5:15 PM - 7:15 PM
[MIS11-P09] Numerical and field investigations of coastal boulder displacement by tsunamis and storm surges in Beppu Bay
Keywords:Coastal boulder, Simulation, Tsunami boulder, Paleo tsunami
Historical records suggest that the 1596 CE Keicho Bungo earthquake (M=7.0±1/4), which occurred on a submarine active fault in the bay, triggered a tsunami with an estimated height of approximately 4–8 m along the coast of Beppu Bay in Oita Prefecture (Hatori, 1985). Furthermore, tsunami deposits in the coastal lowlands indicate that the coastal areas of Beppu Bay have been repeatedly inundated by tsunamis (Yamada et al., 2021). The size of boulders on Itogahama beach was measured during a field survey in 2016, and the boulder weight decreased offshore, indicating that boulders that fell from the cliff behind the tidal flat, which serves as the source area, may have been reworked offshore by tsunamis or storm surges. Goto et al. (2010) reported that the weight of boulders in Ishigaki Island, Okinawa Prefecture, tends to decrease exponentially toward the landward direction, in contrast to the trend observed in our study site. Therefore, it is possible that a tsunami reflected from the cliffs reworked the boulders on the tidal flat, resulting in their present distribution. Numerically identifying the reworked boulders transported by tsunamis may allow us to estimate the magnitude of past earthquakes and tsunamis. This study aims to elucidate the magnitude of the largest earthquake and tsunami that occurred in Beppu Bay in the past. The study consists of the following steps: (1) reinvestigation of the sizes and locations of coastal boulders, (2) estimation of the maximum weight of boulders that could be moved by a typhoon using storm wave calculations, and (3) identification of tsunami boulders and estimation of earthquake and tsunami magnitudes using tsunami calculations. In this presentation, we will report on the research progress, focusing on (1) and (2).
First, a field survey was conducted immediately after the passage of Typhoon No. 2410 to verify whether it had moved the boulders by comparing the location of barnacle communities attached to the boulders with each other, in addition to remeasuring the distribution, axial length, and volume of the boulders. We measured the boulders using a drone to obtain more accurate information on their distribution and weight. The volume of approximately 1000 boulders was calculated from the images taken by the drone. In addition, the orientation and length of the long axis were measured. A comparison of the DSM datasets from 2016 and 2024 taken in the same area showed that boulders larger than 10 tons did not move, but a boulder of about 30 tons located at the center of the boulder distribution area, about 30 m from the cliff, was observed to have rotated by 30°. Furthermore, elevation measurements of the barnacle communities showed no significant differences in height among the boulders, suggesting that this typhoon is unlikely to have moved the boulders through rolling or saltation.
Next, we examined the movement of boulders caused by Typhoon No. 2410 (a once-in-a-few-decades-class typhoon) and the 1959 CE Isewan Typhoon (the largest typhoon ever recorded in this area) using the storm wave simulation model CADMAS-SURF (Arikawa et al., 2019). The calculation results for Typhoon No. 2410 indicated that boulders weighing 30 tons or less were displaced, whereas the simulation for the 1959 CE Isewan typhoon suggested that boulders remained stationary due to wave attenuation near their current positions. These results imply that boulders larger than 30 tons are unlikely to be displaced even by the most extreme typhoons. Consequently, we will conduct tsunami boulder transport calculations focusing on boulders exceeding 30 tons. We will first examine whether boulders of this size could have been moved by a tsunami generated by the 1596 CE Keicho-Bungo earthquake (M=7.0±1/4). Additionally, we aim to estimate the magnitude of the largest possible earthquake and tsunami capable of explaining the current boulder distribution.
First, a field survey was conducted immediately after the passage of Typhoon No. 2410 to verify whether it had moved the boulders by comparing the location of barnacle communities attached to the boulders with each other, in addition to remeasuring the distribution, axial length, and volume of the boulders. We measured the boulders using a drone to obtain more accurate information on their distribution and weight. The volume of approximately 1000 boulders was calculated from the images taken by the drone. In addition, the orientation and length of the long axis were measured. A comparison of the DSM datasets from 2016 and 2024 taken in the same area showed that boulders larger than 10 tons did not move, but a boulder of about 30 tons located at the center of the boulder distribution area, about 30 m from the cliff, was observed to have rotated by 30°. Furthermore, elevation measurements of the barnacle communities showed no significant differences in height among the boulders, suggesting that this typhoon is unlikely to have moved the boulders through rolling or saltation.
Next, we examined the movement of boulders caused by Typhoon No. 2410 (a once-in-a-few-decades-class typhoon) and the 1959 CE Isewan Typhoon (the largest typhoon ever recorded in this area) using the storm wave simulation model CADMAS-SURF (Arikawa et al., 2019). The calculation results for Typhoon No. 2410 indicated that boulders weighing 30 tons or less were displaced, whereas the simulation for the 1959 CE Isewan typhoon suggested that boulders remained stationary due to wave attenuation near their current positions. These results imply that boulders larger than 30 tons are unlikely to be displaced even by the most extreme typhoons. Consequently, we will conduct tsunami boulder transport calculations focusing on boulders exceeding 30 tons. We will first examine whether boulders of this size could have been moved by a tsunami generated by the 1596 CE Keicho-Bungo earthquake (M=7.0±1/4). Additionally, we aim to estimate the magnitude of the largest possible earthquake and tsunami capable of explaining the current boulder distribution.