Earth Planets Space, Vol. 60 (No. 11), pp. 1149-1152, 2008LETTER
Heitaro Kaneda1, Makoto Nakata2, Yoshihiro Hosoo3, Yuichi Sugiyama1, and Yukinobu Okamura1
1Geological Survey of Japan, AIST, Tsukuba 305-8567, Japan
2Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
3International Young Researchers Empowerment Center, Shinshu University, Minami-minowa 399-4598, Japan
(Received November 29, 2007; Revised May 19, 2008; Accepted May 29, 2008; Online published November 18, 2008)
At least 300 tons of subrounded to well-rounded wood blocks emerged onto the seafloor at a water depth of 70-100 m during the 2007 Mw 6.6 Chuetsu-oki, central Japan, earthquake. Radiocarbon dating and taxonomic identification of eight of those wood blocks suggest that they were transported from inland during the middle to late Holocene, buried by subsequent sedimentation, and brought up onto the seafloor in 2007, most likely by submarine liquefaction induced by strong shaking. In particular, all eight blocks gave ages older than 2500 cal yr BP, implying the possibility that the 2007 earthquake was the first earthquake during the last two millennia to have caused shaking strong enough to induce submarine liquefaction in the 2007 meizoseismal area. However, we cannot rule out the possibility of multiple large earthquakes after approximately 2 ka, if the buried wood sources cannot be emptied by a single earthquake. Further studies are required to examine paleoseismic implications of the emergence of these wood blocks in 2007.
Key words: 2007 Chuetsu-oki earthquake, wood block, seafloor, liquefaction, radiocarbon dating, paleoseismology.