11:15 AM - 11:30 AM
[HDS11-08] Investigation of Underwater Ruins in Lake Biwa to Elucidate Past Catastrophic Earthquake-Induced Geohazards
1. Introduction
There are approximately 90-100 underwater archaeological sites along the coast of Lake Biwa (Hayashi et al., 2012). However, the causes of their submergence remain largely unknown. According to data from the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) (Yodo River Basin Committee, 2007), fluctuations in the water level of Lake Biwa over the past several hundred years are believed to have been minimal, suggesting that they are unlikely to be the primary factor in the submergence of these sites. Furthermore, recent ground deformation data from satellite observations covering approximately the past six years indicate that slow subsidence is also unlikely to be a major factor. On the other hand, Hayashi et al. (2012) suggested that some underwater ruins in Lake Biwa may be traces of sudden ground subsidence and collapse triggered by earthquakes.
This study aims to analyze the formation factors of these underwater ruins in relation to earthquake disasters. The area surrounding Lake Biwa contains numerous active faults, and historical records indicate that large earthquakes have occurred in the past (The Headquarters for Earthquake Research Promotion, Japan). Earthquake-induced phenomena such as ground subsidence, liquefaction, and landslides may have contributed to changes in the coastal landscape, leading to the formation of underwater ruins. To verify this hypothesis, field investigations will be conducted using underwater acoustic survey techniques and terrestrial geophysical exploration to clarify the relationship between earthquake disasters and the distribution of these ruins.
Additionally, by comparing these findings with similar cases worldwide, this study aims to elucidate the mechanisms behind the formation of underwater ruins. This will provide a deeper understanding of catastrophic geohazard in coastal geological environment and contribute to the assessment of future earthquake disaster risks for coastal areas.
2. Research Methods and Study Area
This study adopts a comprehensive approach that combines geophysical exploration, soil exploration techniques, direct observations using diving and underwater drones, and archaeological analysis in collaboration with archaeologists. Geophysical exploration will employ sub-bottom profiler, side-scan sonar, and multibeam echosounder, alongside surface wave exploration of the lakebed to capture detailed information on the underwater topography and sedimentary structures. Additionally, boring exploration will be conducted in coastal terrestrial areas.
The study focuses on two sites along the eastern coast of the northern part of Lake Biwa: the Naoesengen site in Maibara City and the Shimosakahamasengen site in Nagahama City. The Naoesengen site has been linked to the 1325 earthquake (Hayashi et al., 2012), while the Shimosakahamasengen site is believed to be associated with the 1586 Tenshō earthquake (Hayashi et al., 2012). Previous surveys at both locations have identified anomalous underwater topography suggesting large-scale ground deformation. However, existing data remain insufficient for fully elucidating the formation mechanisms, necessitating further investigation.
3. Preliminary Results
Initial investigations at the Naoesengen site using underwater acoustic surveys have yielded the following findings: In some areas, the underwater topography appears shallower and extends into the lake in a tongue-like shape, spanning approximately 500 meters. Sub-bottom profiler data revealed strong reflections at depths of 3-6 meters beneath the lakebed, characterized by irregular surfaces.
4. Conclusion
At the time of the presentation, additional comparative analysis with similar cases worldwide will be provided to facilitate further discussion.
There are approximately 90-100 underwater archaeological sites along the coast of Lake Biwa (Hayashi et al., 2012). However, the causes of their submergence remain largely unknown. According to data from the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) (Yodo River Basin Committee, 2007), fluctuations in the water level of Lake Biwa over the past several hundred years are believed to have been minimal, suggesting that they are unlikely to be the primary factor in the submergence of these sites. Furthermore, recent ground deformation data from satellite observations covering approximately the past six years indicate that slow subsidence is also unlikely to be a major factor. On the other hand, Hayashi et al. (2012) suggested that some underwater ruins in Lake Biwa may be traces of sudden ground subsidence and collapse triggered by earthquakes.
This study aims to analyze the formation factors of these underwater ruins in relation to earthquake disasters. The area surrounding Lake Biwa contains numerous active faults, and historical records indicate that large earthquakes have occurred in the past (The Headquarters for Earthquake Research Promotion, Japan). Earthquake-induced phenomena such as ground subsidence, liquefaction, and landslides may have contributed to changes in the coastal landscape, leading to the formation of underwater ruins. To verify this hypothesis, field investigations will be conducted using underwater acoustic survey techniques and terrestrial geophysical exploration to clarify the relationship between earthquake disasters and the distribution of these ruins.
Additionally, by comparing these findings with similar cases worldwide, this study aims to elucidate the mechanisms behind the formation of underwater ruins. This will provide a deeper understanding of catastrophic geohazard in coastal geological environment and contribute to the assessment of future earthquake disaster risks for coastal areas.
2. Research Methods and Study Area
This study adopts a comprehensive approach that combines geophysical exploration, soil exploration techniques, direct observations using diving and underwater drones, and archaeological analysis in collaboration with archaeologists. Geophysical exploration will employ sub-bottom profiler, side-scan sonar, and multibeam echosounder, alongside surface wave exploration of the lakebed to capture detailed information on the underwater topography and sedimentary structures. Additionally, boring exploration will be conducted in coastal terrestrial areas.
The study focuses on two sites along the eastern coast of the northern part of Lake Biwa: the Naoesengen site in Maibara City and the Shimosakahamasengen site in Nagahama City. The Naoesengen site has been linked to the 1325 earthquake (Hayashi et al., 2012), while the Shimosakahamasengen site is believed to be associated with the 1586 Tenshō earthquake (Hayashi et al., 2012). Previous surveys at both locations have identified anomalous underwater topography suggesting large-scale ground deformation. However, existing data remain insufficient for fully elucidating the formation mechanisms, necessitating further investigation.
3. Preliminary Results
Initial investigations at the Naoesengen site using underwater acoustic surveys have yielded the following findings: In some areas, the underwater topography appears shallower and extends into the lake in a tongue-like shape, spanning approximately 500 meters. Sub-bottom profiler data revealed strong reflections at depths of 3-6 meters beneath the lakebed, characterized by irregular surfaces.
4. Conclusion
At the time of the presentation, additional comparative analysis with similar cases worldwide will be provided to facilitate further discussion.