汶川地震后中下地壳及上地幔的粘弹性效应引起的应力变化与芦山地震的发生机制
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摘要
采用多层粘弹性模型计算了2008年5月12日汶川MW7.9地震对周围地区尤其是龙门山断裂带南段的影响,结合地震活动性分析,探讨了2013年4月20日芦山MW6.6地震的发生机制,并对沿龙门山断裂带芦山地震与汶川地震之间的地震空段进行了分析。计算结果表明,由于下部地壳及上地幔的粘弹性效应,芦山地震震源处对应的汶川地震同震库仑应力变化(ΔCFS)为-0.1bar以下量级,其后随时间逐渐增加,在芦山地震前增加到了0.4bar(有效摩擦系数为0.4)或0.6bar(有效摩擦系数为0.2)以上。这表明,芦山地震的发生与汶川地震的非弹性触发密切相关。芦山地震与汶川地震之间的地震破裂空段存在1970年大邑M6.2地震的库仑应力阴影区(下降区),汶川地震及芦山地震未能使阴影区发生根本变化,因此,如没有其他不可知因素,该空段短期内整体发生破裂,引发大地震的可能性不大。
Stress changes due to the co-seismic slip on the source fault of the 2008 MW 7. 9 Whenchuan earthquake and delayed response of inelastic deformation in the lower crust and upper mantle have an important role in the seismicity in Longmenshan area. After the Wenchuan earthquake,seismicity shows progressively increasing in a wide region. However,the south segment of the Longmenshan Fault did not show any significant change in seismicity,where positive Coulomb failure stress change ( ΔCFS) was estimated under the elastic half-space model. Under such a background,the 2013 MW 6. 6 Lushan earthquake occurred. This paper presents some preliminary results based on seismicity analysis and stress analysis using lithology models in which the lower crust and the upper mantle are suggested to be viscoelastic. The Wenchuan earthquake resulted in a miner negative coseismic ΔCFS in the hypocenter region of the Lushan earthquake. As a result of inelastic response the estimated ΔCFS reached the order of 0. 2 ～ 0. 4bar,a value sufficient to trigger earthquakes in critically loaded faults. We thus conclude that the Lushan earthquake provides a case of inelastic triggering of the Wenchuan earthquake. The 1970 M6. 2 Dayi earthquake caused an obvious Coulomb stress shadow in its source area,which partly overlaps to the seismic gap between the ruptures of the Lushan and Wenchuan earthquakes. The stress shadow still exists although the area has been loaded by both the Wenchuan and Lushan earthquakes. We thus suggest that it is less likely that a great earthquake,which ruptures the entire gap,may occur in the near future if there are no other unknown factors.
Stress changes due to the co-seismic slip on the source fault of the 2008 MW 7. 9 Whenchuan earthquake and delayed response of inelastic deformation in the lower crust and upper mantle have an important role in the seismicity in Longmenshan area. After the Wenchuan earthquake,seismicity shows progressively increasing in a wide region. However,the south segment of the Longmenshan Fault did not show any significant change in seismicity,where positive Coulomb failure stress change ( ΔCFS) was estimated under the elastic half-space model. Under such a background,the 2013 MW 6. 6 Lushan earthquake occurred. This paper presents some preliminary results based on seismicity analysis and stress analysis using lithology models in which the lower crust and the upper mantle are suggested to be viscoelastic. The Wenchuan earthquake resulted in a miner negative coseismic ΔCFS in the hypocenter region of the Lushan earthquake. As a result of inelastic response the estimated ΔCFS reached the order of 0. 2 ～ 0. 4bar,a value sufficient to trigger earthquakes in critically loaded faults. We thus conclude that the Lushan earthquake provides a case of inelastic triggering of the Wenchuan earthquake. The 1970 M6. 2 Dayi earthquake caused an obvious Coulomb stress shadow in its source area,which partly overlaps to the seismic gap between the ruptures of the Lushan and Wenchuan earthquakes. The stress shadow still exists although the area has been loaded by both the Wenchuan and Lushan earthquakes. We thus suggest that it is less likely that a great earthquake,which ruptures the entire gap,may occur in the near future if there are no other unknown factors.
引文
①王小龙等,2013,三峡库区及邻近地区地震背景噪声瑞利面波相速度层析成像(待发表)。
①,②同412页①。
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Li H,Su W,Wang C Y,et al.2009.Ambient noise Rayleigh wave tomography in western Sichuan and eastern Tibet[J].Earth Planet Sci Lett,282:201—211.
Matthews M V,Reasenberg P A.1988.Statistical methods for investigating quiescence and other temporal seismicitypatterns[J].Pure App Geophys,126:357—372.
Okada Y.1992.Internal deformation due to shear and tensile faults in a half-space[J].Bull Seismol Soc Am,82(2):1018—1040.
