Double-difference relocation of the 7 September 2012 Yiliang earthquake and its aftershock sequence
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摘要: 本文提出了时域多通道相关检测函数并用其计算波形互相关走时差数据,采用双差定位法对2012年9月7日云南彝良地震和余震序列共944个地震进行重定位,得到652个重定位事件,并与目录数据的结果进行了对比.本文采用了多个准则对走时差数据进行筛选,确保定位结果稳定可靠.得到MS5.7主震的震中为27.516°N,103.951°E,震源深度6.9 km;MS5.6主震的震中为27.543°N,104.023°E,震源深度7.27 km;重定位结果显示,地震序列紧缩为条带状并沿附近断裂走向分布,深度总体分布较重定位前变浅,集中分布在5~8 km,地震群出现轻微倾斜.东西向、南北向、深度和发震时刻的平均相对误差分别为55.2 m,43.0 m,186.7 m和0.01 s,走时残差16ms.研究表明:互相关数据的结果要优于目录数据;震源深度与速度模型存在较大的相关性;确定彝良—会泽断裂为本次彝良地震序列的发震构造.Abstract: The object of this paper is to compare the relocation results between the catalog data and the cross-correlation data of the Yiliang earthquake sequence occurred in September 2011, then discuss the impact on the location of different velocity models and confirm the seismogenic fault structures.We adopted double difference algorithm for earthquake hypocenter relocation by using catalog data and cross-correlation data respectively. We proposed the time domain multi-channel correlation detection function and used it to calculate waveform cross-correlation travel time difference data. In general, the number of the waveform cross-correlation data is less than the catalog data, so the number of relocation earthquakes is small. In order to overcome this shortcoming, we used the catalog data to supplement the waveform cross-correlation data and set the weight of catalog data less than the waveform cross-correlation data. To ensure that the relocation results were stable and reliable, we used multiple criteria to filter the travel time difference data. The proportion of two kinds of data was about 17.5:1 in our cross-correlation data used in relocation. In order to find the most suitable velocity model for the regional crust structure and ensure the relocation results were reliable, we used four velocity models and chose the best results.Among the 944 ML ≥1.0 earthquakes of Yiliang earthquake sequence from September 7 to October 7, 2012, we obtained 652 relocation events. The location of MS5.7 main earthquake was 27.516°N, 103.951°E, and 6.9 km in the depth. The location of MS5.6 main earthquake was 27.543°N, 104.023°E, and 7.27 km in the depth. The Yiliang earthquake sequence was located between Zhaotong-Ludian fault and Yiliang-Huize fault which were both NE trending, and the two main earthquakes occurred at opposite ends of the sequence. Relocation results show that the earthquake sequence was more concentrated along the nearby fault, the seismic strip extended for less than 20 km, the depth distribution became shallower than before and was mainly located between 5 km and 8 km depth and the cluster was slightly tilted. The average relative errors of east-west direction, south-north direction, depth and origin time ware 55.2 m, 43.0 m, 186.7 m and 0.01 s, the travel time residual was 16 ms.We used bootstrap method to evaluate the location errors and the correlations of the four source parameters. The results show that the location errors of the results of cross-correlation data were smaller than catalog data and the location accuracy was more than an order of magnitude higher compared to catalog locations. The cross-correlation data can also reduce the correlations of the source parameters. The location results of different velocity models show that there were significant correlations between focal depth and velocity model and we should choose the velocity model carefully. Based on the distribution of the earthquake sequence and regional modern tectonic stress field, we confirmed that Yiliang-Huize fault was the seismogenic structure of Yiliang earthquake sequence.
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