大陆板内地震的发震机理与地震预报——以汶川地震为例
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摘要
大陆板内地震主要产于新生代厚壳造山带或高原,在平面上呈弥散状分布,在剖面上震源沿中地壳成层分布,为浅源地震。盆山活动断层系统呈规律性组合,盆山挤压边界为逆冲型压性发震断层;盆山走滑转换边界为走滑型扭性发震断层;造山带内部主要是伸展型张性发震断层。大陆板内地壳分层流变作用制约了板内地震的构造物理过程。大陆下地壳韧性流层为地震活动提供了热能,热软化和热融化介质发生缓慢的韧性流动,是孕震构造;中地壳韧-脆性剪切带发生热—应力转换,聚积应力和应变,为蕴震构造;当下地壳流动在中地壳积累的应变超过上地壳特定构造部位介质的应力-应变极限时,上地壳形成脆性发震断层,产生地震。震源出现在上地壳脆性断层与中地壳脆-韧性剪切带的交汇部位,青藏高原的震源深度通常为12~35 km。目前地震预报是世界科学难题,然而,大陆动力学和板内地震的理论突破可能为短临预报提供了新思路。如果大陆下地壳热动力作用是中地壳产生地震和上地壳发生地震的根源,那么上地壳脆性断层活动会释放出地壳深部的热流体和热气体,引起局部的地温异常、水文异常和大气异常。建议从大气到地表再到地下系统地监测活动断层带及邻区的地下水、地表水、大气的温度异常和成分变化,结合观测下地壳流层厚度、地应力-应变变化、重力异常、磁异常、地电异常等,综合评价地壳活动性和发震可能性。2008年5月12日发生的里氏8.0级汶川地震处于龙门山造山带与四川盆地的构造边界上,是典型的大陆板内地震,震源处于映秀-北川断层上,震源深度为12 km。350 km长的地表破裂带呈右行左阶雁行排列在具有逆冲和右行走滑性质的汶川-茂县-青川、映秀-北川和江油-都江堰3条断层带上。下地壳的韧性流动伴随中地壳韧-脆性剪切带应力和应变的积累,产生上地壳脆性活动断层,并控制地表破裂带和滑坡的分布。
The Wenchuan earthquake is a typical intraplate earthquake.Continental intraplate earthquakes are mainly disseminated in the Cenozoic thick-crust orogenic belt or plateau,and distribute in the middle crust as shallow source earthquake in the section.The active faults in continental basin-mountain system exhibit regular pattern,such as reverse or thrust faults on the basin-mountain extrusion boundary,strike-slip faults on the basin-mountain transform boundary,or normal and detachment faults in the extensional orogenic zone interior.While the tectonophysic process of continental intraplate earthquakes is generally controlled by the lamination and rheology of continental crust.Continental lower crust provides heat energy for earthquake,and thermal softening and thermal melting media in lower crust occur slow ductile flow as causative structure.The ductile-brittle transition zone in middle crust appears heat-stress transformation,and accumulate stress and strain.The brittle seismogenic faults in upper crust result from stress and strain accumulation in middle crust caused by lower crustal flow.Seismic source locates at the connection spot between the ductile-brittle shear zone in middle crust and brittle active faults in upper crust,while the focus depth in the Qinghai-Tibet plateau is mostly ound from 12 to 35 km.Earthquake predictioni is a world scientific problem.However,new theory of continental dynamics and intraplate earthquake based on scientific data has probabilities for short-term and imminent prediction.If lower crustal flow causes seismic source of middle crust and brittle fracture of upper crust,the brittle faults of upper crust can release thermal fluid and gas from deep crust,which results in the geothermal anomaly,hydrologic anomaly and atmosphere anomaly in the local areas.It is suggested that thermal anomaly and ingredient variation of underground water,ground water,and atmosphere are systematically monitored along active fault zone and adjacent areas,and activity of crust and probability of earthquake can be appraised according to above thermal anomaly and relative data such as thickness of lower crustal flow layer,stress-strain change,the gravity anomaly,magnetism anomaly and terrestrial electricity anomaly.The Wenchuan earthquake of May 12th,2008 on the tectonic margin between the Longmenshan orogenic belt and Sichuan Basin had a magnitude of 8.0 Ms,with a depth of about 12 km on the Yingxiu-Beichuan fault.A 350 km-long surface rupture zone exhibits right-lateral and left-step features,related to the west-dip Wenchuan-Maoxian-Qingchuan fault,Yingxiu-Beichuan fault and the Jiangyou-Dujiangyan fault shown dextral strike-slip and thrust.The brittle active faults in the upper crust,which control surface rupture zones and landslides,were produced by slowly ductile flow of lower crust with the stress and strain accumulation at the ductile-brittle shear zone of the middle crust.The brttle seismogenic faults may release some heat energy from the lower crust and cause the local thermal anomaly of underground water,ground water and atmosphere.
