数值模型显示的隐伏弯曲断层同震地表位移特征
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摘要
余震精定位资料显示,芦山7.0级地震破裂面可能为弯曲程度较高的三维弯曲断层。相关研究显示,这种弯曲断层的位错方式和破裂面同震应力加卸载模式与普通平直断层有明显不同。文中采用无限半空间位错模型模拟显示,隐伏弯曲断层和平直逆断层引起的地表位移特征相似,但是弯曲断层引起的地表水平位移更接近区域整体的地壳缩短方向,缩短方向水平位移的量明显高于同等规模的平直逆断层,因此能更好地传递断层上盘大范围物质的水平运动。相对于平直断层,弯曲断层下盘水平位移随距离衰减十分明显。同等规模的弯曲断层导致的同震地表抬升小于平直逆断层或左旋逆断层引起的同震抬升,但能造成更明显的地表下降。由于地震规模较小,GPS等低密度空间分布的形变观测可能无法有效分辨芦山地震震源结构是否为弯曲断层。对震源结构的细节研究,还有待于利用高空间密度和高分辨率的形变观测资料。
A possible three-dimensionally highly-curved fault,suspected as the ruptured structure of the Lushan M7. 0 earthquake,is revealed by relocated aftershocks. A recent study shows that obvious differences exist between curved fault and straight fault under the ground in regard to dislocation patterns and co-seismic stress responses on the planes ruptured during an earthquake. Infinite halfspace dislocation models reveal that the characters of surface displacements due to a curved fault are similar to that from straight reverse fault as a whole. Nevertheless,the horizontal displacements due to slip on a curved fault show closer trend parallel to the direction of regional shortening and higher magnitude than that from a straight fault. Subsequently,the curved fault is suggested to be more capable of transferring horizontal movement of hanging-wall materials in large area. Relative to the case on a straight fault,horizontal displacement in foot-wall area of a curved fault decays more with distance from source fault. On the other hand,the curved fault generates obviously less co-seismic uplift while larger and more extensive surface drop somewhere than the reverse fault or left-lateral reverse fault of the same size but with straight fault planes does. For relatively small magnitude of main shock,it is not easy to determine whether the structure of rupture during Lushan earthquake is highly-curved fault or not due to the sparse observations on co-seismic deformation like GPS. Dense and high-resolution observations should be required to survey the features of focal structure in detail.
A possible three-dimensionally highly-curved fault,suspected as the ruptured structure of the Lushan M7. 0 earthquake,is revealed by relocated aftershocks. A recent study shows that obvious differences exist between curved fault and straight fault under the ground in regard to dislocation patterns and co-seismic stress responses on the planes ruptured during an earthquake. Infinite halfspace dislocation models reveal that the characters of surface displacements due to a curved fault are similar to that from straight reverse fault as a whole. Nevertheless,the horizontal displacements due to slip on a curved fault show closer trend parallel to the direction of regional shortening and higher magnitude than that from a straight fault. Subsequently,the curved fault is suggested to be more capable of transferring horizontal movement of hanging-wall materials in large area. Relative to the case on a straight fault,horizontal displacement in foot-wall area of a curved fault decays more with distance from source fault. On the other hand,the curved fault generates obviously less co-seismic uplift while larger and more extensive surface drop somewhere than the reverse fault or left-lateral reverse fault of the same size but with straight fault planes does. For relatively small magnitude of main shock,it is not easy to determine whether the structure of rupture during Lushan earthquake is highly-curved fault or not due to the sparse observations on co-seismic deformation like GPS. Dense and high-resolution observations should be required to survey the features of focal structure in detail.
引文
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Shen Zheng-kang,Sun Jian-bao,Zhang Pei-zhen,et al.2009.Slip maxima at fault junctions and rupturing of barriersduring the2008Wenchuan earthquake[J].Nature Geoscience,2:718—724.
Toda S,Stein R S,Richards-Dinger K,et al.2005.Forecasting the evolution of seismicity in southern California:Animations built on earthquake stress transfer[J].Journal Geophysical Research,110:B05S16.doi:10.1029/2004JB003415.
Wells D,Coppersmith K J.1994.New empirical relationships among magnitude,rupture length,rupture width,rupturearea,and surface displacement[J].Bulletin of Seismological Society of America,84(4):974—1002.
房立华,吴建平,王未来,等.2013.四川芦山MS7.0地震及其余震序列重定位研究[J].科学通报,待刊.FANG Li-hua,WU Jian-ping,WANG Wei-lai,et al.2013.Study on relocation of Lushan MS7.0earthquake,Sichuanand its aftershock sequence[J].Chinese Science Bulletin,(in press)(in Chinese).
王卫民,郝金来,姚振兴.2013.2013年4月20日四川芦山地震震源破裂过程反演初步结果[J].地球物理学报,56(4):1412—1417.WANG Wei-min,HAO Jin-lai,YAO Zhen-xing.2013.Preliminary result for rupture process of Apr.20,2013,Lushanearthquake,Sichuan China[J].Chinese Journal of Geophysics,56(4):1412—1417(in Chinese).
