HR: 0800h
AN: T11A-0353 INVITED     [Abstracts]
TI: Recurrence Intervals of Nankai Earthquakes: comparisons of geological and historical evidence
AU: * Matsuoka, H
EM: matsuoka@cc.kochi-u.ac.jp
AF: Kochi Univ, Akebono-cho 2-5-1, Kochi, 7808520 Japan
AU: Okamura, M
EM: mako-ok@cc.kochi-u.ac.jp
AF: Kochi Univ, Akebono-cho 2-5-1, Kochi, 7808520 Japan
AB: Nankai earthquakes are plate-boundary earthquakes associated with the Nankai subduction zone that have occurred repeatedly during historic times. Nankai earthquakes have been recorded in historical documents a total of nine times since the Hakuho Nankai earthquake in A.D. 687. In order to compare geological and historical evidence of the earthquakes, we investigated core sediments from Tadasuga-Ike pond located on the coast along the Nankai trough. The pond was flooded by a tsunami and subsided coseismically during the last Nankai earthquake in A.D. 1946. Fifteen events were identified in the core sediments using stratigraphic, grain size, microfossil (nannoplankton) analyses and radiocarbon dating. Each of these events was exhibited a characteristic sequence, with the lower part of the sequence consisting of sand layers derived from tsunamis covered with shallow, muddy marine sediments. The marine sediment gradually changed to freshwater sediments before an abrupt change to tsunami sand. These sequences reflected coseismic subsidence and interseismic gradual uplift in this region. Radiocarbon dating showed that the fifteen events occurred between 1300 - 4800 cal BP. Artificial disturbance is likely to have resulted in the absence of the sedimentary record younger than 1300 cal BP. The most recent geological event of 1300 cal BP correlated with the oldest historical event of A.D. 687. Some events may have been lost in the sediment record due to erosion of underlying layers by sand layers from the lower core sediments. The mean recurrence interval of 180 years was calculated from six well-preserved successive events between 1300 and 2200 cal BP. The recurrence interval of 180 years was consistent with the value of 215 years obtained in a recent seismoturbidity study conducted in the Nankai Trough, but much larger than the 114 year-average of the previous four earthquakes (AD1605, 1707, 1854 and 1964). These findings suggest that only relatively large Nankai earthquakes have been recorded in the sediments and that the recurrence intervals of resent historical earthquakes are below the historical average.
DE: 4564 Tsunamis and storm surges
DE: 7221 Paleoseismology (8036)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Tectonophysics [T]
MN: Fall Meeting 2005