铲形逆冲断层地震破裂动力学模型及其在汶川地震研究中的启示
摘要
2008年发生了汶川地震的龙门山断层带是典型的铲形逆冲断层带.利用二维线弹性有限元模型,得到关于铲形逆冲断层带一些具普适性的认识:(1)如果断层带强度不随深度变化,则地震从断层带转换层附近开始发动,破裂沿断层向上传播,当地震蓄积能量足够大时,破裂可以冲破到地表,如汶川地震.(2)一旦到达地表,其最大同震位错将位于断层带出露地表处;且在断层带介质弱化或损伤大致相同条件下,铲形断层倾角越缓,地表的同震位错量越大.(3)与最大同震位错位于地表不同,同震应变能释放最大处位于地壳内十几公里深处的断层带转换层附近.(4)在断层带介质软化或损伤度大致相同条件下,铲形断层倾角越缓,地震对震间期积累应变能的释放越多、范围越广,对地表近断层区域造成的破坏可能越大.(5)冲破到地表的逆冲型大地震会进一步增加铲形断层带下盘转换层以下(约在15～22 km)地壳深处和断层上盘距离断层地表出露点约十几公里处地壳浅部的应变能积累密度;汶川地震之后的余震分布,除了沿主破裂面附近的继续破裂外,也体现了以上地区应变能的继续释放.本文得到的这些认识有助于深入理解汶川地震的动力学机制.
The 12 May 2008 M_w7.9 Wenchuan earthquake occurred on the Longmenshan fault, which is a classic listric-reverse fault.Using a 2-D elastic finite element model,we obtain some generic understanding on listric fault,and rupture process of the Wenchuan earthquake:(1)If fault strength does not vary along depth,the earthquake would initiate from a spot near the brittle-ductile transition zone of the fault,and propagate upward.The rupture could reach the surface when there is enough energy stored in the crust around the fault system.(2)Once the earthquake rupture breaks to the surface,the coseismic slip would be peaked at the surface,as the case of the Wenchuan earthquake;the shallower the dipping angle of the fault is,the greater the dislocation at the surface would be.(3)Unlike the maximum coseismic slip,the maximum strain energy release density would be located at downdip near the brittle-ductile transition zone of the fault at more than 10 km depth.(4)The shallower the dipping angle of the fault is,the more complete the release would be for the strain energy accumulated during the interseismic time period,and the greater destruction the quake would result at the surface for the near-field region at the hanging wall.(5)Thrust earthquakes occurred on a listric fault could not release all the energy accumulated during the interseismic time period,and furthermore higher strain energy density would be loaded at two regions:one is under the brittle-ductile transition zone of the fault,at a depth range of 15～22 km within the foot wall,and the other is in the shallow crust of the hanging wall,in a depth range of 0～5 km and at about 10+ km horizontal distance from the surface break point.Spatial concentration of the Wenchuan aftershocks reflected the continued strain energy release in these related regions in addition to the continued rupture around the main rupture plane.
The 12 May 2008 M_w7.9 Wenchuan earthquake occurred on the Longmenshan fault, which is a classic listric-reverse fault.Using a 2-D elastic finite element model,we obtain some generic understanding on listric fault,and rupture process of the Wenchuan earthquake:(1)If fault strength does not vary along depth,the earthquake would initiate from a spot near the brittle-ductile transition zone of the fault,and propagate upward.The rupture could reach the surface when there is enough energy stored in the crust around the fault system.(2)Once the earthquake rupture breaks to the surface,the coseismic slip would be peaked at the surface,as the case of the Wenchuan earthquake;the shallower the dipping angle of the fault is,the greater the dislocation at the surface would be.(3)Unlike the maximum coseismic slip,the maximum strain energy release density would be located at downdip near the brittle-ductile transition zone of the fault at more than 10 km depth.(4)The shallower the dipping angle of the fault is,the more complete the release would be for the strain energy accumulated during the interseismic time period,and the greater destruction the quake would result at the surface for the near-field region at the hanging wall.(5)Thrust earthquakes occurred on a listric fault could not release all the energy accumulated during the interseismic time period,and furthermore higher strain energy density would be loaded at two regions:one is under the brittle-ductile transition zone of the fault,at a depth range of 15～22 km within the foot wall,and the other is in the shallow crust of the hanging wall,in a depth range of 0～5 km and at about 10+ km horizontal distance from the surface break point.Spatial concentration of the Wenchuan aftershocks reflected the continued strain energy release in these related regions in addition to the continued rupture around the main rupture plane.
