地壳速度结构对极浅源地震深度反演的影响——以荣昌地震为例
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摘要
研究表明,大陆地震很少发生在距地表5km以浅,然而,近年来四川、重庆等地区发生了一系列3km以浅的极浅源地震,据推测可能与新生断层形成或者工业开采活动有关。如果仅凭直达体波震相到时信息,要较好测定极浅源地震深度,需以震中距在2倍震源深度范围内有台站为前提,而目前的台站密度显然不足。在这种情况下,地震的波形信息可为深度测定提供更有效地约束。目前已经广泛采用的CAP(Cut and Paste)方法利用了地震波形信息反演震源机制解、震级和深度,此方法对速度模型虽然没有严格要求,但是速度模型误差太大时不利于地震深度定位。本文以2010年9月10日荣昌地区发生的Mb4.7地震为例,结合远震深度震相约束震源深度,测试了一系列地壳速度模型对CAP方法反演极浅源地震深度的影响。研究发现浅层速度模型误差达10%时,震源深度反演误差不大,但当误差超过50%时,震源深度误差则很大。因此在研究极浅源地震时,需要对浅层速度结构进行较为准确的测定。
As most mature earthquake faults are filled with gouge at shallow part and it's difficult to accumulate pressure,continental earthquakes hardly occurred shallower than 5km in depth.However,several very shallow earthquakes at a depth of about 3km occurred in Sichuan and Chongqing in recent years,which might be related to the formation of new faults or industrial activities. Generally speaking,to locate focal depth accurately using arrival time of direct body waves,stations within the epicentral distance of two times the focal depth are needed. Thus,by now the density of station is not enough to locate the focal depth of very shallow earthquakes( shallower than 5km). Under this circumstance,utilizing the earthquake waveform is very useful to constrain the focal depth. CAP( Cut and Paste) is a method widely used to invert earthquake focal mechanism,magnitude and depth using earthquake waveform. Although this method is not strict with accuracy of crustal velocity structure model,large deviation of the velocity structure model can lead to bad focal depth inversion result. We took the Rongchang earthquake as an example,which happened on September 10,2010 with Mb4. 7,to test the accuracy of focal depth inversion using a series of velocity structure models,with the teleseismic depth phases as a constraint. We found that the deviation of focal depth inversion is small if the deviation of velocity structure model is within 10%. Nevertheless,the deviation is large if the shallow part of velocity structure model has a deviation of about 50%. Thus,it is necessary to ensure the accuracy of shallow part of velocity structure model to locate focal depth of a very shallow earthquake.
As most mature earthquake faults are filled with gouge at shallow part and it's difficult to accumulate pressure,continental earthquakes hardly occurred shallower than 5km in depth.However,several very shallow earthquakes at a depth of about 3km occurred in Sichuan and Chongqing in recent years,which might be related to the formation of new faults or industrial activities. Generally speaking,to locate focal depth accurately using arrival time of direct body waves,stations within the epicentral distance of two times the focal depth are needed. Thus,by now the density of station is not enough to locate the focal depth of very shallow earthquakes( shallower than 5km). Under this circumstance,utilizing the earthquake waveform is very useful to constrain the focal depth. CAP( Cut and Paste) is a method widely used to invert earthquake focal mechanism,magnitude and depth using earthquake waveform. Although this method is not strict with accuracy of crustal velocity structure model,large deviation of the velocity structure model can lead to bad focal depth inversion result. We took the Rongchang earthquake as an example,which happened on September 10,2010 with Mb4. 7,to test the accuracy of focal depth inversion using a series of velocity structure models,with the teleseismic depth phases as a constraint. We found that the deviation of focal depth inversion is small if the deviation of velocity structure model is within 10%. Nevertheless,the deviation is large if the shallow part of velocity structure model has a deviation of about 50%. Thus,it is necessary to ensure the accuracy of shallow part of velocity structure model to locate focal depth of a very shallow earthquake.
