Geological Survey of Japan
Discovering Geological Evidence for a Tsunami
Not in Historical Records on the Pacific Coast
of Chiba Prefecture
- SAWAI Yuki, NAMEGAYA Yuichi
Research Institute of Earthquake and Volcano Geology
Released: September 3, 2021
People should be cautioned about the fact the boundary between the Philippine Sea Plate and the Pacific Plate off the Boso Peninsula is also a source of tsunami waves
Two layers of tsunami deposits were discovered by excavation surveys on the coast of Kujukuri Beach in Chiba Prefecture, and the older tsunami deposits were traces of a historically unknown tsunami that occurred approximately 1,000 years ago. A tsunami inundation simulation revealed that if the plate boundary between the Pacific Plate and the Philippine Sea Plate, located off the east coast of the Boso Peninsula, caused an earthquake, a tsunami inundation on the Kujukuri Beach area would be generated—even with a relatively small amount of slip. The results indicate that, in addition to Sagami Trough and the Japan Trench, the area where the Pacific Plate subducts against the Philippine Sea Plate should also be cautioned to be a place that causes huge earthquakes and tsunamis.
Left: Survey scene in the Hasunuma area of Sanmu City.
Right: A continuous column-shaped sediment sampled using a stratum extractor. The samples were used for dating, etc.
Investigation into the history of occurrences of earthquakes/tsunamis
Coastal low-lying sediments have been recorded to have environmental fluctuations over the past few thousand years and are suitable for examining the history of low-frequency giant tsunamis, which are said to occur once every hundred to a thousand years. The Kujukuri Beach region is adjacent to the Boso Triple Junction where three plates, the Pacific Plate, Continental Plate, and Philippine Sea Plate meet in one place, so it is threatened by earthquakes and tsunamis that occur around the Japan Trench, Sagami Trough, and the Izu-Ogasawara Trench that form at each plate boundary. The 1677 Empo earthquake (Empo year 5) and the 1703 Genroku earthquake (Genroku year 16) are recorded in historical documents as representations of the kind of tsunami damage that occurs along the coast of these sea areas. However, the history of occurrences of earthquakes and tsunamis in eras older than these is not clear.
We found tsunami deposits on the coast of Kujukuri Beach and found the possibility of tsunami inundation caused by an earthquake off the east coast of the Boso Peninsula
An excavation survey was conducted on the coast of Kujukuri Beach, Chiba Prefecture, and two distinct layers of sand were found in the peat layer. Based on fossils and sedimentary structure of the sand layers, both sand layers were determined to be tsunami deposits. The older sand layer was the trace of a historically unknown tsunami that occurred about 1,000 years ago. We examined the origin of this sand layer with a tsunami inundation simulation, and found that if an earthquake occurs at the boundary between the Philippine Sea Plate and the Pacific Plate, even if the amount of slip is small (10 m) compared to other plate boundaries, the tsunami would inundate upto the locations of the tsunami deposits along the Kujukuri Beach area. To now, what has been primarily studied is the repeated occurrences of earthquakes in the Sagami Trough and the Japan Trench. The results of this study indicate that the slip of the boundary between the Philippine Sea Plate and the Pacific Plate off the east coast of the Boso Peninsula may also cause tsunami inundation in the Kujukuri Beach area.
We will continue to investigate and research to clarify the history of great earthquakes and tsunamis
Japan's Pacific coast faces a subduction zone, and has been hit repeatedly by massive earthquakes and tsunamis. We aim to clarify the history of the occurrence of massive earthquakes and tsunamis by continuing geological and geophysical surveys and research.
Annotation
This is a collaboration with Jessica Pilarczyk, Toru Tamura, Koichiro Tanigawa, Dan Matsumoto, Tetsuya Shinozaki, Osamu Fujiwara, Masanobu Shishikura, Yumi Shimada, Tina Dura, Benjamin Horton, Andrew Parnell, Christopher Vane. See details in [Related paper].
Related link
Related paper
Jessica E. Pilarczyk, Yuki Sawai, Yuichi Namegaya, Toru Tamura, Koichiro Tanigawa, Dan Matsumoto, Tetsuya Shinozaki, Osamu Fujiwara, Masanobu Shishikura, Yumi Shimada, Tina Dura, Benjamin P. Horton, Andrew C. Parnell, Christopher H. Vane. A further source of Tokyo earthquakes and Pacific Ocean tsunamis. Nature Geoscience (2021)
Contact for inquiries related to this theme
Subduction Zone Paleoearthquake Research Group,
Research Institute of Earthquake and Volcano Geology
SAWAI Yuki, Leader, Group
NAMEGAYA Yuichi, Senior Researcher
AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567 Japan
E-mail: ievg-webmaster-ml*aist.go.jp (Please convert "*" to "@".)
Web: https://unit.aist.go.jp/ievg/group/subducteq/index_e.html