龙门山断裂带地壳精细结构与汶川地震发震机理
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摘要
利用2001年1月至2008年6月四川固定地震台网和临时地震台站记录到的大量P波到时资料,反演了龙门山断裂带及周边地区的地壳精细三维P波速度模型.结果表明,汶川主震以北和以南地区的结构存在较大差异,以北地区的龙门山断裂带具有很强地壳不均匀性,这与该区发生了大量汶川地震的余震相一致.这些结果有意义地改进了前人对龙门山断裂带仅为不同块体过渡带的认识.汶川主震震源区下方存在有明显低波速异常体,表明流体可能存在于龙门山断裂带内.这些流体可能直接影响汶川大震的形成.本文的成像结果为下地壳流沿龙门山断裂带上浸提供了可靠的地震学证据.
In this study we used a large number of P-wave arrival times recorded by the Sichuan permanent and portable seismic stations during January 2001 and June 2008 to obtain a fine threedimensional P-wave tomographic model of the crust around the Longmenshan fault zone. The results show that there exist obvious differences between south and north of the Wenchuan mainshock in the seismic structure. North of the Wenchuan mainshock there exist much more heterogeneities, where many aftershocks occurred. These results have significantly improved over the previous results suggesting that the Longmenshan fault zone is only a transition zone between different blocks. The source area of the Wenchuan mainshock is underlain by obvious low velocity anomalies, suggesting that there exist fluids within the Longmenshan fault zone. These fluids may affect the occurrence of the large Wenchuan earthquake. The present results provide significantly seismological evidence for the upward intrusion of the lower crustal flow along the Longmenshan fault zone.
In this study we used a large number of P-wave arrival times recorded by the Sichuan permanent and portable seismic stations during January 2001 and June 2008 to obtain a fine threedimensional P-wave tomographic model of the crust around the Longmenshan fault zone. The results show that there exist obvious differences between south and north of the Wenchuan mainshock in the seismic structure. North of the Wenchuan mainshock there exist much more heterogeneities, where many aftershocks occurred. These results have significantly improved over the previous results suggesting that the Longmenshan fault zone is only a transition zone between different blocks. The source area of the Wenchuan mainshock is underlain by obvious low velocity anomalies, suggesting that there exist fluids within the Longmenshan fault zone. These fluids may affect the occurrence of the large Wenchuan earthquake. The present results provide significantly seismological evidence for the upward intrusion of the lower crustal flow along the Longmenshan fault zone.
引文
[1] 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,2008,51:1066-1073 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
[2] 车用太,刘成龙,鱼金子等.汶川M_s8． 0地震的地下流体与宏观异常及地震预测问题的思考.地震地质,2008,30(4) :828-838 Chen Y T, Liu C L, Yu J Z, et al. Underground fluid anomaly and macro anomaly of M_s 8． 0 Wenchuan earthquake and opinions about earthquake prediction. Seismol. Geol. , 2008,30(4) :828-838
[3] Wang C, Han W, Wu J, et al. Crustal structure beneath the eastern margin of the Tibetan Plateau and its tectonic implications. J. Geophys. Res., 2007,112, doi:10． 1029/2005JB003873
[4] Xu L, Rondenay S, van der Hilst R. Structure of the crust beneath the southeastern Tibetan Plateau from teleseismic receiver functions. Phys. Earth Planet. Inter., 2007,165: 176-193
[5] Zhang P, Shen Z, Wang M, et al. Continuous deformation of the Tibetan Plateau from global positioning system data. Geology, 2004,32:809-812
[6] Royden L H, Clark B C, King R W, et al. Surface deformation and lower crustal flow in eastern Tibet. Science, 1997,276:788-790
[7] Clark M K, Royden L H. Topographic ooze:Building the eastern margin of Tibet by lower crustal flow. Geology, 2000,33:525-528
[8] Burchfiel B C, Royden L H, van der Hilst R D, et al. A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today , 2008,18(7) :4-11
[9] Huang J, Zhao D, Zheng S. Lithospheric structure and its relationship to seismic and volcanic activity in southwest China. J. Geophys. Res. , 2002,107, doi:10． 1029/2000JB000137
