利用主动震源检测汶川地震余震引起的浅层波速变化
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摘要
为了监测汶川地震后断裂带附近的波速变化,2008年6月,我们利用快速响应探测系统,在龙门山断裂带北端进行了为期3周的连续观测实验。实验使用电动落锤作为主动震源,GPS连续同步授时的高精度数采作为接收系统。利用记录到的高时间精度观测数据,结合互相关时延检测技术,计算了P波和面波走时的相对变化,来表征地下波速的变化。我们观测到两次面波波速的异常变化,变化幅度最高达到2%,远远大于气压变化所能引起的波速变化。结合该时段内地震活动记录,我们认为面波的波速变化是由在附近发生的两次地震事件的同震效应引起的,该观测结果同理论计算结果相符。
Seismic velocity near the fault zone is sensitive to the stress state within that area.After the Wenchuan earthquake,in order to monitor the velocity variation near fault zone,we conducted a field experiment in the north edge of Longmenshan fault,which lasted three weeks since July 6.In the experiment,an electric hammer was used as a repeatable seismic source and the seismic signals were recorded by eight short period seismometers composed of Guralp 40T sensor and Reftek 130B digitizer.The digitizers were timing continuously with GPS to reduce timing error.We calculate the travel time variation by the combination of absolute time and cross correlation time-delay measurement.The fractal velocity variations were obtained from travel time measurement.Two prominent drops up to～2% were found,which were much more than the possible variation caused by barometric pressure change.We notice two moderate aftershocks with magnitude 4.9 and 4.7 occurring within 10km from the experiment field,when the two surface velocity drops showed up.We speculate the surface velocity drops may be caused by the coseismic effects of these two earthquakes.The theoretical calculation of the stress drop agrees with our measurement.
Seismic velocity near the fault zone is sensitive to the stress state within that area.After the Wenchuan earthquake,in order to monitor the velocity variation near fault zone,we conducted a field experiment in the north edge of Longmenshan fault,which lasted three weeks since July 6.In the experiment,an electric hammer was used as a repeatable seismic source and the seismic signals were recorded by eight short period seismometers composed of Guralp 40T sensor and Reftek 130B digitizer.The digitizers were timing continuously with GPS to reduce timing error.We calculate the travel time variation by the combination of absolute time and cross correlation time-delay measurement.The fractal velocity variations were obtained from travel time measurement.Two prominent drops up to～2% were found,which were much more than the possible variation caused by barometric pressure change.We notice two moderate aftershocks with magnitude 4.9 and 4.7 occurring within 10km from the experiment field,when the two surface velocity drops showed up.We speculate the surface velocity drops may be caused by the coseismic effects of these two earthquakes.The theoretical calculation of the stress drop agrees with our measurement.
引文
陈颙、王宝善、葛洪魁,2007,建立地震发射台的建议,地球科学进展,22(5),441～446.
黄伟传、葛洪魁、杨微、宋丽莉,2009,汶川地震断裂带东北端浅部结构的人工地震探测,地球物理学报,52(2),547～552.
林建民、王宝善、葛洪魁等,2008,大容量气枪震源特征及深部介质中传播的震相分析,地球物理学报,51(1),206～212.
罗桂纯、葛洪魁、王宝善等,2006,气枪震源在地球深部结构探测中的应用研究进展,地球物理学进展,21(2),400～407.
丘学林、陈颙、朱日祥,2007,大容量气枪震源在海陆联测中的应用:南海北部试验结果分析,科学通报,52(4),463～469.
唐杰、王宝善、葛洪魁、陈颙,2009,大容量气枪震源的实验与模拟研究,中国地震,25(1),1～10。
郑秀芬、欧阳飚、张东宁等,2009,“国家数字测震台网数据备份中心”技术系统建设及其对汶川大地震研究的数据支撑,地球物理学报,2009,52(5),1412～1417
Aggarwal Y.P.,Sykes L.R.,Simpson D.W.et al.,1975,Spatial andtemporal variations ints/tpandin P wave residuals at BlueMountain Lake,NewYork:Applicationto earthquake prediction.J.Geophys.Res.,80,718～732.
