1996年四川田湾地震序列揭示的水库诱震结构
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摘要
１９９６年３月２８日田湾地震序列是四川二滩水库地震遥测台网自１９９２年１月正式运行以来，发生在库区的规模最大的一次地震序列，主震震级ＭＬ＝４．０，持续时间近２月，后续余震３３个，其中震级在３．０级以上的余震３个。因该序列地震位于二滩台网监控范围以外，为改善观测台站的分布条件，本文综合使用了二滩水库地震遥测台网，西昌地震遥测台网和四川区域台网约２５个台站的观测资料，利用工业爆破和天然地震资料，计算了该区域的Ｐ波和Ｓ波地壳分层速度模型。以此模型为基础，再采用参考时间平均值法和数值性能极好的单纯形空间迭代寻优的定位方法，修定了该序列的震源位置，得到精度一般小于０．３ｋｍ，最大不超过０．５ｋｍ的极为精确的位置参数（震中经度、震中纬度和震源深度），以到时曲线为准，对解的精度进行了检验。最后，利用定位结果，对田湾序列中ＭＬ≥３．０的４次地震使用Ｐ波初动方向求解了震源机制。结果表明，田湾地震序列非常集中地分布在雅砻江库区及近旁的断层交汇和地形特殊的两个片区。主震及３个大于３．０级的余震都集中发生在田湾断层上，该处的震源深度在１１ｋｍ的深度上呈现出优势层位的分布，该处震中走向横切雅砻江库区。该序列中的地震起初与金河一带南北?
The March 28,1996,Tianwan earthquake sequence has been the biggest one in scale in the reservoir area,lasting about 2 months,since the Ertan Seiemic Network(ESN)ran in Jan.1992.The main shock is M L=4 0,with 33 aftershocks including 3 events greater than M L=3 0.Because the sequence occurred outside the ESN,the observation data from 25 stations of Xichang Telemetric Seismic Network and Sichuan Region Network,not only the ESN,are used to improve station coverage to the sequence.In this paper,the layered crustal velocity model is built using the data of artificial explosion and natural earthquakes;the hypocenters of the sequence are located on basis of the velocity model established above. The precision is between 0 3 km and 0 5 km.Finally,the mechanisms of the 4 events M L≥3 0 are determined using the polarities of P waves.The relocating results indicated the quakes gathered in the reservoir area and across the near faults and topographically special area.The main shock and the 3 bigger aftershocks took place in Tianwan fault with the depths of about 11 km.The hypocenters crossed Yaxijiang reservoir.The quakes of the sequence initially corresponded to those in Jinhe fault,and then moved to Shuiguan dying gradually.From the mechanisms the NNE Tianwan fault is of left lateral strike slip with the pressure from NW direction.On basis of these we analyzed the possibility of reservoir induced earthquakes after the water is filled,and concluded the reservoir area,like Tianwan,should be especially monitored.
The March 28,1996,Tianwan earthquake sequence has been the biggest one in scale in the reservoir area,lasting about 2 months,since the Ertan Seiemic Network(ESN)ran in Jan.1992.The main shock is M L=4 0,with 33 aftershocks including 3 events greater than M L=3 0.Because the sequence occurred outside the ESN,the observation data from 25 stations of Xichang Telemetric Seismic Network and Sichuan Region Network,not only the ESN,are used to improve station coverage to the sequence.In this paper,the layered crustal velocity model is built using the data of artificial explosion and natural earthquakes;the hypocenters of the sequence are located on basis of the velocity model established above. The precision is between 0 3 km and 0 5 km.Finally,the mechanisms of the 4 events M L≥3 0 are determined using the polarities of P waves.The relocating results indicated the quakes gathered in the reservoir area and across the near faults and topographically special area.The main shock and the 3 bigger aftershocks took place in Tianwan fault with the depths of about 11 km.The hypocenters crossed Yaxijiang reservoir.The quakes of the sequence initially corresponded to those in Jinhe fault,and then moved to Shuiguan dying gradually.From the mechanisms the NNE Tianwan fault is of left lateral strike slip with the pressure from NW direction.On basis of these we analyzed the possibility of reservoir induced earthquakes after the water is filled,and concluded the reservoir area,like Tianwan,should be especially monitored.
引文
郭贵安．1997．Prugger方法在汕头地震遥测台网资料定位中的应用．华南地震，17（6）：48～54赵珠，张润生．1987．四川地区地壳上地幔速度结构的初步研究．地震学报，9（2）：154～166赵珠，丁志峰，易桂喜，王建格．1994．西藏地震定位——一种使用单纯形优化的非线性定位方法．地震学报，16（2）：212～219赵珠，陈农．1996．1992年四川力马地震序列期间激发的孕震结构．中国地震，12（1）：64～74赵珠，黄圣睦，张文甫，陈农，胡先明．1996．四川二滩水电站围堰期间诱发地震的初步研究．四川地震，（2）：8～15赵珠，范军，郑斯华，长谷川昭，崛内茂木．1997．龙门山断裂带地壳速度结构和震源位置的精确确定．地震学报，19（6）：615～622
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