of the focal mechanism solutions is relatively small,so the results are reliable.The focal mechanism solution obtained by the amplitude ratio method is quite similar to the average focal mechanism solution obtained by the grid-point trial method,and the3stress axes are exactly located within the discrete area of the corresponding solutions.It can be seen that the solutions obtained by the two methods with different data coincide very well with each other,indicating that both methods are reliable.For further verification,the CMT solution of Harvard University for the largest earthquake occurred in this region in1976is used for comparison.The result shows that the differ-ence of the solution with the solution of this moderate earthquake is very small.Therefore,it not only indi-cates the inheritance of the seismic activity,but also proves that the focal mechanism solutions are not ran-domly distributed. For the M4.3earthquake occurred on April24,2003,55distinct and identifiable initial motion sig-nals recorded at stations with epicentral distance of less than320km were selected.And the maximum dis-placement amplitudes for the vertical-P and-S were obtained from the emulation of the data recorded at11stations with epicentral distance of less than52km.The focal mechanism solution was calculated respec-tively by the above-mentioned two methods.The result shows that the focal mechanism solution of this event is of the dip-slip type.The inconsistent ratio of focal mechanism solution obtained by the grid-point trial method is0.182,the minimum inconsistent ratio is0.127and the cut-off solutions are15.The dis-crete area of the focal mechanism solutions is relatively small,so the results are reliable.The focal mecha-nism solution obtained by the amplitude ratio method is quite similar to the average f