The Wenchuan earthquake is a typical intraplate earthquake.Continental intraplate earthquakes are mainly disseminated in the Cenozoic thick-crust orogenic belt or plateau,and distribute in the middle crust as shallow source earthquake in the section.The active faults in continental basin-mountain system exhibit regular pattern,such as reverse or thrust faults on the basin-mountain extrusion boundary,strike-slip faults on the basin-mountain transform boundary,or normal and detachment faults in the extensional orogenic zone interior.While the tectonophysic process of continental intraplate earthquakes is generally controlled by the lamination and rheology of continental crust.Continental lower crust provides heat energy for earthquake,and thermal softening and thermal melting media in lower crust occur slow ductile flow as causative structure.The ductile-brittle transition zone in middle crust appears heat-stress transformation,and accumulate stress and strain.The brittle seismogenic faults in upper crust result from stress and strain accumulation in middle crust caused by lower crustal flow.Seismic source locates at the connection spot between the ductile-brittle shear zone in middle crust and brittle active faults in upper crust,while the focus depth in the Qinghai-Tibet plateau is mostly ound from 12 to 35 km.Earthquake predictioni is a world scientific problem.However,new theory of continental dynamics and intraplate earthquake based on scientific data has probabilities for short-term and imminent prediction.If lower crustal flow causes seismic source of middle crust and brittle fracture of upper crust,the brittle faults of upper crust can release thermal fluid and gas from deep crust,which results in the geothermal anomaly,hydrologic anomaly and atmosphere anomaly in the local areas.It is suggested that thermal anomaly and ingredient variation of underground water,ground water,and atmosphere are systematically monitored along active fault zone and adjacent areas,and activity of crust and probability of earthquake can be appraised according to above thermal anomaly and relative data such as thickness of lower crustal flow layer,stress-strain change,the gravity anomaly,magnetism anomaly and terrestrial electricity anomaly.The Wenchuan earthquake of May 12th,2008 on the tectonic margin between the Longmenshan orogenic belt and Sichuan Basin had a magnitude of 8.0 Ms,with a depth of about 12 km on the Yingxiu-Beichuan fault.A 350 km-long surface rupture zone exhibits right-lateral and left-step features,related to the west-dip Wenchuan-Maoxian-Qingchuan fault,Yingxiu-Beichuan fault and the Jiangyou-Dujiangyan fault shown dextral strike-slip and thrust.The brittle active faults in the upper crust,which control surface rupture zones and landslides,were produced by slowly ductile flow of lower crust with the stress and strain accumulation at the ductile-brittle shear zone of the middle crust.The brttle seismogenic faults may release some heat energy from the lower crust and cause the local thermal anomaly of underground water,ground water and atmosphere.
引文
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[14]吴立新,刘善军,陈云浩,等.汶川地震前卫星热红外异常与云异常现象[J].科技导报,2008,(10):32-36.
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[17]廖文根,赵永新.中国科学院第十四次院士大会、中国工程院第九次院士大会在京隆重开幕——胡锦涛出席并发表重要讲话[N].人民日报,2008-06-24(1-2).
[2]罗文行,李德威,汪校锋.青藏高原板内地震震源深度分布规律及其成因[J].地球科学:中国地质大学学报,2008,33(5):618-626.
[3]李德威.大陆构造样式及大陆动力学模式初探[J].地球科学进展,1993,8(5):88-93.
[4]李德威.再论大陆构造与动力学[J].地球科学:中国地质大学学报,1995,20(1):19-26.
[5]李德威.大陆下地壳流动——层流还是渠流?[J].地学前缘,2008,15(3):130-139.
[6]Royden L H,Burchifiel B C,King R W,et al.Surface de-formation and lower crustal flow in eastern Tibet[J].Science,1997,276:788-790.
[7]Clark M K,Royden L H.Topographic ooze:Building the east-ern margin of Tibet by lower crustal flow[J].Geology,2000,28:703-706.
[8]Enkelmann E,Ratschbacher L,Jonckheere R,et al.Cenozoicexhumation and deformation of northeastern Tibet and theQinling:Is Tibetan lower crustal flow diverging around the Si-chuan Basin?[J].Geological Society of America Bulletin,2006,118:651-671.
[9]Schoenbohm L M,Burchfiel B C,Liangzhong C.Propagationof surface uplift,lower crustal flow,and Cenozoic tectonics ofthe southeast margin of the Tibetan Plateau[J].Geology,2006,34(10):813-816.
[10]Burchfiel B,Royden L,van der Hilst R,et al.A geologicaland geophysical context for the Wenchuan earthquake of 12May 2008,Sichuan,People's Republic of China[J].GSA To-day,2008,18(7):4-11.
[11]王椿镛,楼海,吕智勇,等.青藏高原东部地壳上地幔S波速度结构—下地壳流的深部环境[J].中国科学:D辑,2008,38(1):22-32.
[12]Geller R J,Kagan Y Y,Kagan Y Y.Earthquakes cannot bepredicted[J].Science,1997,275:1 616-1 617.
[13]强祖基,赁常恭,李玲芝,等.卫星热红外图像亮温异常——短临震兆[J].中国科学:D辑,1998,26(6):564-573.
[14]吴立新,刘善军,陈云浩,等.汶川地震前卫星热红外异常与云异常现象[J].科技导报,2008,(10):32-36.
[15]马未宇.汶川地震前的NCEP异常现象[J].科技导报,2008,(10):37-39.
[16]谌达军,曹南燕.井水突升突降,江水忽冷忽热——汶川地震有前兆[N].长江日报,2008-06-25(5).
[17]廖文根,赵永新.中国科学院第十四次院士大会、中国工程院第九次院士大会在京隆重开幕——胡锦涛出席并发表重要讲话[N].人民日报,2008-06-24(1-2).
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