Aggarwal Y.P.,Sykes L.R.,Armbruster J.,et al.,1973,Premonitory changesin seismic velocities and prediction of earthquakes,Nature,241,101～104.
Aki K.,Richards P.G.,2002,Quantitive Seismology(2nd edition).
Aoi S.,Kunugi T.and Fujiwara H.,2008,Trampoline effect in extreme ground motion.Science,322,727～730.
Birch F.,1960,The velocity of compressional waves in rocks to10kilobars,part1.J.Geophys.Res.,65,1083～1102.
Birch F.,1961,The velocity of compressional waves in rocks to10kilobars,part2.J.Geophys.Res.,66,2199～2224.
Borcherdt R.D.,MalcolmJ.,Johnston S.,et al.,2006,Recordings of the2004Parkfield earthquake on the general earthquakeobservation system array:Implications for earthquake precursors,fault rupture and coseismic strain changes,Bulletin of theSeismological Society of America,96(4b),S73～S89.
Brodsky E.E,Karakostas V.,Kanamori H.,2000,Anewobservationof dynamicallytriggeredregional seismieity:Earthquakesin Greecefollowingthe August,1999Izmit,Turkey earthquake,Geophys Res Letter,27,2741～2744.
Carter G.,1987,Coherence andtime delay estimation.Proceedings of the IEEE,75,236～255.
Cspedes I.,Huang Y,Ophir J.1995,Methodsfor estimation of subsample time delays of digitized echo signals.Ultrasonic Imaging,17,142～171.
De-Fazio T.L.,Aki K.,and Alba J.,1973,Solid earthtide and observed change inthe insitu seismic velocity.J.Geophys.Res.,78,1319～1322.
Huenges E.,Erzinger J.,Kuck J.,et al.,1997,The permeable crust:Geohydraulic properties downto9101mdepth.J.Geophys.Res.,102,18255～18265.
Kaoru Sawazaki,Haruo Sato,Hisashi Nakahara,et al.,2009,Time-lapse changes of seismic velocityinthe shallowground caused bystrong ground motion shock of the2000Western-Tottori Earthquake,Japan as revealed fromcoda deconvolution Analysis.Bulletin ofthe Seismological Society of America,99(1),352～366.
Leary P.C.,Malin P.E.,Phinney R.A.,et al.,1979,Systematic monitoring of millisecondtravel time variations near Palmdale,California,J.Geophys.Res.,84,659～666.
Li Y.G.,Chen P.,Cochran E.S.,et al.,2007,Seismic velocity variations on the San Andreas fault caused by the2004M6Parkfield Earthquake andtheir implications,Earth Planets and Space,59,21～31.
Niu F.,Silver P.G.,Daley T.M.,et al.,2008,Preseismic velocity changes observedfromactive source monitoring at the ParkfieldSAFODdrill site,Nature,454,204～208.
Nur A.and Simmons G.,1969,The effect of saturationonvelocityinlowporosityrocks.Earthand Planetary Science Letters,7(2),183～193.
Nur A.,1971,Effects of stress on velocity anisotropyin rocks with cracks.J.Geophys.Res.,76,2022～2034.
Nur A.,1972,Dilatancy pore fluids and premonitory variations of ts/tptravel times,Bulletin of the Seismological Society of America,62(5),1217～1222.
O Connell R.J.and Budiansky B.,1974,Seismic velocitiesin dry and saturated cracked solids.J.Geophys.Res.,79,5412～5426.
Reasenberg P.and Aki K.,1974,Aprecise continuous measurement of seismic velocity for monitoring in situ stress.J.Geophys.Res.,79,399～406.
Savarensky E.F.1968,Onthe prediction of earthquakes,Tectonophysics,6,17～27.
Schaff D.P.and Beroza G.C.,2004,Coseismic and postseismic velocity changes measured by repeating earthquakes,J.Geophys.Res.,109,B10302.