王卫民,赵连峰,李娟,等.2008.四川汶川8.0级地震震源过程[J].地球物理学报,51(5):1403—1410.WANG Wei-min,ZHAO Lian-feng,LI Juan,et al.2008Rupture process of the MS8.0Wenchuan earthquake ofSichuan,China[J].Chinese Journal of Geophysics,51(5):1403—1410(in Chinese).
徐锡伟,韩竹军,李传友,等.2013.四川芦山7.0级强震:一次典型的盲逆断层型地震[J].科学通报,待刊.XU Xi-wei,HAN Zhu-jun,LI Chuan-you,et al.2013.Lushan MS7.0earthquake:A blind reserve fault earthquake[J].Chinese Science Bulletin,(in press)(in Chinese).
曾祥方,罗艳,韩立波,等.2013.2013年4月20日四川芦山MS7.0地震:一个高角度逆冲地震[J].地球物理学报,56(4):1418—1424.ZENG Xiang-fang,LUO Yan,HAN Li-bo,et al.2013.The Lushan MS7.0earthquake on20April2013:A high-anglethrust event[J].Chinese Journal of Geophysics,56(4):1418—1424(in Chinese).
张勇,冯万鹏,许力生,等.2008.2008年汶川大地震的时空破裂过程[J].中国科学(D辑),38(10):1186—1194.ZHANG Yong,FENG Wan-peng,XU Li-sheng,et al.2008.The space-time of rupture process in Wenchuan earthquakein2008[J].Science in China(Ser D),38(10):1186—1194(in Chinese).
张竹琪,王伟涛,任治坤,等.2013.芦山7.0级地震:特殊的弯曲断层地震[J].科学通报,待刊.ZHANG Zhu-qi,WANG Wei-tao,REN Zhi-kun,et al.2013.Lushan M7.0earthquake:A special earthquake occurringon a curved fault[J].Chinese Science Bulletin,(in press)(in Chinese).
刘成利,郑勇,葛粲,等.2013.2013年芦山7.0级地震的动态破裂过程[J].中国科学(D辑),43(6):1020—1026.LIU Cheng-li,ZHENG Yong,GE Can,et al.2013.Dynamic rupture process of Lushan MS7.0earthquake,2013[J].Science in China(Ser D),43(6):1020—1026(in Chinese).
中国地震局地球物理研究所.2013.2013年4月20日四川芦山7.0级地震(1.0版).http:∥www.cea-igp.ac.cn/jjxcdsxj/266826.shtml.Institute of Geophysics,China Earthquake Administration.2013.The M7.0Lushan,Sichuan earthquake on April20,2013(Version1.0).http:∥www.cea-igp.ac.cn/jjxcdsxj/266826.shtml.
Ji C,Hayes G.2008.Preliminary result of the May12,2008MW7.9eastern Sichuan,China earthquake.http:∥earthquake.usgs.gov/eqcenter/eqinthenews/2008/us2008ryan/finite_fault.php.
Lin J,Stein R S.2004.Stress triggering in thrust and subduction earthquakes,and stress interaction between thesouthern San Andreas and nearby thrust and strike-slip faults[J].Journal of Geophysical Research,109:B02303.doi:10.1029/2003JB002607.
McCaffrey R.2005.Block kinematics of the Pacific-North America plate boundary in the southwestern United Statesfrom inversion of GPS,seismological,and geologic data[J].Journal of Geophysical Research,110:B07401.
Meade B J,Hager B H.2005.Block models of crustal motion in southern California constrained by GPS measurements[J].Journal of Geophysical Research,110:B03403.
Okada Y.1992.Internal deformation due to shear and tensile faults in a half-space[J].Bulletin of SeismologicalSociety of America,82(2):1016—1040.
Reid R.1910.The mechanics of the earthquake,the California earthquake of April18,1906[R].Report of the State Investigation Commission,vol2,Carnegie Institution of Washington,Washington D C.
Savage J C,Gan Wei-jun,Svarc J L.2001.Strain accumulation and rotation in the Eastern California Shear Zone[J].Journal of Geophysical Research,106(B10):21995—22007.
Shen Zheng-kang,Sun Jian-bao,Zhang Pei-zhen,et al.2009.Slip maxima at fault junctions and rupturing of barriersduring the2008Wenchuan earthquake[J].Nature Geoscience,2:718—724.
Toda S,Stein R S,Richards-Dinger K,et al.2005.Forecasting the evolution of seismicity in southern California:Animations built on earthquake stress transfer[J].Journal Geophysical Research,110:B05S16.doi:10.1029/2004JB003415.
Wells D,Coppersmith K J.1994.New empirical relationships among magnitude,rupture length,rupture width,rupturearea,and surface displacement[J].Bulletin of Seismological Society of America,84(4):974—1002.
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