利用雷利波橢圓度反演竹林地區深層山崩的速度構造

Abstract

大規模深層山崩是遍佈全世界的高危害度自然災害之一，往往會造成人們生命與財產的損失。位於南臺灣的竹林地區崩塌地是其中一種具有幾何複雜性的大型深層山崩，具有許多不同滑動行為和深度的子塊體。因此，探討其地下構造對於了解滑動面、斷層帶和可能發生崩塌的潛在弱帶是至關重要的。為此，本研究團隊自2019年7月起在竹林崩塌區架設了包含17個地震站的地震觀測網，以便進行全面的監測。本研究使用DOP-E方法（Berbellini et al., 2019）計算時頻域的極化度（Degree-of-polarization）來有效地從環境噪訊中提取雷利波，並估計雷利波橢圓度及反方位角。由反方位角的分析表明，雷利波能量的來源主要來自荖濃溪的河道噪訊。為了反演的穩定性，我們僅使用來自河道訊號的雷利波橢圓度，並利用鄰居演算法反演每個地震測站地下的一維剪力波速度構造剖面。透過高斯平滑內插出平面空間上的速度剖面，可以對整個崩塌區進行分析與解釋，了解不同子塊體的滑動深度和分層結構。從結果可知，速度剖面的速度對比所顯示的深度和局部鑽井指示的滑動面深度呈現高度相似。我們也將此方法進一步擴展至分析盧壁颱風滑動事件如何影響滑動界面和崩積層的剪力波速度變化的時序研究。

Large-scale deep-seated landslides are one of the most catastrophic natural hazards which threaten lives and property in the world. The Chulin landslide in southern Taiwan is one of such composed of complex morphology, showing multiple sub-blocks with different deformation behaviors. As a result, constraining its subsurface structure is crucial to understand the detailed geometry of sliding interface, fault zones and potential weak zones prone to failure. For this purpose, a seismic network containing 17 stations was installed relatively uniformly across the Chulin landslide area in July 2019 to provide comprehensive observations. We apply a method, named DOP-E (Berbellini et al., 2019), to estimate degree-of-polarization (DOP) in the time-frequency domain to effectively extract the Rayleigh waves and estimate the Rayleigh wave ellipticity from seismic noise. The polarization analysis shows the sources of Rayleigh wave energy vary a lot azimuthally but mainly come from the direction of river bends. For the consistency and stability of inversion, we measure the ellipticity on signals only from the river bend and invert for 1-D shear-wave velocity profile at each station using neighborhood algorithm. The velocity profiles are then spatially mapped over the landslide area through Gaussian smoothing to investigate the possible sliding depth and layering structure of different sub-blocks. The depth variations derived by the major velocity contrast are consistent locally with the sliding depths at four drilling sites. The approach is further extended to analyze the velocity variations of the sliding interface and colluvium after a sliding event during Typhoon Lupit.