用噪声成像方法研究云南地区强震前地震面波速度变化
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摘要
本文利用2008年1月至2010年1月云南地震台网46个台站的连续波形资料,采用背景噪声方法计算云南地区不同台站对之间的面波延迟时间,通过层析成像方法反演得到不同时间段相同时间窗长的面波波速分布。根据试验,将三个月的时间窗长作为云南地区得到稳定信噪比的延迟时间的叠加天数。结果表明,不同时间段得到的波速结构具有明显的变化。这些变化表明:①2008年汶川8.0级地震后,程海断裂带波速增加,能量积累加剧;②2008年攀枝花6.1级地震前攀枝花地区处于速度升高和速度降低的分界带上;③2009年姚安6.0级地震前震中所在区域面波波速升高,而震后出现降低。与传统的动态成像相比,背景噪声方法由于采用固定的台站对,不同时间段的射线固定,这样可以减小由于不同时段地震分布不均一和震源定位而引起的误差,使动态反演结果更加可靠。
Delay times of surface wave for different station pairs in Yunnan region have been calculated by ambient noise method,using continuous waveform records of 46 stations in Yunnan network from Jan.,2008 to Jan.,2010.Then,surface wave velocity distribution of different times in the same window length is obtained by tomography method.According to experiment,we apply window length of three months as the stack days to obtain noise cross-correlation function with stable signal-to-noise ratio(SNR).The results show that the wave velocity structure changes obviously in different times.These changes include ① the wave velocity in Chenghai fault increases due to energy accumulation after the Wenchuan MS8.0 earthquake in 2008;② Panzhihua region is located on the boundary of velocity rising and dropping before the Panzhihua MS6.1 earthquake in 2008;③ Wave velocity increases in the hypocentral area before the Yaoan MS6.0 earthquake in 2009,and reduces after the shock.Comparing with conventional time-dependent seismic tomography,ambient noise method using the fixed rays in different times can reduce the error caused by uneven distribution and hypocenter location errors.
Delay times of surface wave for different station pairs in Yunnan region have been calculated by ambient noise method,using continuous waveform records of 46 stations in Yunnan network from Jan.,2008 to Jan.,2010.Then,surface wave velocity distribution of different times in the same window length is obtained by tomography method.According to experiment,we apply window length of three months as the stack days to obtain noise cross-correlation function with stable signal-to-noise ratio(SNR).The results show that the wave velocity structure changes obviously in different times.These changes include ① the wave velocity in Chenghai fault increases due to energy accumulation after the Wenchuan MS8.0 earthquake in 2008;② Panzhihua region is located on the boundary of velocity rising and dropping before the Panzhihua MS6.1 earthquake in 2008;③ Wave velocity increases in the hypocentral area before the Yaoan MS6.0 earthquake in 2009,and reduces after the shock.Comparing with conventional time-dependent seismic tomography,ambient noise method using the fixed rays in different times can reduce the error caused by uneven distribution and hypocenter location errors.
引文
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[14]程佳,刘杰,甘卫军,等.2009汶川地震同震形变场对川滇地区主要活动断裂地震发生趋势的影响[J].地震学报,31(5):477-490.
[15]Julian B R,Foulger G R.Time-dependent seismic tomography[J].Geophys J Int,2010,182:1 327-1 338.
[16]Brenguier F,Shapiro N M,Campillo M,et al.Towards forecasting volcanic eruptions using seismicnoise[J].Nature Geoscience,2008,1:126-130.
[17]周龙泉,刘杰,张晓东.2003年大姚6.2和6.1级地震前三维波速结构的演化[J].地震学报,2007,29(1):12-20.
[2]房立华,吴建平,吕作勇.华北地区基于噪声的瑞利面波群速度层析成像[J].地球物理学报,2009,52(3):663-67.
[3]李昱,姚华建,刘启元,等.川西地区台阵环境噪声瑞利波相速度层析成像[J].地球物理学报,2010,53(4):842-852.
[4]李军,金星,周峥嵘,等.利用地震噪声准实时监测短周期面波波速变化[J].地震学报,2009,31(6):629-640.
[5]刘志坤,黄金莉.利用背景噪声互相关研究汶川地震震源区地震波速度变化[J].地球物理学报,2010,53(4):853-863.
[6]Sabra K G,Gerstoft P,Roux P,et al.Extracting time domain Green's function estimates from am-bient seismic noise[J].Geophys Res Lett,2005,32(3):L03310.
[7]Bensen G D,Ritzewoller M H,Barmin M P,et al.Processing seismic ambient noise data to obtainreliable broad-band surface wave dispersion measurements[J].Geophys J Int,2007,169:1 239-1 260.
[8]Paige C C,Saunders M A.LSQR,An algorithm for sparse linear equations and sparse linear system[J].ACM Trans Math Software,1982,8:43-71.
[9]周龙泉,刘杰,苏有锦,等.利用S波高频衰减参数对云南地区地壳Q值成像[J].地球物理学报,2009,52(6):1 500-1 507.
[10]Humphreys E,Clayton R W.Adaptation of Back Projection Tomography to Seismic Travel TimeProblems[J].J Geophys Res,1988,93(B2):1 073-1 085.
[11]Inoue H,Fukae Y,Tanabe K,et al.Whole mantle P-wave travel time tomography[J].Phys EarthPlanet Inter,1990,59:294-328.
[12]何正勤,叶太兰,苏伟.云南地区地壳中上部横波速度结构研究[J].地球物理学报,2004,47(5):838-844.
[13]胡鸿翔,高世玉.滇西地区地壳浅部基底速度细结构的研究[J].中国地震,1993,9(4):356-363.
[14]程佳,刘杰,甘卫军,等.2009汶川地震同震形变场对川滇地区主要活动断裂地震发生趋势的影响[J].地震学报,31(5):477-490.
[15]Julian B R,Foulger G R.Time-dependent seismic tomography[J].Geophys J Int,2010,182:1 327-1 338.
[16]Brenguier F,Shapiro N M,Campillo M,et al.Towards forecasting volcanic eruptions using seismicnoise[J].Nature Geoscience,2008,1:126-130.
[17]周龙泉,刘杰,张晓东.2003年大姚6.2和6.1级地震前三维波速结构的演化[J].地震学报,2007,29(1):12-20.
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