[10] Yao H, Beghein C, van der Hilst R. Surface wave array tomography in SE Tibet from ambient seismic noise and two station analysis-Ⅱ. Crustal and upper-mantle structure. Geophys. J. Int. , 2008,173:205-219
[11] Zhao D, Hasegawa A, Horiuchi S. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res., 1992,97:19909-19928
[12] Wang C, Chan W, Mooney W. Three-dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications. J. Geophys. Res. , 2003 ,108,doi:10． 1029/2002JB001973
[13] Paige C, Saunders M. LSQR: an algorithm for sparse linear equations and sparse least squares. Assoc. Comput. Math. Trans. Math. Soft-ware, 1982,8:43-71
[14] 刘超,张勇,许力生等.一种矩张量反演新方法及其对2008年汶川M_s8． 0地震序列的应用.地震学报,2008,30:329-339 Liu C, Zhang Y, Xu L S, et al. A new technique for moment tensor inversion with applications to the 2008 Wenchuan M_s8． 0 earthquake sequence. Acta Seismol. Sinica, 2008,30(4) :329-339
[15] Lei J, Zhao D. Teleseismic evidence for a break-off subducting slab under Eastern Turkey. Earth Planet. Sci. Lett. , 2007,257:14-28
[16] Sun A, Zhao D, Ikeda M, et al. Seismic imaging of southwest Japan using P and PmP data: Implications for arc magmatism and seismotectonics. Gond wana Res. , 2008, 14:535-542
[17] Burchfiel B C, Chen Z, Liu Y, et al. Tectonics of the Longmen Shan and adjacent regions, central China. Int. Geol. Rev., 1995,37:661-735
[18] Cook K L, Royden L H. The role of crustal strength variations in shaping orogenic plateaus, with application to Tibet. J. Geophys. Res. , 2008, 113,doi:10． 1029/ 2007JB005457
[19] Tapponnier P, Xu Z, Roger F, et al. Oblique stepwise rise and growth of the Tibet Plateau. Science, 2001, 294:1671-1677
[20] Bird P. Lateral extrusion of lower crust from under high topography in the isostatic limit.J. Geophys. Res. , 1991,96:10275-10286
[21] Nelson K D, Zhao W, Brown L D, et al. Partially molten middle crust beneath southern Tibet:Synthesis of Project INDEPTH results. Science, 1996, 274:1684-1688
[22] Shapiro N M, Ritzwoller M H, Molnar P, et al. Thinning and flow of Tibetan crust constrained by seismic anisotropy. Science, 2004,305:233-236
[23] Zhao D, Kanamori H, Negishi H, et al. Tomography of the source area of the 1995 Kobe earthquake: Evidence for fluids at the hypocenter? Science, 1996, 274:1891-1894
[24] Mishra O P, Zhao D. Crack density, saturation rate and porosity at the 2001 Bhuj, India, earthquake hypocenter: A fluid driven earthquake? Earth Planet. Sci. Lett. , 2003,212:393-405
[25] Lei J, Xie F, Lan C, et al. Seismic images under the Beijing region inferred from P and PmP data. Phys. Earth Planet. Inter. , 2008,168:134-146
[26] Sibson R H. Implications of fault-valve behavior for rupture nucleation and recurrence. Tectonophysics , 1992,211:283-293
[27] Hickman S, Sibson R, Bruhn R. Introduction to special section: Mechanical involvement of fluids in faulting. J. Geophys. Res. , 1995,100:12831-12840
[28] 黄媛,吴建平,张天忠等.汶川8级大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. Re-determination of the large Wenchuan earthquake (M8． 0) and its sequence of its aftershocks. Science in China (D) , 2008, 38(10) : 1242-1249
[29] Wessel P, Smith W. New version of the Generic Mapping Tools (GMT) version 3． 0 released. EOS Trans. AGU, 1995,76:329-329
[2] 车用太,刘成龙,鱼金子等.汶川M_s8． 0地震的地下流体与宏观异常及地震预测问题的思考.地震地质,2008,30(4) :828-838 Chen Y T, Liu C L, Yu J Z, et al. Underground fluid anomaly and macro anomaly of M_s 8． 0 Wenchuan earthquake and opinions about earthquake prediction. Seismol. Geol. , 2008,30(4) :828-838
[3] Wang C, Han W, Wu J, et al. Crustal structure beneath the eastern margin of the Tibetan Plateau and its tectonic implications. J. Geophys. Res., 2007,112, doi:10． 1029/2005JB003873
[4] Xu L, Rondenay S, van der Hilst R. Structure of the crust beneath the southeastern Tibetan Plateau from teleseismic receiver functions. Phys. Earth Planet. Inter., 2007,165: 176-193
[5] Zhang P, Shen Z, Wang M, et al. Continuous deformation of the Tibetan Plateau from global positioning system data. Geology, 2004,32:809-812
