岷县M_S6.6地震震源机制及构造应力研究
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摘要
利用甘肃省测震台网观测到的主震波形记录的初动符号求解了2013年7月22日岷县MS6.6地震主震的震源机制,并与哈佛及usgs发布的震源机制进行了对比,对这一地区过去发生过的若干次Mb4.5以上的地震也利用初动符号求解了震源机制,发现与MS6.6地震的震源机制形态较为相似。进而利用2010年至2011年甘东南宽频带流动台阵的大量近震资料,对在震中距50km范围内,并且接近岷县MS6.6地震发震断裂临潭-宕昌断裂的中小地震进行了检索,对检索到的地震进行初动识别后得到初动数目大于10个、并且对震源包络良好的中小地震31个,利用小震的震源机制解求解了该区域的区域应力场。结果表明这一区域的主压力方向为NEE方向。
The Minxian MS 6.6 earthquake,the largest destructive earthquake to affect Gansu province,occurred on July 22,2013.Its epicenter at N34.54°,E104.21°was located between the northern fringe fault of western segment of Qinling Mountain and the Lintan-Tanchang fault. After the earthquake occurred,we solved the focal mechanism of the main shock with P wave polarity from the waveforms obtained from the Gansu province seismic network.We then compared the solution with the focal mechanisms published by Harvard and the United States Geological Survey(USGS). During the 10years prior to the Minxian earthquake,several earthquakes stronger than Mb 4.5occurred in this region.We solved the focal mechanisms of these earthquakes with P wave polarity data,and we compared the focal mechanisms with those of the Minxian earthquake.Similar morphologies were revealed,and each contained a larger thrust component. The focal mechanism of a small earthquake is more difficult to solve than that of a large earthquake.However,because small earthquakes occur more often than large earthquakes,it is desirable to refer to the tectonic stress by the focal mechanisms of multiple small earthquakes to decrease the effects of local structures and to highlight the features of the region tectonic stress.Previous methods used to determine tectonic stress from multiple focal mechanisms include grid search.However,because the smallest search grid is 5°,the search grid was too large to meet the exact solution of the tectonic stress.Other methods ignore the data accuracy or cannot give the error of parameters.The newest method obtains the optimal solution of tectonic stress using finer grid parameters of 1°×1°and gives the error of parameters.In this paper,we solve the tectonic stress using this method. A broadband mobile seismometer observation array was conducted in southeastern Gansu in 2010-2011,in which abundant near shock waveforms were obtained.We examined small earthquakes in close proximity to the Lintan-Tanchang fault,which was the triggering seismic fault of the Minxian earthquake,at a distance of less than 50km.To maintain the reasonableness of the focal mechanism,after identifying the P wave polarity of selected earthquakes,we identified 31earthquakes in which the P wave polarity was more than 9and had a good envelope to the source.We solved their focal mechanisms,which we used to solve the tectonic stress in this region.The results show that the direction of the main pressure was 70.12°,which is very close to the results of previous studies.The main pressure was NEE in this region because of the combined action of the Qinghai-Tibetan Plateau,Ordos,and South China blocks.After the Minxian earthquake,a series of aftershocks occurred,from which and we expect to obtain additional data for analyzing the changes of tectonic stress.
The Minxian MS 6.6 earthquake,the largest destructive earthquake to affect Gansu province,occurred on July 22,2013.Its epicenter at N34.54°,E104.21°was located between the northern fringe fault of western segment of Qinling Mountain and the Lintan-Tanchang fault. After the earthquake occurred,we solved the focal mechanism of the main shock with P wave polarity from the waveforms obtained from the Gansu province seismic network.We then compared the solution with the focal mechanisms published by Harvard and the United States Geological Survey(USGS). During the 10years prior to the Minxian earthquake,several earthquakes stronger than Mb 4.5occurred in this region.We solved the focal mechanisms of these earthquakes with P wave polarity data,and we compared the focal mechanisms with those of the Minxian earthquake.Similar morphologies were revealed,and each contained a larger thrust component. The focal mechanism of a small earthquake is more difficult to solve than that of a large earthquake.However,because small earthquakes occur more often than large earthquakes,it is desirable to refer to the tectonic stress by the focal mechanisms of multiple small earthquakes to decrease the effects of local structures and to highlight the features of the region tectonic stress.Previous methods used to determine tectonic stress from multiple focal mechanisms include grid search.However,because the smallest search grid is 5°,the search grid was too large to meet the exact solution of the tectonic stress.Other methods ignore the data accuracy or cannot give the error of parameters.The newest method obtains the optimal solution of tectonic stress using finer grid parameters of 1°×1°and gives the error of parameters.In this paper,we solve the tectonic stress using this method. A broadband mobile seismometer observation array was conducted in southeastern Gansu in 2010-2011,in which abundant near shock waveforms were obtained.We examined small earthquakes in close proximity to the Lintan-Tanchang fault,which was the triggering seismic fault of the Minxian earthquake,at a distance of less than 50km.To maintain the reasonableness of the focal mechanism,after identifying the P wave polarity of selected earthquakes,we identified 31earthquakes in which the P wave polarity was more than 9and had a good envelope to the source.We solved their focal mechanisms,which we used to solve the tectonic stress in this region.The results show that the direction of the main pressure was 70.12°,which is very close to the results of previous studies.The main pressure was NEE in this region because of the combined action of the Qinghai-Tibetan Plateau,Ordos,and South China blocks.After the Minxian earthquake,a series of aftershocks occurred,from which and we expect to obtain additional data for analyzing the changes of tectonic stress.
