利用大地测量资料反演地震震源参数的若干问题
SOME PROBLEMS IN INVERSION OF SEISMIC SOURCE PARAMETERS FROM GEODETIC DATA
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摘要: 本文利用大地测量资料反演地震震源参数的一般理论,归纳为震源模型的建立、最优化计算的实施和观测资料的使用等三部分.文中着重研究了后两部分:提出了利用原始观测资料(地面长度、角度、高差、倾斜、应变的变化值)直接进行反演的严密方法;改进了单纯形最优化计算方法,加速了迭代的收敛并给出了收敛准则;按逐渐趋近法进行观测资料的标准化,保证了标准化后的各类资料都属于同一正态分布.文中根据地震前、后的大地测量资料,对1976年唐山7.8级地震的震源参数进行了反演,并对成果的稳定性和可靠性进行了检验,得到地震断层长度为112公里,走向为北东56,倾角为90,断层破裂至地面,断层面延深(宽)15公里,水平错距(右旋)2.51米,垂直错距(东南盘下降)0.70米,断层迹线中点位置为北纬3936'6、东经11811'4.由此计算得到的地震矩为1.451027达因厘米,应力降为37.2巴,应变降为5.610-5.这个结果与根据地震波及震时其它地球物理资料求得的结果相近,因此认为本次地震前(或后),发生相当于主震规模的断层蠕动的可能性不大.Abstract: In this paper, the general theory of the source parameter inversion from geodetic data is considered to be consisted of modeling of the earthquake source, the realization of optimization and the utilization of the geodetic data. Mainly the latter two are studied in detail. A rigorous method of direct inversion from the original observation data (e.g. variations in length, angle, elevation difference, tilt and strain on the earth's surface) is put forward. The modified simple method accelerates the convergence of iteration. The convergence tests are given. The normalization of observation data according to sequential approximation guarantees that data of different observations are of the same normal distribution.The source parameters of the 1976 Tangshan earthquake are inversed from geodetic data. The stability and reliability of the results are tested. The results show right lateral slip of 2.51m and vertical slip of 0.70m (southeast side down) on a fault plane which dips 90. The strike, width and upper margin of the earthquake fault are N56E, 112 km, 15 km and 0 km respectively. The middle point position of the fault is N3936'.6, E11811',4. The seismic moment, stress drop and strain drop are 1.45 1027 dyne, cm, 37.2 bar and 5.6 10-5 respectively. This result is nearly the same as that deduced from seismic waves and other geophysical data observed during the earthquake. Therefore, it is concluded that there was rare possibility that fault creep, comparable with the faulting of the main shock, could have taken place both before and after the earthquake.