1:45 PM - 3:15 PM
[SSS09-P04] Shallow microtremor array survey for landslide risk assessment: Preliminary analysis on the region with pyroclastic flow deposits to the east of the Aso caldera
Keywords:slope disaster, Aso, tephra, microtremor array, exploration
The National Institute of Advanced Industrial Science and Technology (AIST) is currently developing geological database for landslide risk assessment, focusing on the northern Kyushu region based on the intellectual infrastructure development plan of METI (2021). The Japan Landslide Society (2014) proposed seismic response analysis based on 2D/3D elastoplastic FEM as one of the methods for fine examinations on identified risk areas of landslides caused by earthquakes. They also suggest developing an approach to assess the frequency characteristics of ground motions based on microtremor H/V spectra. Therefore, a research group including the presenter in the above project team of AIST is planning to model shallow (up to several tens of meters in depth) S-wave velocity structure based on microtremor array survey as the fundamental data for seismic response analysis. We are also considering measuring the microtremor H/V spectra simultaneously during the microtremor array survey.
Specifically, focusing on a pyroclastic flow deposit area (Hoshizumi et al., 2015) of about 10 x 15 km that extends to the east of the Aso Caldera, we plan to conduct an dense evaluation of 1D shallow S-wave velocity structure model and ground resonance frequencies. In this region, thick layers of loam (volcanic ash soil), up to 30 m thick in part, cover the stiff-soil ground formed by the fusion of pyroclastic flow deposits Aso-4B (90ka), which shows flat to gentle slopes with 3° or less in a distance scale of about 10 km, but locally the slopes are 15-30° or more (GSI, 1994). Consequently, the risk of either mudslides or steep slope failures is alarming at multiple places in that region (Kumamoto Prefecture, 2023).
It is said, in general, that microtremor array surveys are less applicable in mountainous areas far from urban areas, especially in rock/stiff-soil sites (Horike, 1993; Geophysical Exploration Society of Japan, 2008). Our target is mountainous or inter-mountainous areas. In addition, although the area is generally covered by a thick layer of volcanic ash (i.e., soft) soil, it is significantly undulating on a local scale, resulting in the stiff-soil ground from the surface locally, which may make the microtremor array survey inapplicable. Therefore, we conducted preliminary observations and analysis to check the applicability of microtremor array survey in this area before the dense evaluation.
A 25-km survey line was set from the east of Aso Caldera (at an elevation of about 800 m) towards east, or the eastwards, vicinity of Taketa City Hall (at an elevation of about 260 m). Specifically, this survey line runs eastward parallel to National Route 57. Microtremor array observations were conducted at 13 sites with 1-3 km intervals along the survey line (Fig. 1). L-shaped array, consisting of two independent linear arrays (24 m long), were adopted as a measure of very low SNR at narrow/stiff-soil sites (Cho 2023).
The survey results were generally successful showing applicability of our microtremor-survey approach. The S-wave structure model obtained at a Hi-net station close to Taketa City Hall showed a good agreement with the PS logging data at that station. A vertical section that were created by interpolating the 1D S-wave velocity structure models obtained at 13 sites showed thick and thin soft surface layers on the western and eastern sides, respectively, reasonably corresponding to the distribution of Aso-3 and Aso-4 (Fig. 1). This trend was also evident in the distribution of average S-wave velocities to the depths of 10, 20, and 30 m.
We also conducted experimental surveys. A small array consisting of a very small array with a radius of 0.6m (Cho et al., 2013) and 15-m linear array ("bearded array". Yamada et al., 2018) were conducted at a site of Aso-4 area. The results of this array were generally consistent with the results of the above L-shaped array. This site has PS logging data and the S-wave velocity profile obtained by the microtremor surveys were consistent with the PS logging.
Currently, we have a plan to conduct a dense microtremor array survey, using the very small array for the main part of the target region (i.e., the "bearded array" or a one similar to that) and L-shaped arrays depending on necessity.
Specifically, focusing on a pyroclastic flow deposit area (Hoshizumi et al., 2015) of about 10 x 15 km that extends to the east of the Aso Caldera, we plan to conduct an dense evaluation of 1D shallow S-wave velocity structure model and ground resonance frequencies. In this region, thick layers of loam (volcanic ash soil), up to 30 m thick in part, cover the stiff-soil ground formed by the fusion of pyroclastic flow deposits Aso-4B (90ka), which shows flat to gentle slopes with 3° or less in a distance scale of about 10 km, but locally the slopes are 15-30° or more (GSI, 1994). Consequently, the risk of either mudslides or steep slope failures is alarming at multiple places in that region (Kumamoto Prefecture, 2023).
It is said, in general, that microtremor array surveys are less applicable in mountainous areas far from urban areas, especially in rock/stiff-soil sites (Horike, 1993; Geophysical Exploration Society of Japan, 2008). Our target is mountainous or inter-mountainous areas. In addition, although the area is generally covered by a thick layer of volcanic ash (i.e., soft) soil, it is significantly undulating on a local scale, resulting in the stiff-soil ground from the surface locally, which may make the microtremor array survey inapplicable. Therefore, we conducted preliminary observations and analysis to check the applicability of microtremor array survey in this area before the dense evaluation.
A 25-km survey line was set from the east of Aso Caldera (at an elevation of about 800 m) towards east, or the eastwards, vicinity of Taketa City Hall (at an elevation of about 260 m). Specifically, this survey line runs eastward parallel to National Route 57. Microtremor array observations were conducted at 13 sites with 1-3 km intervals along the survey line (Fig. 1). L-shaped array, consisting of two independent linear arrays (24 m long), were adopted as a measure of very low SNR at narrow/stiff-soil sites (Cho 2023).
The survey results were generally successful showing applicability of our microtremor-survey approach. The S-wave structure model obtained at a Hi-net station close to Taketa City Hall showed a good agreement with the PS logging data at that station. A vertical section that were created by interpolating the 1D S-wave velocity structure models obtained at 13 sites showed thick and thin soft surface layers on the western and eastern sides, respectively, reasonably corresponding to the distribution of Aso-3 and Aso-4 (Fig. 1). This trend was also evident in the distribution of average S-wave velocities to the depths of 10, 20, and 30 m.
We also conducted experimental surveys. A small array consisting of a very small array with a radius of 0.6m (Cho et al., 2013) and 15-m linear array ("bearded array". Yamada et al., 2018) were conducted at a site of Aso-4 area. The results of this array were generally consistent with the results of the above L-shaped array. This site has PS logging data and the S-wave velocity profile obtained by the microtremor surveys were consistent with the PS logging.
Currently, we have a plan to conduct a dense microtremor array survey, using the very small array for the main part of the target region (i.e., the "bearded array" or a one similar to that) and L-shaped arrays depending on necessity.