引文
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[11]冯建刚,蒋长胜,韩立波,等.1970年以来甘肃台网地震观测报告收集整理及其重新定位研究[J].西北地震学报,2012,34(3):289-293.FENG Jian-gang,JIANG Chang-sheng,HAN Li-bo,et al.Collection and Reestablish Work for the Earthquake Observation Reports of the Gansu Seismographic Network since 1970and Earthquake Relocation[J].Northwestern Seismological Journal,2012,34(3):289-293.(in Chinese)
[12]龙海英,聂晓红,唐兰兰.新疆乌苏5.1级地震前波速比异常震例研究[J].地震研究,2011,34(2):126-130.LONG Hai-ying,NIE Xiao-hong,TANG Lan-lan.Study of the Abnormity of Wave-velocity Ratio before MS5.1 Wusu Earthquake in Xinjiang[J].Journal of Seismological Research,2011,34(2):126-130.(in Chinese)
[13]Whitcomb J H,Garmany J D,Anderson D L.Earthquake Prediction:Variation of Seismic Velocities before the San Fernando Earthquake[J].Science,1973,180:632-641.
[14]Gupta I N.Premonitory Seismic-wave Phenomena before Earthquakes near Fairview Peak,Nevada[J].BSSA,1975,65(2):425-437.
[2]冯建刚,代炜,董治平,等.青藏块体东北缘平均波速比的测定及研究[J].西北地震学报,2009,31(1):61-65.FENG Jian-gang,DAI Wei,DONG Zhi-ping,et al.Determination and Study on Average Seismic Velocity Ratio(Vp/Vs)in Northeastern Margin of Qinghai-Tibet Block[J].Northwestern Seismological Journal,2009,31(1):61-65.(in Chinese)
[3]傅征祥,程燕.北京和张家口地区地壳平均波速比的联合测定[J].地震,1988,8(4):28-34.FU Zheng-xiang,CHENG Yan.Joint Determination of Crust Wave Velocity Ratio in Beijing and Zhangjiakou Region[J].Earthquake,1988,8(4):28-34.(in Chinese)
[4]Aggarwal Y P,Sykes L R,Ambruster J,et al.Premonitory Changes in Seismic Velocities and Prediction of Earthquakes[J].Nature,1973,241(5385):101-104.
[5]冯德益.地震波速异常[M].北京:地震出版社,1981.FENG De-yi.Anomalies of Seismic Velocity[M].Beijing:Seismological Press,1981.(in Chinese)
[6]王林瑛,李艳娥,李芳,等.汶川地震前后地震波速比和视速度变化特征的研究[J].地震学报,2011,33(1):1-14.WANG Lin-ying,LI Yan-e,LI Fang,et al.Temporal Variation of Vp/Vs,Vp and Vs before and after 2008 Wenchuan Earthquake[J].Acta Seismologica Sinica,2011,33(1):1-14.(in Chinese)
[7]黎明晓,刘杰.云南地区地震序列的波速比(Vp/Vs)异常研究[J].地震,2006,26(1):26-34.LI Ming-xiao,LIU Jie.Study on Velocity Ratio(Vp/Vs)Anomaly of Earthquake Sequences in Yunnan Region[J].Earthquake,2006,26(1):26-34.(in Chinese)
[8]黎明晓,张晓东.应用多台法测定华北地区地壳的平均波速比[J].地震,2004,24(1):163-169.LI Ming-xiao,ZHANG Xiao-dong.Determining Average Seismic Velocity Ratios(Vp/Vs)in the Curst in North China Region by Multi-station Method[J].Earthquake,2004,24(1):163-169.(in Chinese)
[9]李善邦.中国地震[M].北京:地震出版社,1981.LI Shan-bang.Chinese Earthquake[M].Beijing:Seismological Press,1981.(in Chinese)
[10]王志铄,李源.汶川地震余震区波速比与强余震的关系[J].山西地震,2012,1:2.WANG Zhi-shuo,LI Yuan.The Relationship between Wave Velocity Ratio and Strong Aftershocks in Wenchuan Aftershock Zone[J].Earthquake Research in Shanxi,2012,1:2.(in Chinese)
[11]冯建刚,蒋长胜,韩立波,等.1970年以来甘肃台网地震观测报告收集整理及其重新定位研究[J].西北地震学报,2012,34(3):289-293.FENG Jian-gang,JIANG Chang-sheng,HAN Li-bo,et al.Collection and Reestablish Work for the Earthquake Observation Reports of the Gansu Seismographic Network since 1970and Earthquake Relocation[J].Northwestern Seismological Journal,2012,34(3):289-293.(in Chinese)
[12]龙海英,聂晓红,唐兰兰.新疆乌苏5.1级地震前波速比异常震例研究[J].地震研究,2011,34(2):126-130.LONG Hai-ying,NIE Xiao-hong,TANG Lan-lan.Study of the Abnormity of Wave-velocity Ratio before MS5.1 Wusu Earthquake in Xinjiang[J].Journal of Seismological Research,2011,34(2):126-130.(in Chinese)
[13]Whitcomb J H,Garmany J D,Anderson D L.Earthquake Prediction:Variation of Seismic Velocities before the San Fernando Earthquake[J].Science,1973,180:632-641.
[14]Gupta I N.Premonitory Seismic-wave Phenomena before Earthquakes near Fairview Peak,Nevada[J].BSSA,1975,65(2):425-437.
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