用CAP方法反演2010年6月5日山西阳曲M_S4.6地震震源机制解
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摘要
利用CAP方法反演了2010年6月5日阳曲MS4.6地震震源机制解,得到震级MW为4.5,节面I走向213°、倾角47°、滑动角-161°,节面II走向109°,倾角76°,滑动角-44°,属于倾滑型;精确定位显示震中处于石岭关隆起区,CAP反演和精定位结果推断本次地震的震源深度为17～20km。震源机制解节面参数与震中附近的山根底断裂和系舟山西麓断裂产状存在差异,这两条断裂不是阳曲地震的发震断裂,由于现场野外地质考察未发现地表断裂,不排除本次地震为隐伏断层活动的结果。
On June 5,2010,a magnitude MS4.6 earthquake occurred in the Yangqu region,which was located on the Shilingguan uplift between the Taiyuan basin and the Xinding basin in the Shanxi rift.This was a moderately strong earthquake felt in Yangqu and surrounding areas.The earthquake ruptured on the Shangendi fault,which controlled the tectonic activities in the east side of the Taiyuan basin.Based on the seismicity distribution,it appears that this earthquake was also located in the seismic gap of the Taiyuan basin.To better understand the seismotectonics of this region,we relocated this event and further studied its focal mechanism based on the local broadband waveform data recorded by the regional seismic network in the Shanxi Province.In this work,the earthquake was initially relocated by means of the absolute relocating method,and then the focal mechanism and focal depth were determined by the "Cut and Paste"(CAP)method.We inversed the focal mechanism using waveforms in seven stations near the epicenter with high SNR ratios and clear P wave polarization,and the stations were evenly azimuthally distributed.The results of the focal mechanism and focal depth were testified by comparing the observed records and the synthetic waveforms.The results indicate that the Yangqu earthquake was a normal fault earthquake with a small percentage of strike-slip components.The best double couple solution is 213°,47°,and-161°for strike,dip,and slip angles,respectively,for one nodal plane,and 109°,76°,-44°for another.The moment magnitude was MW4.5,and the estimated focal depth was 17～20km,as determined by the CAP method and absolute relocation method,which suggests that this event was a shallow earthquake.The focal depth of the Yangqu earthquake was consistent with the relocation of small to moderate earthquakes in the Shanxi rift.The Yangqu earthquake occurred about 6km away from the Shangendi fault,which is the eastern boundary fault of the Taiyuan basin and is about 9.7km away from the Xizhoushan-Xilu fault,which is the western boundary fault of the Xinding basin.The focal mechanism of the Yangqu earthquake was dramatically different with the tectonic settings of the Shangendi fault and the Xizhoushan-Xilu fault near the epicenter.Based on the relocated location,the Yangqu earthquake should occur in the Shilingguan uplift.However,there is no evidence to show that the Shangendi fault and the Xizhoushan xilu fault extend into the uplift region;furthermore,there is no fracture that is consistent with the mechanism solution of the Yangqu earthquake.Thus,combining previous studies and our results,we suggest that the Shangendi fault and the Xizhoushan-Xilu fault are not the seismogenic fault of the Yangqu earthquake.Because the energy is too weak to transport the rupture to the surface,no surface rupture has been found in the field survey.Therefore,we suggest that a buried fault may be the seismogenic structure of this event.This conclusion still needs further investigationwith ambient noise imaging,active fault detecting,a dense network of high dynamicrange seismometers,and geological field work in Yangqu and its surrounding areas.
On June 5,2010,a magnitude MS4.6 earthquake occurred in the Yangqu region,which was located on the Shilingguan uplift between the Taiyuan basin and the Xinding basin in the Shanxi rift.This was a moderately strong earthquake felt in Yangqu and surrounding areas.The earthquake ruptured on the Shangendi fault,which controlled the tectonic activities in the east side of the Taiyuan basin.Based on the seismicity distribution,it appears that this earthquake was also located in the seismic gap of the Taiyuan basin.To better understand the seismotectonics of this region,we relocated this event and further studied its focal mechanism based on the local broadband waveform data recorded by the regional seismic network in the Shanxi Province.In this work,the earthquake was initially relocated by means of the absolute relocating method,and then the focal mechanism and focal depth were determined by the "Cut and Paste"(CAP)method.We inversed the focal mechanism using waveforms in seven stations near the epicenter with high SNR ratios and clear P wave polarization,and the stations were evenly azimuthally distributed.The results of the focal mechanism and focal depth were testified by comparing the observed records and the synthetic waveforms.The results indicate that the Yangqu earthquake was a normal fault earthquake with a small percentage of strike-slip components.The best double couple solution is 213°,47°,and-161°for strike,dip,and slip angles,respectively,for one nodal plane,and 109°,76°,-44°for another.The moment magnitude was MW4.5,and the estimated focal depth was 17～20km,as determined by the CAP method and absolute relocation method,which suggests that this event was a shallow earthquake.The focal depth of the Yangqu earthquake was consistent with the relocation of small to moderate earthquakes in the Shanxi rift.The Yangqu earthquake occurred about 6km away from the Shangendi fault,which is the eastern boundary fault of the Taiyuan basin and is about 9.7km away from the Xizhoushan-Xilu fault,which is the western boundary fault of the Xinding basin.The focal mechanism of the Yangqu earthquake was dramatically different with the tectonic settings of the Shangendi fault and the Xizhoushan-Xilu fault near the epicenter.Based on the relocated location,the Yangqu earthquake should occur in the Shilingguan uplift.However,there is no evidence to show that the Shangendi fault and the Xizhoushan xilu fault extend into the uplift region;furthermore,there is no fracture that is consistent with the mechanism solution of the Yangqu earthquake.Thus,combining previous studies and our results,we suggest that the Shangendi fault and the Xizhoushan-Xilu fault are not the seismogenic fault of the Yangqu earthquake.Because the energy is too weak to transport the rupture to the surface,no surface rupture has been found in the field survey.Therefore,we suggest that a buried fault may be the seismogenic structure of this event.This conclusion still needs further investigationwith ambient noise imaging,active fault detecting,a dense network of high dynamicrange seismometers,and geological field work in Yangqu and its surrounding areas.
