17:15 〜 19:15
[SCG49-P05] Borehole Investigation for Understanding the Seismic and Flood Hazards in the Nakagawa Lowland
キーワード：中川低地、軟弱地盤、強震動、洪水、ボーリング調査
The Nakagawa Lowland region suffered significant damage during the 1923 Kanto earthquake (Moroi & Takemura, 2002), partly due to seismic motion amplification caused by thick soft alluvial deposits (Sekiguchi et al., 2014). Additionally, traces of river flooding are evident in this lowland area, with large-scale inundation recorded during the 1947 Typhoon Kathleen. Sea level changes over the past 10,000 years and the resulting geomorphological evolution of the Kanto Plain have influenced both seismic and flood hazards in this region.
It is crucial to determine the level of strong ground motion and flooding that should be anticipated in lowland areas with soft ground. Therefore, this study conducted all-core boring surveys to obtain data that contribute to the analysis of strong ground motion characteristics of soft ground and the historical occurrence of floods.
All-core drilling survey
A 42-meter deep all-core drilling survey collected samples for facies observations, age dating, soil tests, dynamic deformation tests, PS logging, and microtremor measurements. Core observations revealed changes in the depositional environment. Estuarine system deposits are revealed from 30.7m to 38.2m, with the top layer corresponding to the maximum marine flooding surface, approximately 6,900 years ago. Prodelta silt deposits of the delta system were identified from 22.5m to 30.7m, while alternating sand-silt layers from delta-front deposits were observed between 12.9m and 22.5m. Modern river deposits, including channel sands and floodplain muds, were found between 4.0m and 12.9m and 1.2m to 4.0m, respectively.
Seismic hazard investigation
PS logging over a 37-meter depth measured P-wave and S-wave velocities. Comparison with nearby boreholes confirmed consistent S-wave velocity and N-value trends. A soft layer (Vs = 110-160 m/s) over 30 m thick was identified, deposited when much of the Kanto Plain was below sea level. Microtremor observations at three depths (surface, 14 m, 40 m) showed H/V spectral peaks around 5 s, likely due to deep velocity structures below the engineering bedrock. Peaks around 1 s (at surface and 14 m) correlated with alluvial sediments, while a 0.2 s peak (at surface) was linked to delta-front deposits. Undisturbed samples from 7.9-8.9m, 15.0-15.9m, and 25.0-25.9m were analyzed for nonlinear properties, forming a one-dimensional ground model.
Flood hazard investigation
A sand layer resembling the "inverse grading structure" of flood deposits (Masuda & Iseya, 1985) was found at 3.40-3.70m within modern river deposits and floodplain mud. Radiocarbon dating indicates that the age at a depth of 3.00 m is approximately 1,920 years ago, while the age at a depth of 4.00m is approximately 2,160 years ago.
This confirms event deposits from floods after the Kanto Plain emerged above sea level (~3,000 years ago). Furthermore, five shallow borehole surveys were conducted within a 2 km radius of this site, reaching depths of about 5 m. Preliminary analysis identified a clayey layer likely to be a flood deposit. These findings support reconstructing large-scale flood histories via age dating.
Future work
Using the one-dimensional ground model developed near the survey sites, it is necessary to verify nonlinear behavior and evaluate the accuracy of the model and appropriate assessment methods to assess the impact of widely distributed soft ground. Additionally, installing strong-motion observation stations is essential for empirically evaluating the nonlinear behavior of the ground during earthquakes.
To clarify the timing and scale of floods that inundated the lowland areas of the Kanto Plain, it is necessary to increase the number of borehole survey sites. Furthermore, methods for extracting flood deposits other than the "inverse grading structure," effective site selection, and efficient borehole survey operations must also be examined.
Acknowledgments
We are deeply grateful to the City of Koshigaya for their considerable support during the borehole surveys.
It is crucial to determine the level of strong ground motion and flooding that should be anticipated in lowland areas with soft ground. Therefore, this study conducted all-core boring surveys to obtain data that contribute to the analysis of strong ground motion characteristics of soft ground and the historical occurrence of floods.
All-core drilling survey
A 42-meter deep all-core drilling survey collected samples for facies observations, age dating, soil tests, dynamic deformation tests, PS logging, and microtremor measurements. Core observations revealed changes in the depositional environment. Estuarine system deposits are revealed from 30.7m to 38.2m, with the top layer corresponding to the maximum marine flooding surface, approximately 6,900 years ago. Prodelta silt deposits of the delta system were identified from 22.5m to 30.7m, while alternating sand-silt layers from delta-front deposits were observed between 12.9m and 22.5m. Modern river deposits, including channel sands and floodplain muds, were found between 4.0m and 12.9m and 1.2m to 4.0m, respectively.
Seismic hazard investigation
PS logging over a 37-meter depth measured P-wave and S-wave velocities. Comparison with nearby boreholes confirmed consistent S-wave velocity and N-value trends. A soft layer (Vs = 110-160 m/s) over 30 m thick was identified, deposited when much of the Kanto Plain was below sea level. Microtremor observations at three depths (surface, 14 m, 40 m) showed H/V spectral peaks around 5 s, likely due to deep velocity structures below the engineering bedrock. Peaks around 1 s (at surface and 14 m) correlated with alluvial sediments, while a 0.2 s peak (at surface) was linked to delta-front deposits. Undisturbed samples from 7.9-8.9m, 15.0-15.9m, and 25.0-25.9m were analyzed for nonlinear properties, forming a one-dimensional ground model.
Flood hazard investigation
A sand layer resembling the "inverse grading structure" of flood deposits (Masuda & Iseya, 1985) was found at 3.40-3.70m within modern river deposits and floodplain mud. Radiocarbon dating indicates that the age at a depth of 3.00 m is approximately 1,920 years ago, while the age at a depth of 4.00m is approximately 2,160 years ago.
This confirms event deposits from floods after the Kanto Plain emerged above sea level (~3,000 years ago). Furthermore, five shallow borehole surveys were conducted within a 2 km radius of this site, reaching depths of about 5 m. Preliminary analysis identified a clayey layer likely to be a flood deposit. These findings support reconstructing large-scale flood histories via age dating.
Future work
Using the one-dimensional ground model developed near the survey sites, it is necessary to verify nonlinear behavior and evaluate the accuracy of the model and appropriate assessment methods to assess the impact of widely distributed soft ground. Additionally, installing strong-motion observation stations is essential for empirically evaluating the nonlinear behavior of the ground during earthquakes.
To clarify the timing and scale of floods that inundated the lowland areas of the Kanto Plain, it is necessary to increase the number of borehole survey sites. Furthermore, methods for extracting flood deposits other than the "inverse grading structure," effective site selection, and efficient borehole survey operations must also be examined.
Acknowledgments
We are deeply grateful to the City of Koshigaya for their considerable support during the borehole surveys.