引文
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[13] 朱艾斓,徐锡伟,刁桂苓等.汶川Ms8． 0地震部分余震重新定位及地震构造初步分析.地震地质,2008,30(3) :759～767 Zhu A L,Xu X W,Diao G L,et al.Relocation of the Ms8． 0 Wenchuan earthquake sequence in part:preliminary seismotectonic analysis.Seismology and Geology (in Chinese),2008,30(3) :759～767
[14] 周永胜,何昌荣.汶川地震区的流变结构与发震高角度逆断层滑动的力学条件.地球物理学报,2009,52(2) :474～484 Zhou Y S,He C R.The rheological structures of crust and mechanics of high angle reverse fault slip for Wenchuan MsS.0 earthquake.Chinese J.Geophys.(in Chinese),2009,52(2) :474～484
[15] Hu C,Zhou Y,Cai Y,Wang C.Study of earthquake triggering in a heterogeneous crust using a new finite element model.Seismological Research Letters ,2009 ,80(5) ;795～803,doi:10． 1785/gssrl.80． 5． 799
[16] http://www.fegensoft.com/[2010-06-25]
[17] http://www.kepu.net.cn/gb/special/2009/0512/03_kj/03_lsl_02sll.html[2010-06-25]
[18] 王卫民,赵连锋,李娟等.四川汶川8． o级地震震源过程.地球物理学报,2008,51(5) :1403～1410 Wang W M,Zhao L F,Li J,et al.Rupture process of the Ms8． 0 Wenchuan earthquake of Sichuan,China.Chinese J.Geophys.(in Chinese),2008,51(5) :1403～1410
[19] Ammon C J,Chen J,Thio H K.Rupture process of the 2004 Sumatra-Andaman Earthquake.Science,2005,308(5725) :1133～1139,DOI:10． 1126/science.1112260
[20] Wald D J,Heaton T H,Hudnut K W.The slip history of the 19 9 4 Northridge,California,Earthquake determined from strong ground motion,teleseismic,GPS,and leveling data.Bull.Seism.Soc.Am.,1996,86:S49～S70
[21] Scholz C.The Mechanics of Earthquakes and Faulting-2nd Edition,Cambridge University Press,2002
[22] 陈九辉,刘启元,李顺成等.汶川Ms8． 0地震余震序列重新定位及其地震构造研究.地球物理学报,2009,52(2) :390～397 Chen J H,Liu Q Y,Li S C,et al.Seismotectonic study by relocation of the Wenchuan Ms8． 0 earthquake sequence.Chinese J.Geophys.(in Chinese),2009,52(2) :390～397
[23] 黄 嫒,吴建平,张天中等.汶川8． o级大地震及其余震序列重定位研究.中国科学(D辑),2008,38(10) :1242～1249 Huang Y,Wu J P,Zhang T Z,et al.The study of the Wenchuan Ms8． 0 earthquake,2008 and the relocation of it's aftershocks' sequence.Science in China Series D (in Chinese),2008,38(10) :1242～1249
[24] 朱守彪,张培震.2008年汶川Ms8． 0地震发生过程的动力学机制研究.地球物理学报,2009,52(2) :418～427 Zhu S B,Zhang P Z.A study on the dynamical mechanis of the Wenchuan Ms8． 0 earthquake,2008． Chinese J.Geophys.(in Chinese),2009,52(2) :418～427
[25] Zhu S B,Zhang P Z.Numeric modeling of the strain accumulation and release of the 2008 Wenchuan,Sichuan,China.Bull.Seismol.Soc.Am.,2010,100(5B):2825～2839,doi10． 1785/0120090351
