区域最小完整性震级时空分布的确定──以龙门山断裂带为例
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摘要
文中首先介绍了估计MC的拟合效果测试法,然后采用该方法确定龙门山断裂带及其附近最小完整性震级的时空分布,最后总结和讨论了该方法的优点以及在应用中存在的问题。研究表明,拟合效果测试法基于G-R关系的幂率分布假设,通过由小到大不断调整起算震级和对观测数据进行拟合,将能最好拟合观测数据的G-R关系的起算震级作为最小完整性震级MC。该方法还通过定义一个拟合评价函数来衡量对观测资料的拟合优度。应用结果表明,龙门山断裂带及其附近地区的最小完整性震级MC从1970年以来呈现减小的趋势,反映监测能力的逐步提高;由拟合效果测试法估计MC值的空间分布,总体上与区域台网理论监测能力的分布相当,但可获得更多的分布细节。另外,由于拟合效果测试法是一种基于地震活动性的算法,在应用局部地区较短时段的地震资料时,可能会过低或过高估计最小完整性震级MC。在计算出MC的基础上,对研究区进行了b值的时间扫描,结果反映2008年汶川8.0级地震前3~8年,龙门山断裂带及其邻近地区背景小震活动的b值似乎略有升高。
In this paper,the goodness-of-fit test(GFT) method for estimating the minimum magnitude of completeness(MC) is introduced first and then applied to determine the temporal-spatial distribution of MC of the Longmenshan fault zone,Sichuan,and its surrounding area.Finally,advantage and problems of the method in application are summarized and discussed.Our study suggests that based on the assumption that the G-R relationship has the power-low distribution,the GFT method is used to estimate a minimum magnitude of completeness MC through fitting a G-R relationship for a set of observed data by adjusting various minimum magnitudes gradually from small to big ones and eventually taking that minimum magnitude when the fitted G-R relationship can match best the set of observed data to be the result.Also,in the method a fit evaluation function is defined and used to measure the goodness of fitting.Our application results show that in the studied region the minimum magnitudes of completeness MC show a tendency of decreasing gradually with time from 1970,reflecting a gradual improvement in the seismic monitoring ability of the region.The spatial distribution of MC are basically comparable with that of the theoretical monitoring ability of the regional seismic network,and have more details about the distribution.In addition,we also found that,as a seismicity-based arithmetic,the GFT method may underestimate or overestimate MC when seismic data for limited areas and for short time-period are used.Based on the estimated MC,b-value scanning is carried out for the studied region.The result shows that b-values of background small earthquakes along the Longmenshan fault zone and its nearby area seem to have slightly risen 3 to 8 years before the 2008 Wenchuan M8.0 earthquake.
In this paper,the goodness-of-fit test(GFT) method for estimating the minimum magnitude of completeness(MC) is introduced first and then applied to determine the temporal-spatial distribution of MC of the Longmenshan fault zone,Sichuan,and its surrounding area.Finally,advantage and problems of the method in application are summarized and discussed.Our study suggests that based on the assumption that the G-R relationship has the power-low distribution,the GFT method is used to estimate a minimum magnitude of completeness MC through fitting a G-R relationship for a set of observed data by adjusting various minimum magnitudes gradually from small to big ones and eventually taking that minimum magnitude when the fitted G-R relationship can match best the set of observed data to be the result.Also,in the method a fit evaluation function is defined and used to measure the goodness of fitting.Our application results show that in the studied region the minimum magnitudes of completeness MC show a tendency of decreasing gradually with time from 1970,reflecting a gradual improvement in the seismic monitoring ability of the region.The spatial distribution of MC are basically comparable with that of the theoretical monitoring ability of the regional seismic network,and have more details about the distribution.In addition,we also found that,as a seismicity-based arithmetic,the GFT method may underestimate or overestimate MC when seismic data for limited areas and for short time-period are used.Based on the estimated MC,b-value scanning is carried out for the studied region.The result shows that b-values of background small earthquakes along the Longmenshan fault zone and its nearby area seem to have slightly risen 3 to 8 years before the 2008 Wenchuan M8.0 earthquake.
引文
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[11]Rydelek P A,I S Sacks.Testing the Completeness of earthquake Catalogues and the hypothesis ofself-similarity[J].Nature,1989,337:251-253.
[12]Ogata Y,Katsura K.Analysis of temporal and spatial heterogeneity of magnitude frequency distri-bution inferred from earthquake catalogues[J].Geophys J Int,1993,113:121-138.
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[14]Cao A M,Gao S S.Temporal variation of seismic b-values beneath northeastern Japan island arc[J].Geophys Res Let,2002,29(9):doi:10.1029/2001GL013775.
[15]Woessner J,Wiemer S.Assessing the quality of earthquake catalogues:Estimating the magnitude ofcompleteness and Its uncertainty[J].Bull Seism Soc Am,2005,95(2):684-698.
