Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi-net tiltmeters
Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi-net tiltmeters
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DOI：
10.1029/2007jb005395
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发表时间：
2008-10-02
影响因子：
3.9
通讯作者：
Obara, K.
中科院分区：
文献类型：
--
作者：
Nishida, K.;Kawakatsu, H.;Obara, K.
通过将合成互谱拟合到观测到的环境地震噪声互谱，提出了一种三维S波速度层析成像的三步法。我们采用这种方法的Hi-net倾斜仪在日本的679个站从2004年6月至2004年12月的记录。首先，我们计算了每对台站的径向分量和横向分量之间的归一化互谱。第一步是局部1-D S波速度反演为每个站假设小的横向非均匀性下的100公里的一个站的圆。我们测量的频散曲线的基本瑞利波，基本洛夫波，和第一泛音的洛夫波通过拟合的合成交叉谱之间的观测到的圆内的站对。我们对测量的频散曲线进行了反演，以获得一维S波速度模型。第二步是反演观测到的互谱，得到路径平均的一维S波速度结构。第三步是使用射线近似法反演所得到的路径平均结构，以获得3-D S波速度结构（从地表到50 km深度的0.1度x0.1度x1 km网格）。结果表明，从地表到20 km深处，沿构造线沿着存在明显的低速异常。特别是，沿着日高山脉，我们观察到的S波扰动更极端的-20%。它们还显示了九州和东北火山下的低速异常。在四国西南部，我们的结果显示出一个明显的低速异常对应的增生带（四万十带）。在20公里以下，我们观察到日本中心的低速异常，这表明地壳很厚。
We developed a three-step method for three-dimensional (3-D) S wave velocity tomography by fitting synthetic cross spectra to the observed ones of ambient seismic noise. We applied this method to the recording of Hi-net tiltmeters in Japan at 679 stations from June 2004 to December 2004. First, we calculated normalized cross spectra between radial components and those between transverse components for every pair of stations. The first step is local 1-D S wave velocity inversion for each station assuming small lateral heterogeneity under a 100-km circle of a station. We measured the dispersion curves of fundamental Rayleigh waves, fundamental Love waves, and first overtone of Love waves by fitting the synthetic cross spectra to the observed ones between pairs of stations within the circle. We inverted the measured dispersion curves for obtaining a 1-D S wave velocity model. The second step is the inversion of the observed cross spectra for obtaining path-averaged 1-D S wave velocity structure. The third step is the inversion of the resultant path-averaged structures for obtaining 3-D S wave velocity structure (0.1 degrees x 0.1 degrees x 1 km grid from the surface to a depth of 50 km) using ray approximation. The resultant S wave velocity structures show clear low-velocity anomalies along tectonic lines from the surface to a depth of 20 km. In particular, along the Hidaka mountain range, we observed S wave perturbation more extreme than -20%. They also show low-velocity anomalies under volcanoes in Kyusyu and Tohoku. In the southwestern part of Shikoku, our results show a clear low-velocity anomaly corresponding to an accretional belt (Shimanto belt). Below 20 km, we observe a low-velocity anomaly in the center of Japan, which suggests a thick crust.