[2] 刘启元,李昱,陈九辉等.汶川Ms8． 0地震:地壳上地幔S波速度结构的初步研究.地球物理学报,2009,52(2) :309～319 Liu Q Y,Li Y,Chen J H,et al.Wenchuan Ms8． 0 earthquake:preliminary study of the S-wave velocity structure of the crust and upper mantle.Chinese J.Geophys.(in Chinese),2009,52(2) :309～319
[3] Wang C Y,Han W B,Wu J P,et al.Crustal structure beneath the eastern margin of the Tibetan Plateau and its tectonic implications.Journal of Geophysical Research ,2007,112:B07307,doi:10． 1029/2005JB003873
[4] 王阎昭,王恩宁,沈正康等.基于GPS资料约束反演川滇地区主要断裂现今活动速率.中国科学(D辑),2008,38(5) :582～597 Wang Y Z,Wang E N,Shen Z K,et al.GPS-constrained inversion of present-day slip rates along major faults of the Sichuan-Yunnan region,China.Science in China (Series D)(in Chinese),2008,38(5) :582～597
[5] Zhang P Z,Wen X Z,Shen Z K,Chen J H.Oblique,highangle,listric-reverse faulting and associated development of strain:The Wenchuan Earthquake of May 12,2008,Sichuan,China.Annu.Rev.Earth Planet.Sci.,2010,38:353～382
[6] 朱艾斓,徐锡伟,周永胜等.川西地区小震重新定位及其活动构造意义.地球物理学报,2005,48(3) :629～636 Zhu A I,Xu X W,Zhou Y S,et a1． Relocation of sinail earthquakes in western Sichuan,China and its implications for active tectonics.Chinese J.Geophys.(in Chinese),2005,48(3) :629～636
[7] Jia D,Li Y Q,Lin A M.Structural model of 2008 M?7． 9 Wenchuan earthquake in the rejuvenated Longmen Shan thrust belt,China.Tectonophysics,2009,491:174～184,doi:10． 1016/j.tecto.2009． 08． 040
[8] 邓起东,陈社发,赵小麟.龙门山及其邻区的构造和地震活动及动力学.地震地质,1994,16(4) :389～403 Deng Q D,Chen S F,Zhao X L.The tectonics,earthquake activities and dynamics in Longmengshan and its adjacent area.Seismology and Geology (in Chinese),1994,16(4) :389～403
[9] Shen Z K,Sun J B,Zhang P Z,et al.Slip maxima at fault junctions and rupturing of barriers during the 2008 Wenchuan earthquake.Narure Geoscience ,2009 ,2 :718～724 ,DOI:10． 1038/NGEO636
[10] Robert A,Zhu J,Vergne J,et al.Crustal structures in the area of the 2008 Sichuan earthquake from seismologic and gravimetric data.Tectonophysics,2009,491:205-210,dio:10． 1016/j.tecto.2009． 11． 010
[11] 张培震,徐锡伟,闻学泽等.2008年汶川8． 0级地震发震断裂的滑动速率、复发周期和构造成因.地球物理学报,2008,51(4) :1066～1073 Zhang P Z,Xu X W,Wen X Z,et al.Slip rates and recurrence intervals of the Longmen Shan active fault zone,and tectonic implications for the mechanism of the May 12 Wenchuan earthquake,2008,Sichuan,China.Chinese J.Geophys.(in Chinese),2008,51(4) :1066～1073
[12] 朱介寿.汶川地震的岩石圈深部构造与动力学背景.成都理工大学学报(自然科学版),2008,35(4) :348～355 Zhu J S.The Wenchuan earthquake occurrence background in deep structure and dynamic of lithosphere.Journal of Chengdu University of Technology (Science & Technology Edition) (in Chinese),2008,35(4) :348～355