Scholz C.H.,1968,Microfracturing andthe inelastic deformation of rockin compression.J.Geophys.Res.,73,1417～1432.
Semenov A.N.,1969,Variationinthe travel time of transverse and longitudinal waves before violent earthquakes,Bull.Acad.Sci.USSR,Phys.Solid Earth,3,245～248.
Silver P.G.,Daley T.M.,Niu F.,et al.,2007,Active source monitoring of cross-well seismic travel time for stress-inducedchanges.Bulletin of the Seismological Society of America,97,281～293.
Walker and Trahey G.,1995,A fundamental limit on delay estimation using partially correlated speckle signals.Ultrasonics,Ferroelectrics and Frequency Control,IEEE Transactions,42(2),301～308.
Walsh J.B.,1965,The effect of cracks onthe compressibility of rock.J.Geophys.Res.,70,381～389.
Wang B.S.,Zhu P.,Chen Y.,et al.,2008,Continuous subsurface velocity measurement withcoda waveinterferometry,J.Geophys.Res.,113,B2313.
Yamamura K.,Sano O.,Utada H.,et al.,2003,Long-termobservation of in situ seismic velocity and attenuation,J.Geophys.Res.,108(B6),2317.
Yukutake H.,Nakajima T.and Doi K.,1988,In situ measurements of elastic wave velocity in a mine,and the effects of water andstress ontheir variation,Tectonophysics,149,165～175.
黄伟传、葛洪魁、杨微、宋丽莉,2009,汶川地震断裂带东北端浅部结构的人工地震探测,地球物理学报,52(2),547～552.
林建民、王宝善、葛洪魁等,2008,大容量气枪震源特征及深部介质中传播的震相分析,地球物理学报,51(1),206～212.
罗桂纯、葛洪魁、王宝善等,2006,气枪震源在地球深部结构探测中的应用研究进展,地球物理学进展,21(2),400～407.
丘学林、陈颙、朱日祥,2007,大容量气枪震源在海陆联测中的应用:南海北部试验结果分析,科学通报,52(4),463～469.
唐杰、王宝善、葛洪魁、陈颙,2009,大容量气枪震源的实验与模拟研究,中国地震,25(1),1～10。
郑秀芬、欧阳飚、张东宁等,2009,“国家数字测震台网数据备份中心”技术系统建设及其对汶川大地震研究的数据支撑,地球物理学报,2009,52(5),1412～1417
Aggarwal Y.P.,Sykes L.R.,Simpson D.W.et al.,1975,Spatial andtemporal variations ints/tpandin P wave residuals at BlueMountain Lake,NewYork:Applicationto earthquake prediction.J.Geophys.Res.,80,718～732.
Aggarwal Y.P.,Sykes L.R.,Armbruster J.,et al.,1973,Premonitory changesin seismic velocities and prediction of earthquakes,Nature,241,101～104.
Aki K.,Richards P.G.,2002,Quantitive Seismology(2nd edition).
Aoi S.,Kunugi T.and Fujiwara H.,2008,Trampoline effect in extreme ground motion.Science,322,727～730.
Birch F.,1960,The velocity of compressional waves in rocks to10kilobars,part1.J.Geophys.Res.,65,1083～1102.
Birch F.,1961,The velocity of compressional waves in rocks to10kilobars,part2.J.Geophys.Res.,66,2199～2224.
Borcherdt R.D.,MalcolmJ.,Johnston S.,et al.,2006,Recordings of the2004Parkfield earthquake on the general earthquakeobservation system array:Implications for earthquake precursors,fault rupture and coseismic strain changes,Bulletin of theSeismological Society of America,96(4b),S73～S89.
Brodsky E.E,Karakostas V.,Kanamori H.,2000,Anewobservationof dynamicallytriggeredregional seismieity:Earthquakesin Greecefollowingthe August,1999Izmit,Turkey earthquake,Geophys Res Letter,27,2741～2744.
Carter G.,1987,Coherence andtime delay estimation.Proceedings of the IEEE,75,236～255.