引文
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[8]张辉、徐辉、姚军,等.祁连山中东段基于CAP方法的中小地震震源机制研究[J].山西地震,2010,(1):25-29.ZHANG Hui,XU Hui,YAO Jun,et al.Small-moderate Earthquakes in Middle-east Segment of Qilian Mountain[J].Earthquake Research in Shanxi,2010,(1):25-29.(in Chinese)
[9]吕坚、郑勇、倪四道,等.2005年11月26日九江-瑞昌MS5.7 、MS4 .8地震的震源机制解与发震构造研究[J].地球物理学报,2008,5(1):158-164.LV Jian,ZHENG Yong,NI Si-dao,et al.Focal Mechanisms and Seismogenic Structures of the MS5 .7and MS4 .8Jiujiang-Ruichang Earthquakes of Nov.26,2005[J].Chinese Journal of Geophysics,2008,5(1):158-164.(in Chinese)
[10]Zheng Y,Ma H S,Lv J,et al.Source Mechanism of Strong Aftershocks(MS≥5.6)of the 2008/05/12 Wenchuan Earthquake and the Implication for Seismotectonics[J].Science in China:Series D,2009,52(6):739-753.
[11]Zheng Y,Ni S D,Xie Z J,et al.Strong Aftershocks in the Northern Segment of the Wenchuan Earthquake Rupture Zone and Their Seismotectonic Implications[J].Earth PlanetsSpace,2010,62(11):881-886.
[12]宋美琴,郑勇,葛粲,等.山西地震带中小震精确位置及其显示的山西地震构造特征[J].地球物理学报,2012,55(2):513-525 .SONG Mei-qin,ZHENG Yong,GE Can,et al.Relocation of Small to Moderate Earthquakes in Shanxi Province and Its Relation to the Seismogenic Structures[J].Chinese Journal of Geophysics,2012,55(2):513-525.(in Chinese)
[13]宋美琴,李斌,吕芳,等.2010年1月24日山西河津-万荣MS4 .8地震特征[J].地震地磁观测与研究.2012,33(1):18-22 .SONG Mei-qin,LI Bin,LV Fang,et al.Characteristics of Hejin-Wanrong MS4.8Earthquake on January 24,2010[J].Seismological and Geomagnetic Observation and Research,2012,33(1):18-22.(in Chinese)
[14]Zhu L P,Rivera L A.A Note on the Dynamic and Static Displacements from A Point Source in Multilayered Media[J].Geophys.J.Int.,2002,148:619-627
[15]谢祖军,金笔凯,郑勇,等.近远震波形反演2013年芦山地震震源参数[J].中国科学:地球科学,2013,43:1010-1019.XIE Zu-jun,JIN Bi-kai,ZHENG Yong,et al.Source Parameters Inversion of the 2013Lushan Earthquake by Combining Teleseismic Waveforms and Local Seismograms[J].Science China:Earth Sciences,2013,43:1010-1019.(in Chinese)
[16]赵金仁,张先康,张成科,等.山西五台山地区地壳深部结构特征研究[J].地球物理学报,2006,49(1):123-129.ZHAO Jin-ren,ZHANG Xian-kang,ZHANG Cheng-ke,et al.Features of Deep Crustal Structure Beneath the Wutai Mountain Area of Shanxi Province[J].Chinese Journal of Geophysics,2006,49(1):123-129.(in Chinese)
[2]M7专项组.中国大陆大地震中-长期危险性研究[M].北京:地震出版社,2012.Working Group of M7.Study on the Mid-to Long-term Potential of Large Earthquakes on the Chinese Continent[M].Beijing:Seismological Press,2012.(in Chinese)
[3]Zhao L S,Helmberger D V.Source Estimation from Broadband Regional Seismograms[J].Bull.Seis.Soc.Amer.,1994,84(1):91-104.
