浙江珊溪水库地震精细定位及构造研究
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摘要
珊溪水库地震是近年来发生在少震、弱震的浙江省南部地区的显著地震事件,地震活动表现出分组的间隙性特征,台网定位的地震震中没有显示出分布的优势方向,发震构造不清楚。本研究基于虚波走时方程,将非线性方程线性化,求解获得震源位置及虚波速度初值,之后计算初值走时残差,将走时残差的标准偏差扩大2倍,并据此计算其经度、纬度、深度及虚波速度的最大误差,构造遗传种群,利用遗传算法进行二次定位,获得地震定位的最终结果。利用该方法,对浙江珊溪水库震群进行重新定位,结果表明:本次震群基本上集中发生在一个近垂直、北西向的断层面内,断层面法向与正北向夹角为46°,与正东向夹角为44°,与垂直向夹角为87°。计算小震综合面解获得的结果与定位结果基本一致。震源深度平均4.7km,最大深度9.5km,最浅1.7km,地震震中沿北西向呈狭窄条带状分布,沿北西向地震震源深度趋势性增大,在条带西北端有大约3.5km的地震不连续空段。利用Borun模型计算的小震震源参数结果表明,该震群地震震源参数没有表现出明显有别于构造地震的特征,地震应力降多在0.33MPa左右,平均应力降为0.88MPa,根据应力降空间分布的情况判断,该地震条带西北端地震活动不连续段处于应力降的低值区。
The Shanxi reservoir earthquake was a significant seismic event in southern Zhejiang Province in recent years,an area with fewer and weaker earthquakes.The earthquake activity had the intermittent characteristics of groups.The epicenters of Seismic Network positioning showed no advantage of the direction distribution and the seismogenic structure was not clear.In the study,the nonlinear virtual travel time equation was linearized and solved,and the source position,initial virtual velocity and travel time residuals were obtained.Then by doubling the standard deviation as a time residuals,the maximum error generated from longitude,latitude,depth and virtual velocity was calculated.The genetic population was structured using the maximum error and the end result of earthquake location was obtained by genetic algorithm.The result of repositioning of the Shanxi reservoir earthquake with this method shows that the earthquake is largely concentrated in a nearly vertical,north west of the fault plane,whose normal and north angle of 46°,and east angle of 44°,and vertical angle of 87°.The result is in agreement with that of small earthquake comprehensive mechanism solution.The average depth of the earthquake was 4.7km,the maximum depth 9.5km,and the minimum depth 1.7km.The epicenter was of northwest narrow band distribution,and focal depth increased along the northwest direction.There was a seismic discontinuities space segment at about 3.5km the northwest end of the strip.The characteristics of source parameters obtained by using the Borun model was not significantly different from that tectonic earthquakes.Seismic stress drop was about 0.33MPa,and the average stress drop 0.88MPa.According to the stress drop spatial distribution,the seismic discontinuities space segment at the northwest end of the strip is a low stress drop one.
The Shanxi reservoir earthquake was a significant seismic event in southern Zhejiang Province in recent years,an area with fewer and weaker earthquakes.The earthquake activity had the intermittent characteristics of groups.The epicenters of Seismic Network positioning showed no advantage of the direction distribution and the seismogenic structure was not clear.In the study,the nonlinear virtual travel time equation was linearized and solved,and the source position,initial virtual velocity and travel time residuals were obtained.Then by doubling the standard deviation as a time residuals,the maximum error generated from longitude,latitude,depth and virtual velocity was calculated.The genetic population was structured using the maximum error and the end result of earthquake location was obtained by genetic algorithm.The result of repositioning of the Shanxi reservoir earthquake with this method shows that the earthquake is largely concentrated in a nearly vertical,north west of the fault plane,whose normal and north angle of 46°,and east angle of 44°,and vertical angle of 87°.The result is in agreement with that of small earthquake comprehensive mechanism solution.The average depth of the earthquake was 4.7km,the maximum depth 9.5km,and the minimum depth 1.7km.The epicenter was of northwest narrow band distribution,and focal depth increased along the northwest direction.There was a seismic discontinuities space segment at about 3.5km the northwest end of the strip.The characteristics of source parameters obtained by using the Borun model was not significantly different from that tectonic earthquakes.Seismic stress drop was about 0.33MPa,and the average stress drop 0.88MPa.According to the stress drop spatial distribution,the seismic discontinuities space segment at the northwest end of the strip is a low stress drop one.