[16]Sereno T J Jr,Bratt S R.Seismic detection capability at NORESS and implications for the detectionthreshold of a hypothetical network in the Soviet Union[J].J Geophys Res,1989,94:10 397-10 414.
[17]Harvey D,Hansen R.Contributions of IRIS data to nuclear monitoring[J].IRIS Newsletter,1994,13:1 051-1 076.
[18]Gomberg J.Seismicity and detection location threshold in the southern Great Basin seismic network[J].J Geophys Res,1991,96:16 401-16 414.
[19]Aki K.Maximum likelihood estimate of b in the formula logN=a-bMand its confidence limits[J].Bull Earthquake Re Inst,1965,43:237-239.
[20]Shi Y,Bolt B A.The standard error of the magnitude-frequency b value[J].Bull Seism Soc Am,1982,72:1 677-1 687.
[21]Bender B.Maximum likelihood estimation of b-values for magnitude grouped data[J].Bull SeismSoc Am,1983,73:831-851.
[22]陆远忠,李胜乐,邓志辉,等.基于GIS的地震分析预报系统[M].四川:成都地图出版社,2002.68-69.
①http://www.earthquake.ethz.ch/software/zmap
[2]Gutenberg R C,Richter F.Frequency of earthquakes in california[J].Bull Seism Soc Am,1944,34:185-188.
[3]Mogi K.Magnitude-frequency relation for elastic shocks accompanying fractures of various materialsand some related problems in earthquakes[J].Bull Earthq Res Inst,1962,40:831-853.
[4]Scholz C H.The frequency-magnitude relation to microfracturing in rock and its relation to earth-quakes[J].Bull Seism Soc Am,1968,58:399-415.
[5]茂木清夫.日本的地震预报[M].庄灿涛译.北京:地震出版社,1981.
[6]闻学泽,徐锡伟,龙锋,等.中国大陆东部中一弱活动断层潜在地震最大震级评估的震级-频度关系模型[J].地震地质,2007,29(2):236-253.
[7]吴小平,许宇栋,黄雍.地震活动参数b值与地震分维关系的再探讨[J].地震研究,1998,21(1):88-93.
[8]张智,吴开统,焦远碧,等.用b值横截距预报强余震震级的方法探讨[J].中国地震,1989,5(4):59-69.
[9]Youngs R R,Coppersmith K J.Implications of fault slip rates and earthquake recurrence models toprobabilistic seismic hazard estimates[J].Bull Seism Soc Am,1985,75(4):939-964.
[10]An Weiping,Qin Changyuan.On factors influencing b value[J].Journal of geodesy and geodynam-ics,2007,27(3):85-90.
[11]Rydelek P A,I S Sacks.Testing the Completeness of earthquake Catalogues and the hypothesis ofself-similarity[J].Nature,1989,337:251-253.
[12]Ogata Y,Katsura K.Analysis of temporal and spatial heterogeneity of magnitude frequency distri-bution inferred from earthquake catalogues[J].Geophys J Int,1993,113:121-138.
[13]Wiemer S,Wyss M.Minimum magnitude of complete reporting in earthquake catalogs:Examplesfrom Alaska,the Western United States,and Japan[J].Bull Seism Soc Am,2000,90:859-869.
[14]Cao A M,Gao S S.Temporal variation of seismic b-values beneath northeastern Japan island arc[J].Geophys Res Let,2002,29(9):doi:10.1029/2001GL013775.
[15]Woessner J,Wiemer S.Assessing the quality of earthquake catalogues:Estimating the magnitude ofcompleteness and Its uncertainty[J].Bull Seism Soc Am,2005,95(2):684-698.
[16]Sereno T J Jr,Bratt S R.Seismic detection capability at NORESS and implications for the detectionthreshold of a hypothetical network in the Soviet Union[J].J Geophys Res,1989,94:10 397-10 414.
[17]Harvey D,Hansen R.Contributions of IRIS data to nuclear monitoring[J].IRIS Newsletter,1994,13:1 051-1 076.
[18]Gomberg J.Seismicity and detection location threshold in the southern Great Basin seismic network[J].J Geophys Res,1991,96:16 401-16 414.
[19]Aki K.Maximum likelihood estimate of b in the formula logN=a-bMand its confidence limits[J].Bull Earthquake Re Inst,1965,43:237-239.
[20]Shi Y,Bolt B A.The standard error of the magnitude-frequency b value[J].Bull Seism Soc Am,1982,72:1 677-1 687.
[21]Bender B.Maximum likelihood estimation of b-values for magnitude grouped data[J].Bull SeismSoc Am,1983,73:831-851.
[22]陆远忠,李胜乐,邓志辉,等.基于GIS的地震分析预报系统[M].四川:成都地图出版社,2002.68-69.
①http://www.earthquake.ethz.ch/software/zmap
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