3:00 PM - 3:15 PM
[MIS17-06] Numerical modeling of turbidite formation process for paleoearthquake reconstruction -An Example from the seas around Kikai Island-
Keywords:Turbidite, Shinsei-maru, Ryukyu Trench, Kikai Island, Slope stability analysis, Turbidity current modeling
Geological traces are important to clarify frequencies and maximum magnitudes of prehistoric earthquakes. In addition to tsunamis and uplifts associated with earthquakes, strong motion is left slope failure or turbidity current deposits (turbidites) as geological traces and thus have the potential to reveal past seismic motions. However, there has been no study that reconstructs the process from slope failure (surface resuspension), turbidity current generation, sediment transport, and turbidite formation caused by seismic motion. It is necessary to understand the formation process of turbidites formed by observed earthquakes with known epicenters and magnitudes, and to apply this understanding to paleoearthquake history research. In this study, core samples and exploration data were obtained using the research vessels, and we aim to understand the process from seismic motion to turbidite deposition by reproducing observed turbidite distributions using physical models (seismic motion prediction, slope stability analysis, and turbidity current simulation).
The study area is located around Kikai Island in the central Ryukyu Trench, and three research cruises have been conducted to collect multiple corer samples and acoustic survey data to obtain information on the turbidite distribution. Two turbidite layers were identified up to 150 years ago in two basins. The depositional ages based on the 210Pb and 137Cs dating are correlated with the 1986 (M6.1) and 1911 (M8.0) earthquakes. The 1911 Kikai-jima earthquake is the largest earthquake ever recorded in the Central Ryukyu Trench, and other earthquakes are about M6-7 at most with a frequency of once every five years.
The potential of the two age-correlating earthquakes to generate turbidity currents compared to other earthquakes was examined based on slope stability analysis and ground motion prediction equation. For the slope stability analysis, the simple Bishop method was used to obtain the seismic acceleration that cause slope instability. The maximum areal acceleration was obtained from the formula regressed on S-net data based on the distance from the source fault, Mw, focal depth, and crustal thickness (Nakanishi and Takemura, 2024). Among the 12 earthquakes with Mw > 5.5 around the Kikai Island, the events that caused seismic motion that destabilized the slope facing the basin were consistent with the two events contrasted from the depositional ages.
The unstable slope derived from above analysis was used as the initial area of turbidity current generation, and the distribution of turbidite was obtained from numerical simulations using 'turb2d' (Naruse, 2020). The calculated results can roughly reproduce the observed layer thickness of a few centimeters. The seismic events inferred from the age correlation were shown to be triggering factors with physical implications for the generation and flow process of turbidity currents. Since the turbidite distribution can be explained by a physical model based on seismic characteristics, it is expected to be possible to reconstruct paleoearthquakes from geological traces by solving an inverse problem with the turbidite distribution as an input.
The study area is located around Kikai Island in the central Ryukyu Trench, and three research cruises have been conducted to collect multiple corer samples and acoustic survey data to obtain information on the turbidite distribution. Two turbidite layers were identified up to 150 years ago in two basins. The depositional ages based on the 210Pb and 137Cs dating are correlated with the 1986 (M6.1) and 1911 (M8.0) earthquakes. The 1911 Kikai-jima earthquake is the largest earthquake ever recorded in the Central Ryukyu Trench, and other earthquakes are about M6-7 at most with a frequency of once every five years.
The potential of the two age-correlating earthquakes to generate turbidity currents compared to other earthquakes was examined based on slope stability analysis and ground motion prediction equation. For the slope stability analysis, the simple Bishop method was used to obtain the seismic acceleration that cause slope instability. The maximum areal acceleration was obtained from the formula regressed on S-net data based on the distance from the source fault, Mw, focal depth, and crustal thickness (Nakanishi and Takemura, 2024). Among the 12 earthquakes with Mw > 5.5 around the Kikai Island, the events that caused seismic motion that destabilized the slope facing the basin were consistent with the two events contrasted from the depositional ages.
The unstable slope derived from above analysis was used as the initial area of turbidity current generation, and the distribution of turbidite was obtained from numerical simulations using 'turb2d' (Naruse, 2020). The calculated results can roughly reproduce the observed layer thickness of a few centimeters. The seismic events inferred from the age correlation were shown to be triggering factors with physical implications for the generation and flow process of turbidity currents. Since the turbidite distribution can be explained by a physical model based on seismic characteristics, it is expected to be possible to reconstruct paleoearthquakes from geological traces by solving an inverse problem with the turbidite distribution as an input.