引文
[1]浙江省地震局,2006～2008,浙江省地震观测报告。
[2]浙江省地震局,2006,珊溪水库活断层调查报告。
白志明、王椿镛,2006,下扬子地壳P波速度结构:符离集-奉贤地震测深剖面再解释,科学通报,51(21),2534～2541。
陈运泰、黄立人、林邦慧等,1979,用大地测量资料反演的1976年唐山地震的位错模式,地球物理学报,22(3),201～217。
陈运泰、林邦慧、林中洋等,1975,根据地面形变的观测研究1966年邢台地震的震源过程,地球物理学报,28(3),164～182。
陈运泰、吴忠良、王培德等,2000,数字地震学,1～14,北京:地震出版社。
许忠准、阎明、赵仲和,1983,由多个小地震推断的华北地区构造应力场的方向,地震学报,5(3),268～279。
杨智娴、陈运泰等,2002,张北-尚义地震序列的重新定位和发震构造,地震学报,24(4),366～377。
于俊谊、朱新运,2008,浙江珊溪水库地震震源参数研究,中国地震,24(4),379～387。
朱新运、陈运泰,2007,用Lg波资料反演场地效应与地震波衰减参数,地震学报,29(6),569～580。
朱新运、于俊谊,2008,基于MATLAB的小震震源参数计算软件研制,西北地震学报,30(4),380～384。
Billings,S.D.,Kennett,B.L.N.,Sambridge,M.S.,1994,Hypocenter genetic algorithms incorporating problem-specific information,Geophys.J.Int.,118(3),693～706.
Crosson,R.S.,1976,Crustal structure modeling of earthquake data,1,Simultameous least squares estimation of hypocenter andvelocity parameters.J.Geophys.Res.,81(17),3036～3046.
Geiger,L.,1912,Probability method for the determination of earthquake picenters from arrival time only.Bull.St.Louis Univ.,8,60～71.
Gupta,H.K.,2001,Short-term earthquake forecasting may be feasible at Koyna,India,Tectonophysics,338,353～357.
Gupta,H.K.,2002,A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna,India,Earth-Sci.Rev.,58,279～310.
Gupta,H.K.,2005,artificial water reservoirs-triggered earthquakes with special emphasis at Koyna,Current Science,88(10),1628～1631.
Gupta,H.K.,Shashidhar,D.,Periera,M.,et al,2006,Prediction of an M～4 earthquake in Koyna region comes true,Jour.Geol.Soc.India,68,149～150.
Holland,J.H.,1975,Adaptation in Natural and Artificial Systems,Ann Arbor,MI:The University of Michigan Press.
Sambridge,M.S.,Gallaghen,K.,1993,Hypocenter location using genetic algorithnics,Bull.Seismol.Soc.Am.,83,1467～1491.
Waldhauser,F0,Ellsworth,W.L.,2000,A double-difference earthquake location algorithm:method and application to the Northern Hayward Fault,California,Bull.Seism.Soc.Am.,90,1353～1368.
[2]浙江省地震局,2006,珊溪水库活断层调查报告。
白志明、王椿镛,2006,下扬子地壳P波速度结构:符离集-奉贤地震测深剖面再解释,科学通报,51(21),2534～2541。
陈运泰、黄立人、林邦慧等,1979,用大地测量资料反演的1976年唐山地震的位错模式,地球物理学报,22(3),201～217。
陈运泰、林邦慧、林中洋等,1975,根据地面形变的观测研究1966年邢台地震的震源过程,地球物理学报,28(3),164～182。
陈运泰、吴忠良、王培德等,2000,数字地震学,1～14,北京:地震出版社。
许忠准、阎明、赵仲和,1983,由多个小地震推断的华北地区构造应力场的方向,地震学报,5(3),268～279。
杨智娴、陈运泰等,2002,张北-尚义地震序列的重新定位和发震构造,地震学报,24(4),366～377。
于俊谊、朱新运,2008,浙江珊溪水库地震震源参数研究,中国地震,24(4),379～387。
朱新运、陈运泰,2007,用Lg波资料反演场地效应与地震波衰减参数,地震学报,29(6),569～580。
朱新运、于俊谊,2008,基于MATLAB的小震震源参数计算软件研制,西北地震学报,30(4),380～384。
Billings,S.D.,Kennett,B.L.N.,Sambridge,M.S.,1994,Hypocenter genetic algorithms incorporating problem-specific information,Geophys.J.Int.,118(3),693～706.
Crosson,R.S.,1976,Crustal structure modeling of earthquake data,1,Simultameous least squares estimation of hypocenter andvelocity parameters.J.Geophys.Res.,81(17),3036～3046.
Geiger,L.,1912,Probability method for the determination of earthquake picenters from arrival time only.Bull.St.Louis Univ.,8,60～71.
Gupta,H.K.,2001,Short-term earthquake forecasting may be feasible at Koyna,India,Tectonophysics,338,353～357.
Gupta,H.K.,2002,A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna,India,Earth-Sci.Rev.,58,279～310.
Gupta,H.K.,2005,artificial water reservoirs-triggered earthquakes with special emphasis at Koyna,Current Science,88(10),1628～1631.
Gupta,H.K.,Shashidhar,D.,Periera,M.,et al,2006,Prediction of an M～4 earthquake in Koyna region comes true,Jour.Geol.Soc.India,68,149～150.
Holland,J.H.,1975,Adaptation in Natural and Artificial Systems,Ann Arbor,MI:The University of Michigan Press.
Sambridge,M.S.,Gallaghen,K.,1993,Hypocenter location using genetic algorithnics,Bull.Seismol.Soc.Am.,83,1467～1491.
Waldhauser,F0,Ellsworth,W.L.,2000,A double-difference earthquake location algorithm:method and application to the Northern Hayward Fault,California,Bull.Seism.Soc.Am.,90,1353～1368.
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