Cspedes I.,Huang Y,Ophir J.1995,Methodsfor estimation of subsample time delays of digitized echo signals.Ultrasonic Imaging,17,142～171.
De-Fazio T.L.,Aki K.,and Alba J.,1973,Solid earthtide and observed change inthe insitu seismic velocity.J.Geophys.Res.,78,1319～1322.
Huenges E.,Erzinger J.,Kuck J.,et al.,1997,The permeable crust:Geohydraulic properties downto9101mdepth.J.Geophys.Res.,102,18255～18265.
Kaoru Sawazaki,Haruo Sato,Hisashi Nakahara,et al.,2009,Time-lapse changes of seismic velocityinthe shallowground caused bystrong ground motion shock of the2000Western-Tottori Earthquake,Japan as revealed fromcoda deconvolution Analysis.Bulletin ofthe Seismological Society of America,99(1),352～366.
Leary P.C.,Malin P.E.,Phinney R.A.,et al.,1979,Systematic monitoring of millisecondtravel time variations near Palmdale,California,J.Geophys.Res.,84,659～666.
Li Y.G.,Chen P.,Cochran E.S.,et al.,2007,Seismic velocity variations on the San Andreas fault caused by the2004M6Parkfield Earthquake andtheir implications,Earth Planets and Space,59,21～31.
Niu F.,Silver P.G.,Daley T.M.,et al.,2008,Preseismic velocity changes observedfromactive source monitoring at the ParkfieldSAFODdrill site,Nature,454,204～208.
Nur A.and Simmons G.,1969,The effect of saturationonvelocityinlowporosityrocks.Earthand Planetary Science Letters,7(2),183～193.
Nur A.,1971,Effects of stress on velocity anisotropyin rocks with cracks.J.Geophys.Res.,76,2022～2034.
Nur A.,1972,Dilatancy pore fluids and premonitory variations of ts/tptravel times,Bulletin of the Seismological Society of America,62(5),1217～1222.
O Connell R.J.and Budiansky B.,1974,Seismic velocitiesin dry and saturated cracked solids.J.Geophys.Res.,79,5412～5426.
Reasenberg P.and Aki K.,1974,Aprecise continuous measurement of seismic velocity for monitoring in situ stress.J.Geophys.Res.,79,399～406.
Savarensky E.F.1968,Onthe prediction of earthquakes,Tectonophysics,6,17～27.
Schaff D.P.and Beroza G.C.,2004,Coseismic and postseismic velocity changes measured by repeating earthquakes,J.Geophys.Res.,109,B10302.
Scholz C.H.,1968,Microfracturing andthe inelastic deformation of rockin compression.J.Geophys.Res.,73,1417～1432.
Semenov A.N.,1969,Variationinthe travel time of transverse and longitudinal waves before violent earthquakes,Bull.Acad.Sci.USSR,Phys.Solid Earth,3,245～248.
Silver P.G.,Daley T.M.,Niu F.,et al.,2007,Active source monitoring of cross-well seismic travel time for stress-inducedchanges.Bulletin of the Seismological Society of America,97,281～293.
Walker and Trahey G.,1995,A fundamental limit on delay estimation using partially correlated speckle signals.Ultrasonics,Ferroelectrics and Frequency Control,IEEE Transactions,42(2),301～308.
Walsh J.B.,1965,The effect of cracks onthe compressibility of rock.J.Geophys.Res.,70,381～389.
Wang B.S.,Zhu P.,Chen Y.,et al.,2008,Continuous subsurface velocity measurement withcoda waveinterferometry,J.Geophys.Res.,113,B2313.
Yamamura K.,Sano O.,Utada H.,et al.,2003,Long-termobservation of in situ seismic velocity and attenuation,J.Geophys.Res.,108(B6),2317.
Yukutake H.,Nakajima T.and Doi K.,1988,In situ measurements of elastic wave velocity in a mine,and the effects of water andstress ontheir variation,Tectonophysics,149,165～175.
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