[6] Royden L H, Clark B C, King R W, et al. Surface deformation and lower crustal flow in eastern Tibet. Science, 1997,276:788-790
[7] Clark M K, Royden L H. Topographic ooze:Building the eastern margin of Tibet by lower crustal flow. Geology, 2000,33:525-528
[8] Burchfiel B C, Royden L H, van der Hilst R D, et al. A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today , 2008,18(7) :4-11
[9] Huang J, Zhao D, Zheng S. Lithospheric structure and its relationship to seismic and volcanic activity in southwest China. J. Geophys. Res. , 2002,107, doi:10． 1029/2000JB000137
[10] Yao H, Beghein C, van der Hilst R. Surface wave array tomography in SE Tibet from ambient seismic noise and two station analysis-Ⅱ. Crustal and upper-mantle structure. Geophys. J. Int. , 2008,173:205-219
[11] Zhao D, Hasegawa A, Horiuchi S. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res., 1992,97:19909-19928
[12] Wang C, Chan W, Mooney W. Three-dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications. J. Geophys. Res. , 2003 ,108,doi:10． 1029/2002JB001973
[13] Paige C, Saunders M. LSQR: an algorithm for sparse linear equations and sparse least squares. Assoc. Comput. Math. Trans. Math. Soft-ware, 1982,8:43-71
[14] 刘超,张勇,许力生等.一种矩张量反演新方法及其对2008年汶川M_s8． 0地震序列的应用.地震学报,2008,30:329-339 Liu C, Zhang Y, Xu L S, et al. A new technique for moment tensor inversion with applications to the 2008 Wenchuan M_s8． 0 earthquake sequence. Acta Seismol. Sinica, 2008,30(4) :329-339
[15] Lei J, Zhao D. Teleseismic evidence for a break-off subducting slab under Eastern Turkey. Earth Planet. Sci. Lett. , 2007,257:14-28
[16] Sun A, Zhao D, Ikeda M, et al. Seismic imaging of southwest Japan using P and PmP data: Implications for arc magmatism and seismotectonics. Gond wana Res. , 2008, 14:535-542
[17] Burchfiel B C, Chen Z, Liu Y, et al. Tectonics of the Longmen Shan and adjacent regions, central China. Int. Geol. Rev., 1995,37:661-735
[18] Cook K L, Royden L H. The role of crustal strength variations in shaping orogenic plateaus, with application to Tibet. J. Geophys. Res. , 2008, 113,doi:10． 1029/ 2007JB005457
[19] Tapponnier P, Xu Z, Roger F, et al. Oblique stepwise rise and growth of the Tibet Plateau. Science, 2001, 294:1671-1677
[20] Bird P. Lateral extrusion of lower crust from under high topography in the isostatic limit.J. Geophys. Res. , 1991,96:10275-10286
[21] Nelson K D, Zhao W, Brown L D, et al. Partially molten middle crust beneath southern Tibet:Synthesis of Project INDEPTH results. Science, 1996, 274:1684-1688
[22] Shapiro N M, Ritzwoller M H, Molnar P, et al. Thinning and flow of Tibetan crust constrained by seismic anisotropy. Science, 2004,305:233-236
[23] Zhao D, Kanamori H, Negishi H, et al. Tomography of the source area of the 1995 Kobe earthquake: Evidence for fluids at the hypocenter? Science, 1996, 274:1891-1894
[24] Mishra O P, Zhao D. Crack density, saturation rate and porosity at the 2001 Bhuj, India, earthquake hypocenter: A fluid driven earthquake? Earth Planet. Sci. Lett. , 2003,212:393-405
[25] Lei J, Xie F, Lan C, et al. Seismic images under the Beijing region inferred from P and PmP data. Phys. Earth Planet. Inter. , 2008,168:134-146
[26] Sibson R H. Implications of fault-valve behavior for rupture nucleation and recurrence. Tectonophysics , 1992,211:283-293
[27] Hickman S, Sibson R, Bruhn R. Introduction to special section: Mechanical involvement of fluids in faulting. J. Geophys. Res. , 1995,100:12831-12840
[28] 黄媛,吴建平,张天忠等.汶川8级大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. Re-determination of the large Wenchuan earthquake (M8． 0) and its sequence of its aftershocks. Science in China (D) , 2008, 38(10) : 1242-1249
[29] Wessel P, Smith W. New version of the Generic Mapping Tools (GMT) version 3． 0 released. EOS Trans. AGU, 1995,76:329-329
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