Parsons T,Ji C,Kirby E.2008.Stress changes from the2008Wenchuan earthquake and increased hazard in the SichuanBasin[J].Nature,454:509—510.
Reasenberg P A,Simpson R W.1992.Response of regional seismicity to the static stress change produced by the LomaPrieta earthquake[J].Science,255:1687—1690.
Royden L H,Burchfiel B C,King R W,et al.1997.Surface deformation and lower crustal flow in eastern Tibet[J].Sci-ence,276:788—790.
Ryder I,Bürgmann R,Pollitz F.2011.Lower crustal relaxation beneath the Tibetan plateau and Qaidam Basin followingthe2001Kokoxili earthquake[J].Geophys J Int,187:613—630.
Tapponnier P,Xu Z,Roger F,et al.2001.Oblique stepwise rise and growth of the Tibet plateau[J].Science,294:1671—1677.
Toda S,Lin J,Meghraoui M,et al.2008.12May2008M=7.9Wenchuan,China,earthquake calculated to increase fail-ure stress and seismicity rate on three major fault systems[J].J Geophys Res,35:L17305.doi:10.1029/2008GL034903.
Wan Y,Shen Z.2010.Static Coulomb stress changes on faults caused by the2008MW7.9Wenchuan,China earthquake[J].Tectonophysics,491:105—118.
Wang R,Lorenzo-Martin F,Roth F.2006.PSGRN/PSCMP,a new code for calculating co-and post-seismic deformation,geoid and gravity changes based on the viscoelastic-gravitational dislocation theory[J].Computers&Geosciences,32:527—541.
Wang W,Zhao L,Li J,et al.2008.Rupture process of the MS8.0Wenchuan earthquake of Sichuan,China[J].Chin JGeophys,51:1403—1410(in Chinese).
Wang Z,Wang X,Huang R,Huang W.2012.Structural heterogeneities in southeast Tibet:Implications for regional flowin the lower crust and upper mantle[J].International Journal of Geophysics.doi:10.1155/2012/975497.
Wei G,Chen G,Du S,et al.2008.Petroleum systems of the oldest gas field in China:Neoproterozoic gas pools in theWeiyuan gas field,Sichuan Basin[J].Marine and Petroleum Geology,25:371—386.
Xie C,Zhu Y,Lei X-L,et al.2010.Pattern of stress change and its effect on seismicity rate caused by MS8.0Wen-chuan earthquake[J].Sci China Earth Sci,53:1260—1270.
Xu H,Wei G,Jia C,et al.2012.Tectonic evolution of the Leshan-Longnusi paleo-uplift and its control on gas accumula-tion in the Sinian strata[J].Petroleum Exploration and Development,39(4):436—446.
Zhang Z,Yuan X,Chen Y,et al.2010.Seismic signature of the collision between the east Tibetan escape flow and theSichuan Basin[J].Earth Planet Sci Lett,292:254—264.
①,②同412页①。
①同412页①。
邓起东,冉勇康,杨晓平,等.2007.1︰400,000中国活动构造图[M].北京:地震出版社.DENG Qi-dong,RAN Yong-kang,YANG Xiao-ping,et al.2007.Map of Active Tectonics in China[M].SeismologicalPress,Beijing(in Chinese).
雷兴林,马胜利,闻学泽,等.2008.地表水体对断层应力与地震时空分布影响的综合分析:以紫坪铺水库为例[J].地震地质,30(4):1046—1064.LEI Xing-lin,MA Sheng-li,WEN Xue-ze,et al.2008.Integrated analysis of stress and regional seismicity by surfaceloading:A case study of Zipingpu reservoir[J].Seismology and Geology,30(4):1046—1064(in Chinese).
刘启元,李昱,陈九辉,等.2009.汶川8.0地震:地壳上地幔s波速度结构的初步研究[J].地球物理学报,52(2):309—319.LIU Qi-yuan,LI Yu,CHEN Jiu-hui,et al.2009.Wenchuan M8.0earthquake:Preliminary study of the S-wave velocitystructure of the crust and upper mantle[J].Chinese J Geophys,52(2):309—319(in Chinese).
Bird P.1991.Lateral extrusion of lower crust from under high topography in the isostatic limit[J].J Geophys Res,96:10275—10286.
Burchfiel B C,Chen Z,Liu Y,et al.1995.Tectonics of the Longmen Shan and adjacent regions[J].Int Geol Rev,37(8):661—735.
Burchfiel B C.2004.Presidential address:New technology,new geological challenges[J].GSA Today,14:4—9.
Burchfiel B C,Royden L H,van der Hilst R D,et al.2008.A geological and geophysical context for the Wenchuanearthquake of12May2008,Sichuan,People's Republic of China[J].GSA Today,18(7):4—11.