[13] 朱艾斓,徐锡伟,刁桂苓等.汶川Ms8． 0地震部分余震重新定位及地震构造初步分析.地震地质,2008,30(3) :759～767 Zhu A L,Xu X W,Diao G L,et al.Relocation of the Ms8． 0 Wenchuan earthquake sequence in part:preliminary seismotectonic analysis.Seismology and Geology (in Chinese),2008,30(3) :759～767
[14] 周永胜,何昌荣.汶川地震区的流变结构与发震高角度逆断层滑动的力学条件.地球物理学报,2009,52(2) :474～484 Zhou Y S,He C R.The rheological structures of crust and mechanics of high angle reverse fault slip for Wenchuan MsS.0 earthquake.Chinese J.Geophys.(in Chinese),2009,52(2) :474～484
[15] Hu C,Zhou Y,Cai Y,Wang C.Study of earthquake triggering in a heterogeneous crust using a new finite element model.Seismological Research Letters ,2009 ,80(5) ;795～803,doi:10． 1785/gssrl.80． 5． 799
[16] http://www.fegensoft.com/[2010-06-25]
[17] http://www.kepu.net.cn/gb/special/2009/0512/03_kj/03_lsl_02sll.html[2010-06-25]
[18] 王卫民,赵连锋,李娟等.四川汶川8． o级地震震源过程.地球物理学报,2008,51(5) :1403～1410 Wang W M,Zhao L F,Li J,et al.Rupture process of the Ms8． 0 Wenchuan earthquake of Sichuan,China.Chinese J.Geophys.(in Chinese),2008,51(5) :1403～1410
[19] Ammon C J,Chen J,Thio H K.Rupture process of the 2004 Sumatra-Andaman Earthquake.Science,2005,308(5725) :1133～1139,DOI:10． 1126/science.1112260
[20] Wald D J,Heaton T H,Hudnut K W.The slip history of the 19 9 4 Northridge,California,Earthquake determined from strong ground motion,teleseismic,GPS,and leveling data.Bull.Seism.Soc.Am.,1996,86:S49～S70
[21] Scholz C.The Mechanics of Earthquakes and Faulting-2nd Edition,Cambridge University Press,2002
[22] 陈九辉,刘启元,李顺成等.汶川Ms8． 0地震余震序列重新定位及其地震构造研究.地球物理学报,2009,52(2) :390～397 Chen J H,Liu Q Y,Li S C,et al.Seismotectonic study by relocation of the Wenchuan Ms8． 0 earthquake sequence.Chinese J.Geophys.(in Chinese),2009,52(2) :390～397
[23] 黄 嫒,吴建平,张天中等.汶川8． o级大地震及其余震序列重定位研究.中国科学(D辑),2008,38(10) :1242～1249 Huang Y,Wu J P,Zhang T Z,et al.The study of the Wenchuan Ms8． 0 earthquake,2008 and the relocation of it's aftershocks' sequence.Science in China Series D (in Chinese),2008,38(10) :1242～1249
[24] 朱守彪,张培震.2008年汶川Ms8． 0地震发生过程的动力学机制研究.地球物理学报,2009,52(2) :418～427 Zhu S B,Zhang P Z.A study on the dynamical mechanis of the Wenchuan Ms8． 0 earthquake,2008． Chinese J.Geophys.(in Chinese),2009,52(2) :418～427
[25] Zhu S B,Zhang P Z.Numeric modeling of the strain accumulation and release of the 2008 Wenchuan,Sichuan,China.Bull.Seismol.Soc.Am.,2010,100(5B):2825～2839,doi10． 1785/0120090351
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