Monitoring Coseismic Temporal Changes of Shallow Material during Strong Ground Motion with Interferometry and Autocorrelation

Temporal changes of seismic velocities generated at the subsurface by the 2011 M-w 9 Tohoku earthquake in Japan are analyzed using interferometry and autocorrelation of waveforms recorded by surface and borehole sensors at the KiK-net station IBRH16. Spectral ratios and interferometry of data recorded in moving time windows at the surface and a depth of 300 m exhibit clear time delays and reduction of the predominant frequency during the strong ground motion followed by partial recovery in the waveform coda. Converting the time-delay evolution to velocity changes implies about 30% velocity reduction in the structure between the two sensors. Calculating temporal evolution with autocorrelation of data at the surface sensor in the moving time window indicates about 50% velocity reduction, reflecting changes of shallower material below the station. Computing autocorrelations of data recorded by the surface sensor with the Stockwell transform allows monitoring temporal changes with higher resolution that approaches the sampling rate of the waveforms. Using this technique, we estimate about 60% drop of seismic velocity below the surface sensor during strong ground motion followed again by partial recovery in the coda. These results provide fundamental information on in situ dynamic properties of soils and damaged shallow rocks that complement laboratory measurements, with important implications for constitutive equations of material degradation and healing and nonlinear site effects.

利用干涉测量法以及日本KiK - net台站IBRH16的地表和井下传感器所记录波形的自相关，分析了2011年日本东北9.0级地震在地下产生的地震波速的时间变化。地表和300米深处移动时间窗口内记录数据的频谱比和干涉测量结果显示，在强烈地面运动期间存在明显的时间延迟以及主频降低，随后在波形尾波中部分恢复。将时间延迟演变转换为速度变化意味着两个传感器之间的结构速度降低了约30%。通过移动时间窗口内地表传感器数据的自相关计算时间演变，表明速度降低了约50%，反映了台站下方较浅物质的变化。利用斯托克韦尔变换计算地表传感器记录数据的自相关，可以以更高的分辨率监测时间变化，接近波形的采样率。利用这种技术，我们估计在强烈地面运动期间地表传感器下方的地震波速下降了约60%，随后在尾波中再次部分恢复。这些结果提供了关于土壤和受损浅层岩石原位动态特性的基本信息，补充了实验室测量结果，对材料退化和愈合的本构方程以及非线性场地效应具有重要意义。