Diao F,Xiong X,Wang R.2010.Mechanisms of transient postseismic deformation following the2001MW7.8Kunlun(China)earthquake[J].Pure Appl Geophys,168:767—779.
Fu B,Shi P,Guo H,et al.2011.Surface deformation related to the2008Wenchuan earthquake,and mountain buildingof the Longmen Shan,eastern Tibetan plateau[J].Journal of Asian Earth Sciences,40:805—824.
Hubbard J,Shaw J H,Klinger Y.2010.Structural setting of the2008MW7.9Wenchuan earthquake,China[J].BullSeismol Soc Am,100:2713—2735.
Johnson K M,Hilley G E,Burgmann R.2007.Influence of lithosphere viscosity structure on estimates of fault slip ratein the Mojave region of the San Andreas Fault system[J].J Geophys Res,112:B07408.doi:10.1029/2006JB004842.
Lei X L.2012.Dragon-Kings in rock fracturing:Insights gained from rock fracture tests in laboratory[J].EuropeanPhysical Journal Special Topics,205:217—230.doi:10.1140/epjst/e2012-01572-8.
Lei X L,Tamagawa T,Tezuka K,et al.2011.Role of drainage conditions in deformation and fracture of porous rocks un-der triaxial compression in the laboratory[J].Geophys Res Lett,38:L24310.doi:10.1029/2011GL049888.
Li H,Su W,Wang C Y,et al.2009.Ambient noise Rayleigh wave tomography in western Sichuan and eastern Tibet[J].Earth Planet Sci Lett,282:201—211.
Matthews M V,Reasenberg P A.1988.Statistical methods for investigating quiescence and other temporal seismicitypatterns[J].Pure App Geophys,126:357—372.
Okada Y.1992.Internal deformation due to shear and tensile faults in a half-space[J].Bull Seismol Soc Am,82(2):1018—1040.
Parsons T,Ji C,Kirby E.2008.Stress changes from the2008Wenchuan earthquake and increased hazard in the SichuanBasin[J].Nature,454:509—510.
Reasenberg P A,Simpson R W.1992.Response of regional seismicity to the static stress change produced by the LomaPrieta earthquake[J].Science,255:1687—1690.
Royden L H,Burchfiel B C,King R W,et al.1997.Surface deformation and lower crustal flow in eastern Tibet[J].Sci-ence,276:788—790.
Ryder I,Bürgmann R,Pollitz F.2011.Lower crustal relaxation beneath the Tibetan plateau and Qaidam Basin followingthe2001Kokoxili earthquake[J].Geophys J Int,187:613—630.
Tapponnier P,Xu Z,Roger F,et al.2001.Oblique stepwise rise and growth of the Tibet plateau[J].Science,294:1671—1677.
Toda S,Lin J,Meghraoui M,et al.2008.12May2008M=7.9Wenchuan,China,earthquake calculated to increase fail-ure stress and seismicity rate on three major fault systems[J].J Geophys Res,35:L17305.doi:10.1029/2008GL034903.
Wan Y,Shen Z.2010.Static Coulomb stress changes on faults caused by the2008MW7.9Wenchuan,China earthquake[J].Tectonophysics,491:105—118.
Wang R,Lorenzo-Martin F,Roth F.2006.PSGRN/PSCMP,a new code for calculating co-and post-seismic deformation,geoid and gravity changes based on the viscoelastic-gravitational dislocation theory[J].Computers&Geosciences,32:527—541.
Wang W,Zhao L,Li J,et al.2008.Rupture process of the MS8.0Wenchuan earthquake of Sichuan,China[J].Chin JGeophys,51:1403—1410(in Chinese).
Wang Z,Wang X,Huang R,Huang W.2012.Structural heterogeneities in southeast Tibet:Implications for regional flowin the lower crust and upper mantle[J].International Journal of Geophysics.doi:10.1155/2012/975497.
Wei G,Chen G,Du S,et al.2008.Petroleum systems of the oldest gas field in China:Neoproterozoic gas pools in theWeiyuan gas field,Sichuan Basin[J].Marine and Petroleum Geology,25:371—386.
Xie C,Zhu Y,Lei X-L,et al.2010.Pattern of stress change and its effect on seismicity rate caused by MS8.0Wen-chuan earthquake[J].Sci China Earth Sci,53:1260—1270.
Xu H,Wei G,Jia C,et al.2012.Tectonic evolution of the Leshan-Longnusi paleo-uplift and its control on gas accumula-tion in the Sinian strata[J].Petroleum Exploration and Development,39(4):436—446.
Zhang Z,Yuan X,Chen Y,et al.2010.Seismic signature of the collision between the east Tibetan escape flow and theSichuan Basin[J].Earth Planet Sci Lett,292:254—264.
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