引文
陈伟文、倪四道、汪贞杰等,2012,2010年高雄地震震源参数的近远震波形联合反演,地球物理学报,55(7),2319~2328。
崇加军、倪四道、曾祥方,2010,sP L,一个近距离确定震源深度的震相,地球物理学报,11(53),2620~2630。
丁丹、倪四道、田晓峰等,2010,地震相关的声音现象进展研究,华南地震,30(2),46~53。
房明山、杜安陆、董孝平等,1995,用sP n震相测定近震震源深度,地震地磁观测与研究,16(5),13~18。
高立新、刘芳、赵蒙生等,2007,用sP n震相计算震源深度的初步分析与应用,西北地震学报,29(3),213~244。
罗艳、倪四道、曾祥方等,2010,汶川地震余震区东北端一个余震序列的地震学研究,中国科学:D辑,40(6),677~687。
王小龙、马胜利、雷兴林等,2012,重庆荣昌诱发地震区精细速度结构及2010年ML5.1地震序列精确定位,地震地质,34(2),348~358。
谢军、倪四道、曾祥方,2012,四川盆地中部浅层地壳一维剪切波速度结构初步研究,四川地震,143(2),20~24。
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,52(5),1412~1417。
Bent A L,Perry H K C,2002,Depths of eastern Canadian earthquakes from regional data,Seism Res Lett,73(2),273~284
Bock G,Grfinthal G,Wylegalla K,1996,The 1985/86 Western Bohemia earthquakes:Modelling source parameters witsynthetic seismograms,Tectonophysics,261(1~3),139~146.
Dawson J,Cummins P,Tregoning P et al,2008,Shallow intraplate earthquakes in Western Australia observed by Interfero\metriSynthetic Aperture Radar,Journal of Geophysical Research-Solid Earth,113,B11408,DOI:10.1029/2008JB005807.
Dziewonski,A,Bloch S,Landisman M,1969,A technique for the analysis of transient seismic signals,Bull Seism Soc Am,59(1),427~444.
Ehgdahl E R,Van der Hilst R,Buland R,1998,Global teleseismic earthquake relocation with improved travel times anprocedures for depth determination,Bull Seism Soc Am,88(3),722~743.
Herrmann,R B,1973,Some aspects of band-passltering of surface waves,Bull Seism Soc Am,63(2),663~671.
Herrmann,R B,Ammon C J,2004,Surface waves,receiver functions and crustal structure,in Computer Programs iSeismology,Version 3.30,Saint Louis University.http://www.eas.slu.edu/People/RBHerrmann/CPS330.html(lasaccessed December 12,2006).
Kikuchi M,Kanamori H,1982,Inversion of complex body waves,Bull Seism Soc Am,72(2),491~506.
Mori J,1991,Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Rancand Superstition Hills,California,earthquakes,Bull Seism Soc Am,81(2),508~523.
Saikia C K,2000,A method for path calibration using regional and teleseismic broadband seismograms:application to the 21 Ma1997 Jabalpur,India earthquake(MW5 8),Curr Sci,79(9~10),1301~1315.
Scholz C H,2002,The mechanics of earthquakes and faulting,Cambridge:Cambridge University Press.
Wang Z,Chong J,Ni S,et al,2011,Determination of focal depth by two waveform-based methods:A case study for the 2008Panzhihua earthquake,Earthq Sci,24(4),321~328.
Wei S,Konca A,Luo Y,et al,2008,Regional wave propagation beneath western US,2008 IRIS Workshop,poster.
Uski M,Hyvonena T,Korjaa A,et al,2003,Focal mechanisms of three earthquakes in Finland and their relation to surfacfaults,Tectonophysics,363(1~2),141~157.
Zhao L S,Helmberger D,1994,Source estimation from broadband regional seismograms,Bull Seism Soc Am,84(1),91~104
Zhu L,Helmberger D,1996,Advancement in source estimation techniques using broadband regional seismogram,Bull Seism SoAm,86(5),1634~1641.
Zhu L,Rivera L A,2002,A note on the dynamic and static displacements from a point source in multilayered media,GeophysicaJournal International,148(3),619~627.
