SEDIMENTOLOGICAL SIGNATURES OF REGIONAL SEISMICITY IN WESTERN SHIKOKU, JAPAN

The Nankai Trough in southwestern Japan causes great intraplate earthquakes every 100-150 years. Although historical documents are unreliable regarding the magnitude of the resulting tsunamis, sediments preserved in back-barrier lakes allow past storm events to be reconstructed. In these sediments tsunamis are characterized by anomalously coarse deposits with elemental signatures similar to those observed along the site's barrier beach, with chronologies obtained from Cesium-137, heavy metal and Carbon-14 ages. The event of record is believed to be the 1707 Hoei tsunami which has been observed in several back-barrier lakes in Shikoku. Lake Sunokawa which is also located on Shikoku, as well as Lake Ryujin, a lake across the Bungo Channel in Kyushu, both exhibit the 1707 event in their sediments, providing a lateral extent of the tsunami. In addition, lithological changes at other sites in the Bungo Channel at around 1000 years ago suggest that they were caused by a tsunami derived from the Median Tectonic Line. Lagoonal sediments and the barrier beach at Lake Sunokawa in its current form date to 1000 years BP and suggest a major environmental change at this time. However, Lake Sunokawa is closer to the mouth of the Bungo Channel and the Nankai Trough, suggesting that the event at 1000 years BP was less regional than earlier thought.