[4]Zhu L P,Helmberger D V.Advancement in Source Estimation Techniques Using Broadband Regional Seismograms[J].Bull.Seis.Soc.Amer.,1996,86(5):1634-1641.
[5]刘薇,张晓清,石玉成,等.利用格林函数库计算2008年青海大柴旦6.4级地震的余震震源机制[J].西北地震学报,2012,34(2):154-160.LIU Wei,ZHANG Xiao-qing,SHI Yu-cheng,et al.Using Green Function Database and Quick Moment Tensor Inversion Calculating the Focal Mechanism Solution of Aftershocks of Dachaidan MS6.4Earthquake in 2008in Qinghai Province[J].Northwestern Seismological Journal,2012,34(2):154-160.(inChinese)
[6]张辉.2012年5月11日甘肃肃南MS4 .9地震震源机制解[J].西北地震学报,2012,34(2):207-208.ZHANG Hui.Focal Mechanism of the Sunan MS4 .9Earthquake in Gansu Province on May 11,2012[J].Northwestern Seismological Journal,2012,34(2):207-208.(in Chinese)
[7]赵凌云、邓津、陈俊华,等.基于CAP方法的震源机制研究[J].长江科学院院报,2010,27(5):81-84.ZHAO Ling-yun,DENG Jin,CHEN Jun-hua,et al.Study on Source Mechanism of MS4 .1and MS3 .2Earthquakes in Hubei-Zigui with CAP Method[J].Journal of Yangtze River Scientific Research Institute,2010,27(5):81-84.(in Chinese)
[8]张辉、徐辉、姚军,等.祁连山中东段基于CAP方法的中小地震震源机制研究[J].山西地震,2010,(1):25-29.ZHANG Hui,XU Hui,YAO Jun,et al.Small-moderate Earthquakes in Middle-east Segment of Qilian Mountain[J].Earthquake Research in Shanxi,2010,(1):25-29.(in Chinese)
[9]吕坚、郑勇、倪四道,等.2005年11月26日九江-瑞昌MS5.7 、MS4 .8地震的震源机制解与发震构造研究[J].地球物理学报,2008,5(1):158-164.LV Jian,ZHENG Yong,NI Si-dao,et al.Focal Mechanisms and Seismogenic Structures of the MS5 .7and MS4 .8Jiujiang-Ruichang Earthquakes of Nov.26,2005[J].Chinese Journal of Geophysics,2008,5(1):158-164.(in Chinese)
[10]Zheng Y,Ma H S,Lv J,et al.Source Mechanism of Strong Aftershocks(MS≥5.6)of the 2008/05/12 Wenchuan Earthquake and the Implication for Seismotectonics[J].Science in China:Series D,2009,52(6):739-753.
[11]Zheng Y,Ni S D,Xie Z J,et al.Strong Aftershocks in the Northern Segment of the Wenchuan Earthquake Rupture Zone and Their Seismotectonic Implications[J].Earth PlanetsSpace,2010,62(11):881-886.
[12]宋美琴,郑勇,葛粲,等.山西地震带中小震精确位置及其显示的山西地震构造特征[J].地球物理学报,2012,55(2):513-525 .SONG Mei-qin,ZHENG Yong,GE Can,et al.Relocation of Small to Moderate Earthquakes in Shanxi Province and Its Relation to the Seismogenic Structures[J].Chinese Journal of Geophysics,2012,55(2):513-525.(in Chinese)
[13]宋美琴,李斌,吕芳,等.2010年1月24日山西河津-万荣MS4 .8地震特征[J].地震地磁观测与研究.2012,33(1):18-22 .SONG Mei-qin,LI Bin,LV Fang,et al.Characteristics of Hejin-Wanrong MS4.8Earthquake on January 24,2010[J].Seismological and Geomagnetic Observation and Research,2012,33(1):18-22.(in Chinese)
[14]Zhu L P,Rivera L A.A Note on the Dynamic and Static Displacements from A Point Source in Multilayered Media[J].Geophys.J.Int.,2002,148:619-627
[15]谢祖军,金笔凯,郑勇,等.近远震波形反演2013年芦山地震震源参数[J].中国科学:地球科学,2013,43:1010-1019.XIE Zu-jun,JIN Bi-kai,ZHENG Yong,et al.Source Parameters Inversion of the 2013Lushan Earthquake by Combining Teleseismic Waveforms and Local Seismograms[J].Science China:Earth Sciences,2013,43:1010-1019.(in Chinese)
[16]赵金仁,张先康,张成科,等.山西五台山地区地壳深部结构特征研究[J].地球物理学报,2006,49(1):123-129.ZHAO Jin-ren,ZHANG Xian-kang,ZHANG Cheng-ke,et al.Features of Deep Crustal Structure Beneath the Wutai Mountain Area of Shanxi Province[J].Chinese Journal of Geophysics,2006,49(1):123-129.(in Chinese)
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