17:15 〜 18:45
[HDS08-P01] 斜面災害リスク評価のための浅部微動アレイ探査（その２）：阿蘇カルデラから東部の火砕流堆積域における稠密探査
★招待講演
キーワード：斜面災害、阿蘇、火山灰、微動アレイ
We conducted a dense microtremor array survey at the eastern foot of the Aso Caldera as part of a slope disaster risk assessment in northern Kyushu based on the intellectual infrastructure plan of the Ministry of Economy, Trade and Industry (METI). The objective of this study is to obtain the S-wave velocity and the resonance frequency of the ground as the fundamental data to evaluate slope disaster risks in this area. The background of the study, preliminary observations (conducted on December 2022 and June 2023), and the analysis results were reported in the JpGU2023 (Cho et al., 2023). In this report, we describe the entitled, final survey.
The dense microtremor observation was conducted from September to November 2023 in an area of about 17 km by 15 km just outside the eastern margin of the Aso Caldera at an elevation of about 800 m. In the preliminary analysis last year, the microtremor array survey was conducted along a 25-km survey line from the eastern margin of the caldera to Taketa City, Oita Prefecture along National Route 57. In this study, the microtremor array observations were conducted at 184 sites within the above wide area on the western part of the above survey line. We selected the above area, especially focusing on the thick loam layers (volcanic ash soil) that cover the stiff-soil ground formed by the fusion of pyroclastic flow deposits Aso-4B (90ka). The whole area, however, includes regions without Aso 4 deposits, where the Aso 3 pyroclastic flow deposits appear at the ground surface (i.e., in the northern and southern parts of the central to the eastern side of the analysis area). We arranged the observation points so that the density of the observation points is similar everywhere in the target area.
The goals of this survey were to evaluate at each observation point the average S-wave velocities to the depths of 10, 20, and 30 m (i.e., AVS10, 20, 30), 1D S-wave velocity (Vs) structure model to depths of several tens of meters, and the resonance frequencies (peak frequency of H/V spectrum) of the ground. The area under analysis systematically decreases in elevation from about 800 m to 400 m toward the east, and ridges and valleys generally run from west to east. The topography is locally undulating and very complex, however. Therefore, we have to be careful not to overinterpret the analysis results obtained at spatially discrete, albeit "dense", microtremor observation points. We created maps of AVSs and ground resonance frequencies based on spatial interpolation, with the aim of understanding general trends, by clearly indicating the location of each observation point. In addition, a number of Vs cross sections were created based on spatial interpolation of the 1D Vs structure, so that we can assess the general 3D Vs distribution in the area, for help in understanding the soil structure.
It was revealed that the top soft layers tend to be thicker in the western part and thinner in the eastern part of the analysis area that we are interested in, namely, in the area with comparatively thick volcanic-ash soil covering the stiff-soil ground formed by the fusion of pyroclastic flow deposits Aso-4B. More specifically, the surface volcanic-ash layers have Vs of 100-200 m/s or less, and the layer with Vs less than 200 m/s tended to be thicker in the western part and thinner in the eastern part. Such a distribution of top soft-layer thickness is qualitatively consistent with the thickness distribution of the Aso-4 pyroclastic flow deposit that is described by Hoshizumi et al. (2015). The distribution of layer thickness is also consistent with borehole data existing in that area, though the number of boreholes is as small as about 40. The resonance frequency (i.e., peak frequency of H/V spectrum) of the shallow ground in the areas of interest tends to take lower values in the eastern part (i.e., values less than 3Hz) than in the western part (i.e., values from 3 to 4 Hz). This indicates that the shallow ground on the eastern part resonates at a slightly lower resonant frequency than the western part due to the impedance contrast between stiffer layers than those on the western part, as well as the resonant surface layer is thicker on the eastern part than those on the western part.
The dense microtremor observation was conducted from September to November 2023 in an area of about 17 km by 15 km just outside the eastern margin of the Aso Caldera at an elevation of about 800 m. In the preliminary analysis last year, the microtremor array survey was conducted along a 25-km survey line from the eastern margin of the caldera to Taketa City, Oita Prefecture along National Route 57. In this study, the microtremor array observations were conducted at 184 sites within the above wide area on the western part of the above survey line. We selected the above area, especially focusing on the thick loam layers (volcanic ash soil) that cover the stiff-soil ground formed by the fusion of pyroclastic flow deposits Aso-4B (90ka). The whole area, however, includes regions without Aso 4 deposits, where the Aso 3 pyroclastic flow deposits appear at the ground surface (i.e., in the northern and southern parts of the central to the eastern side of the analysis area). We arranged the observation points so that the density of the observation points is similar everywhere in the target area.
The goals of this survey were to evaluate at each observation point the average S-wave velocities to the depths of 10, 20, and 30 m (i.e., AVS10, 20, 30), 1D S-wave velocity (Vs) structure model to depths of several tens of meters, and the resonance frequencies (peak frequency of H/V spectrum) of the ground. The area under analysis systematically decreases in elevation from about 800 m to 400 m toward the east, and ridges and valleys generally run from west to east. The topography is locally undulating and very complex, however. Therefore, we have to be careful not to overinterpret the analysis results obtained at spatially discrete, albeit "dense", microtremor observation points. We created maps of AVSs and ground resonance frequencies based on spatial interpolation, with the aim of understanding general trends, by clearly indicating the location of each observation point. In addition, a number of Vs cross sections were created based on spatial interpolation of the 1D Vs structure, so that we can assess the general 3D Vs distribution in the area, for help in understanding the soil structure.
It was revealed that the top soft layers tend to be thicker in the western part and thinner in the eastern part of the analysis area that we are interested in, namely, in the area with comparatively thick volcanic-ash soil covering the stiff-soil ground formed by the fusion of pyroclastic flow deposits Aso-4B. More specifically, the surface volcanic-ash layers have Vs of 100-200 m/s or less, and the layer with Vs less than 200 m/s tended to be thicker in the western part and thinner in the eastern part. Such a distribution of top soft-layer thickness is qualitatively consistent with the thickness distribution of the Aso-4 pyroclastic flow deposit that is described by Hoshizumi et al. (2015). The distribution of layer thickness is also consistent with borehole data existing in that area, though the number of boreholes is as small as about 40. The resonance frequency (i.e., peak frequency of H/V spectrum) of the shallow ground in the areas of interest tends to take lower values in the eastern part (i.e., values less than 3Hz) than in the western part (i.e., values from 3 to 4 Hz). This indicates that the shallow ground on the eastern part resonates at a slightly lower resonant frequency than the western part due to the impedance contrast between stiffer layers than those on the western part, as well as the resonant surface layer is thicker on the eastern part than those on the western part.