5:15 PM - 7:15 PM
[MIS11-P02] Tracing tsunami deposits in coastal lowlands using ground-penetrating radar: a case study of Iburi region, Hokkaido
Keywords:Paleoearthquake, Kuril Trench, Japan Trench
The reliability of the numerical tsunami analysis depends largely on the topographic data used. For the numerical analysis of historical and prehistoric tsunamis, it is necessary to reconstruct the paleotopography, and one way is to use the data obtained from field surveys (Sugawara, 2019). Considering the formation process of tsunami deposits, it is considered that the subsurface of tsunami deposits corresponds to the paleotopography because tsunami deposits are deposited on the ground at the time of tsunami attack. Although limited to inland locations where no evidence of tsunami erosion can be seen, tracing the subsurface of tsunami deposits can help to trace and correct partial paleotopography. The GPR (Ground Penetrating Radar) method, which can acquire data nondestructively and continuously, is useful for obtaining the distribution of tsunami deposits over a wide area. While many surveys of tsunami deposits have already been conducted using GPR, there are some challenges. For example, there are issues related to GPR resolution, errors caused when correcting for depth, problems with measurement equipment and data processing, and the reliability of interpretation techniques. Therefore, there is a need for better understanding and robust interpretation of GPR data.
In Atsuma Town, Yufutsu District, Hokkaido, the study area of this research, a single event sand layer interpreted as tsunami deposits from the 17th century has been identified. Additionally, a precise tephrostratigraphy has been established (Takashimizu et al., 2007; Takashimizu et al., 2013). In addition, a trench survey has also been conducted to reveal local horizontal variations in the tsunami sediment layer. A survey line was set up beside this trench, and GPR surveys were conducted using 300 MHz and 800 MHz high-frequency antennas. Depths of metal stakes inserted into the trench wall were used to correct the GPR data. We conducted an excavation survey using a peat sampler and grain size analysis of the collected samples using a sieve to try to capture the correspondence between each reflective surface in the GPR image and the strata boundary.
Comparison of the GPR survey and the excavation results at the trench side survey line shows that the upper edge of the weakly reflective area in the GPR output corresponds to the subsurface of the tsunami deposit. Nanayama et al. (2007) stated that although the peat layer has many voids and high water content, it is relatively homogeneous, and therefore, there is no factor that causes a reflection surface. Based on this interpretation, we traced the boundary between the subsurface of the tsunami sediment layer (or equivalent tephra layer) and the peat layer on longer survey lines around the trench. While we were able to trace the reflection surfaces that corresponded to the subsurface of the tsunami deposit in many survey lines, there were some uncertainties in the interpretation due to ringing and GPR resolution. In addition, the GPR surveys along the side of the trench showed differences in the output results, which may have been caused by the presence or absence of water in the trench, suggesting that the surrounding environment affects the survey using GPR.
In Atsuma Town, Yufutsu District, Hokkaido, the study area of this research, a single event sand layer interpreted as tsunami deposits from the 17th century has been identified. Additionally, a precise tephrostratigraphy has been established (Takashimizu et al., 2007; Takashimizu et al., 2013). In addition, a trench survey has also been conducted to reveal local horizontal variations in the tsunami sediment layer. A survey line was set up beside this trench, and GPR surveys were conducted using 300 MHz and 800 MHz high-frequency antennas. Depths of metal stakes inserted into the trench wall were used to correct the GPR data. We conducted an excavation survey using a peat sampler and grain size analysis of the collected samples using a sieve to try to capture the correspondence between each reflective surface in the GPR image and the strata boundary.
Comparison of the GPR survey and the excavation results at the trench side survey line shows that the upper edge of the weakly reflective area in the GPR output corresponds to the subsurface of the tsunami deposit. Nanayama et al. (2007) stated that although the peat layer has many voids and high water content, it is relatively homogeneous, and therefore, there is no factor that causes a reflection surface. Based on this interpretation, we traced the boundary between the subsurface of the tsunami sediment layer (or equivalent tephra layer) and the peat layer on longer survey lines around the trench. While we were able to trace the reflection surfaces that corresponded to the subsurface of the tsunami deposit in many survey lines, there were some uncertainties in the interpretation due to ringing and GPR resolution. In addition, the GPR surveys along the side of the trench showed differences in the output results, which may have been caused by the presence or absence of water in the trench, suggesting that the surrounding environment affects the survey using GPR.