5:15 PM - 6:45 PM
[HQR05-P06] Using microtremors to discriminate two similar types of slightly elevated topographies: A case of the Pleistocene Omiya terrace from Holocene natural levees, Saitama, Japan
Keywords:microtremor observation, terrace, natural levee, Post-glacial deposits
Pleistocene terraces scattered within alluvial lowlands as isolated hills due to erosion and burial can become very similar to natural levees and difficult to have distinguished them each other based on morphology alone. Distinguishing between those two is important not only for the geologic and geographic history, but is also for disaster prevention, as they have different vulnerability to earthquakes or liquefaction because of difference in geologic components.
Geographic classification is generally based on aerial photos, but as mentioned above, when topography looks similar, indistinct, or not typical, excavation surveys such as drilling are required. However, drilling surveys have limitations in terms of areal data collection in addition to begin time-consuming and expensive. Therefore, we investigated a non-destructive and handy method to discriminate between terraces and natural levees from ground properties based on microtremor observation data.
The survey area is northern part of the Omiya Upland to Kazo Lowland, which extends across Kazo to Kuki cities in Saitama Prefecture. It is composed of an alluvial lowland and the Pleistocene (MIS 5c) Omiya terrace, and parts of the terrace are scattered as isolated topographical highs in the alluvial lowland. In the alluvial lowland, well-developed natural levees are distributed. The Omiya terrace and natural levees have almost the same relative height against the backmarsh.
The geomorphic classification of this area is already described in detail in the geological map "Konosu" (Naya and Yasuhara, 2014). On the other hand, microtremor observations are carried out on grid points at about 1 km intervals in the study area (Senna et al., 2023). Array size of those observations is small enough, enabling high-resolution evaluation of ground properties.
First, as a preliminary analysis, the data of Senna et al. (2023) were classified by geomorphology and confirm the difference between their trends. The results indicate that the S-wave velocities are almost the same down to a few meters below the ground surface on the Omiya terrace and the natural levee but became different deeper. It suggests that the S-wave velocities just below the ground surface are same in the alluvial natural levee and Pleistocene loam layers on the Omiya terraces but are different at depth or have systematic variation in their thickness. This difference is expected to be used as a clue to distinguish between the Omiya terraces and the natural levees in the alluvial lowland.
In order to verify the prospects from the preliminary analysis described above, microtremor observations with micro array were carried out in December 2023 at on the Omiya terraces and the natural levees, where the geography has been confirmed by the soil cane survey by Naya and Yasuhara (2014). Totally eight sites were selected, four each on the Omiya terraces and on natural levees, across Shiraoka, Kuki, Kazo and Konosu cities, Saitama Prefecture. The basal depth of the surface unconsolidated layers (Holocene and loam) was estimated from H/V spectra and S-wave velocities as around 6 m deep from the ground surface on the Omiya terrace and around 12 m deep on the natural levee, both are consistent with the subsurface structure. The phase velocities show a clear difference between the Omiya terrace and the natural levee, for example, those at frequencies of 8.5-9.0 Hz are all faster than 190 m/s on the Omiya terrace, and all are slower than 140 m/s on the natural levee. This indicates that microtremor observation data can be used to discriminate between the Omiya terrace and the natural levee under the right conditions.
Practically, the examination of the grid data of Senna et al. (2023) for 93 sites in the study area showed that the Omiya terrace and the natural levee could be discriminated with 80 % probability when a frequency of 7 Hz and a phase velocity of 170 m/s were used as a threshold.
The remaining 20 % of the data in which the estimation from microtremor observation data are inconsistent with the geographic classification will be discussed to determine the factors of the difference, and the conditions under which this method can be applied will be discussed as a next stage. We will also consider the application of this method to other areas.
Geographic classification is generally based on aerial photos, but as mentioned above, when topography looks similar, indistinct, or not typical, excavation surveys such as drilling are required. However, drilling surveys have limitations in terms of areal data collection in addition to begin time-consuming and expensive. Therefore, we investigated a non-destructive and handy method to discriminate between terraces and natural levees from ground properties based on microtremor observation data.
The survey area is northern part of the Omiya Upland to Kazo Lowland, which extends across Kazo to Kuki cities in Saitama Prefecture. It is composed of an alluvial lowland and the Pleistocene (MIS 5c) Omiya terrace, and parts of the terrace are scattered as isolated topographical highs in the alluvial lowland. In the alluvial lowland, well-developed natural levees are distributed. The Omiya terrace and natural levees have almost the same relative height against the backmarsh.
The geomorphic classification of this area is already described in detail in the geological map "Konosu" (Naya and Yasuhara, 2014). On the other hand, microtremor observations are carried out on grid points at about 1 km intervals in the study area (Senna et al., 2023). Array size of those observations is small enough, enabling high-resolution evaluation of ground properties.
First, as a preliminary analysis, the data of Senna et al. (2023) were classified by geomorphology and confirm the difference between their trends. The results indicate that the S-wave velocities are almost the same down to a few meters below the ground surface on the Omiya terrace and the natural levee but became different deeper. It suggests that the S-wave velocities just below the ground surface are same in the alluvial natural levee and Pleistocene loam layers on the Omiya terraces but are different at depth or have systematic variation in their thickness. This difference is expected to be used as a clue to distinguish between the Omiya terraces and the natural levees in the alluvial lowland.
In order to verify the prospects from the preliminary analysis described above, microtremor observations with micro array were carried out in December 2023 at on the Omiya terraces and the natural levees, where the geography has been confirmed by the soil cane survey by Naya and Yasuhara (2014). Totally eight sites were selected, four each on the Omiya terraces and on natural levees, across Shiraoka, Kuki, Kazo and Konosu cities, Saitama Prefecture. The basal depth of the surface unconsolidated layers (Holocene and loam) was estimated from H/V spectra and S-wave velocities as around 6 m deep from the ground surface on the Omiya terrace and around 12 m deep on the natural levee, both are consistent with the subsurface structure. The phase velocities show a clear difference between the Omiya terrace and the natural levee, for example, those at frequencies of 8.5-9.0 Hz are all faster than 190 m/s on the Omiya terrace, and all are slower than 140 m/s on the natural levee. This indicates that microtremor observation data can be used to discriminate between the Omiya terrace and the natural levee under the right conditions.
Practically, the examination of the grid data of Senna et al. (2023) for 93 sites in the study area showed that the Omiya terrace and the natural levee could be discriminated with 80 % probability when a frequency of 7 Hz and a phase velocity of 170 m/s were used as a threshold.
The remaining 20 % of the data in which the estimation from microtremor observation data are inconsistent with the geographic classification will be discussed to determine the factors of the difference, and the conditions under which this method can be applied will be discussed as a next stage. We will also consider the application of this method to other areas.