Sudden drop of seismic velocity after the 2004 Mw 6.6 mid-Niigata earthquake, Japan, observed with Passive Image Interferometry
Sudden drop of seismic velocity after the 2004 Mw 6.6 mid-Niigata earthquake, Japan, observed with Passive Image Interferometry
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DOI：
10.1029/2008jb005869
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发表时间：
2009-06
影响因子：
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通讯作者：
U. Wegler;H. Nakahara;C. Sens‐Schönfelder;M. Korn;K. Shiomi
中科院分区：
文献类型：
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作者：
U. Wegler;H. Nakahara;C. Sens‐Schönfelder;M. Korn;K. Shiomi
[1] 被动图像干涉测量（PII）是一种地震学方法，利用环境地震噪声来监测地球内平均剪切波速度的时间演化。首先，两个地震仪之间的弹性格林张量是根据在一定时期内记录的地震噪声的互相关来计算的。其次，将不同时间段构建的地震图视为地震多次波，并利用其尾波中的小时移来反演平均剪切波速度的相对变化。当应用于 2004 年日本新泻 Mw = 6.6 地震的震源区（质心深度 5 公里）时，我们使用了距离震中不到 25 公里的 6 个地震仪记录的噪声。地震前后两个月的每日反演显示，新泻中期地震时平均剪切波速突然下降了百分之零点几。使用 0.1–0.5 Hz 和 2–8 Hz 两个频段的噪声，我们发现速度下降的幅度相似，这表明变化并不局限于浅层地下。这种速度下降的可能解释是地震后地壳应力的下降，由于强烈的地面运动而导致浅层地下层的非线性场地响应，或者由于在地震源区产生新的裂缝而导致的结构弱化。
[1] Passive Image Interferometry (PII) is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within the Earth. First, the elastic Green's tensor between two seismometers is computed from the cross correlation of seismic noise recorded during a certain period. Second, the constructed seismograms of different time periods are treated as earthquake multiplets, and small time shifts in their coda are used to invert a relative change in mean shear wave velocity. When applied to the source region of the 2004 Mw = 6.6 mid-Niigata earthquake, Japan (centroid depth 5 km), we used noise recorded at six seismometers located at a distance of less than 25 km from the epicenter. Daily inversions during the 2 months before and after the earthquake show a sudden drop of mean shear wave velocity of some tenths of a percent at the time of the mid-Niigata earthquake. Using noise in two frequency bands, 0.1–0.5 Hz and 2–8 Hz, we find similar amplitudes for the velocity drop, which indicates that changes are not restricted to the shallow subsurface. Possible interpretations of this velocity decrease are a decrease of crustal stress after the earthquake, a nonlinear site response in the shallow subsurface layer due to strong ground motion, or structural weakening due to the creation of new fractures in the source area of the earthquake.