崇加军、倪四道、曾祥方,2010,sP L,一个近距离确定震源深度的震相,地球物理学报,11(53),2620~2630。
丁丹、倪四道、田晓峰等,2010,地震相关的声音现象进展研究,华南地震,30(2),46~53。
房明山、杜安陆、董孝平等,1995,用sP n震相测定近震震源深度,地震地磁观测与研究,16(5),13~18。
高立新、刘芳、赵蒙生等,2007,用sP n震相计算震源深度的初步分析与应用,西北地震学报,29(3),213~244。
罗艳、倪四道、曾祥方等,2010,汶川地震余震区东北端一个余震序列的地震学研究,中国科学:D辑,40(6),677~687。
王小龙、马胜利、雷兴林等,2012,重庆荣昌诱发地震区精细速度结构及2010年ML5.1地震序列精确定位,地震地质,34(2),348~358。
谢军、倪四道、曾祥方,2012,四川盆地中部浅层地壳一维剪切波速度结构初步研究,四川地震,143(2),20~24。
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,52(5),1412~1417。
Bent A L,Perry H K C,2002,Depths of eastern Canadian earthquakes from regional data,Seism Res Lett,73(2),273~284
Bock G,Grfinthal G,Wylegalla K,1996,The 1985/86 Western Bohemia earthquakes:Modelling source parameters witsynthetic seismograms,Tectonophysics,261(1~3),139~146.
Dawson J,Cummins P,Tregoning P et al,2008,Shallow intraplate earthquakes in Western Australia observed by Interfero\metriSynthetic Aperture Radar,Journal of Geophysical Research-Solid Earth,113,B11408,DOI:10.1029/2008JB005807.
Dziewonski,A,Bloch S,Landisman M,1969,A technique for the analysis of transient seismic signals,Bull Seism Soc Am,59(1),427~444.
Ehgdahl E R,Van der Hilst R,Buland R,1998,Global teleseismic earthquake relocation with improved travel times anprocedures for depth determination,Bull Seism Soc Am,88(3),722~743.
Herrmann,R B,1973,Some aspects of band-passltering of surface waves,Bull Seism Soc Am,63(2),663~671.
Herrmann,R B,Ammon C J,2004,Surface waves,receiver functions and crustal structure,in Computer Programs iSeismology,Version 3.30,Saint Louis University.http://www.eas.slu.edu/People/RBHerrmann/CPS330.html(lasaccessed December 12,2006).
Kikuchi M,Kanamori H,1982,Inversion of complex body waves,Bull Seism Soc Am,72(2),491~506.
Mori J,1991,Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Rancand Superstition Hills,California,earthquakes,Bull Seism Soc Am,81(2),508~523.
Saikia C K,2000,A method for path calibration using regional and teleseismic broadband seismograms:application to the 21 Ma1997 Jabalpur,India earthquake(MW5 8),Curr Sci,79(9~10),1301~1315.
Scholz C H,2002,The mechanics of earthquakes and faulting,Cambridge:Cambridge University Press.
Wang Z,Chong J,Ni S,et al,2011,Determination of focal depth by two waveform-based methods:A case study for the 2008Panzhihua earthquake,Earthq Sci,24(4),321~328.
Wei S,Konca A,Luo Y,et al,2008,Regional wave propagation beneath western US,2008 IRIS Workshop,poster.
Uski M,Hyvonena T,Korjaa A,et al,2003,Focal mechanisms of three earthquakes in Finland and their relation to surfacfaults,Tectonophysics,363(1~2),141~157.
Zhao L S,Helmberger D,1994,Source estimation from broadband regional seismograms,Bull Seism Soc Am,84(1),91~104
Zhu L,Helmberger D,1996,Advancement in source estimation techniques using broadband regional seismogram,Bull Seism SoAm,86(5),1634~1641.
Zhu L,Rivera L A,2002,A note on the dynamic and static displacements from a point source in multilayered media,GeophysicaJournal International,148(3),619~627.
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