Japan Geoscience Union Meeting 2015

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS25] tsunami deposit

Sun. May 24, 2015 9:00 AM - 10:45 AM 201B (2F)

Convener:*Kazuhisa Goto(International Research Institute of Disaster Science (IRIDeS),Tohoku University), Masanobu Shishikura(Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology), Yuichi Nishimura(Graduate School of Science, Hokkaido University), Chair:Kazuhisa Goto(International Research Institute of Disaster Science (IRIDeS),Tohoku University), Yasuhiro Takashimizu(Mathematical and Natural Sciences, Institute of Humanities, Social Sciences and Education, Niigata University)

9:15 AM - 9:30 AM

[MIS25-02] Evaluating the magnitude of late Holocene mega-tsunamis on Ishigaki Island based on an analysis of molluscan assemblages

*Akihisa KITAMURA1, Mami ITO2, Masataka ANDO3, Ryoya IKUTA1, Masayuki IKEDA1, Takafumi IMAI2 (1.Faculty of Science, Shizuoka University, 2.Graduate School of Science, Shizuoka University, 3.Center for Integrated Research and Education of Natural hazards, Shizuoka University)

Keywords:large tsunami, Ishigaki Island, late Holocene, analysis of molluscan assemblages

Four ancient tsunami deposits have been identified on Ishigaki Island, Okinawa, Japan. Three tsunami deposits (T-I, T-II, and T-IV) are calcareous sand beds which have sharp erosional bases and normal graded structure, while one tsunami deposit (T-III) is buried tsunami boulders between tsunami deposits T-II and T-IV. The youngest tsunami deposit T-I, was caused by the AD 1771 Meiwa tsunami, which had a wave height of at least 10 m in the study area. The depositional ages of the three older tsunami deposits (T-II, T-III, and T-IV) are 790-610 cal. yrs BP, 1494-1258 cal. yrs BP and 2502-2287 to 1494-1258 cal. yrs BP, respectively. The elevations of the landward margins of sandy tsunami deposits T-I, T-II, and T-IV are up to 9, 6, and 8 m, respectively. This study examines the influence of the local topography, such as beach ridge and reef system, on the runup of these ancient tsunamis based on a comparison of molluscan assemblages in the tsunami deposits T-I, T-II, and T-IV and those found in recent beach deposits. The results show that the conditions in the reef system off the study area have remained largely unchanged since the occurrence of tsunami T-IV. Beach ridge during the occurrence of tsunami T-IV did not become well developed relative to those during the occurrence of tsunami T-I and T-II. These findings suggest that the magnitude of tsunami T-II were smaller than those of tsunamis T-I and T-IV, while the magnitude of tsunami T-IV did not reach that of tsunami T-I (AD 1771 Meiwa tsunami).