Geologic evidence for great earthquakes during the last 1500 years along the eastern Nankai Trough, central Japan
Abstract
Coastal marsh sequence reveals that great plate boundary earthquakes have caused the coastal area to subside along the eastern Nankai Trough. We analyzed eight drilling cores obtained from a coastal marsh neighboring the plate boundary. The marsh is located on the Philippine Sea plate that subducts beneath Southwest Japan Arc on Eurasian plate in northwestward direction. The plate boundary extends in northeast direction from the Nankai Trough through Suruga Trough to the Fujikawa-kako fault zone in central Japan. The marsh stratigraphy consists of alternation of dark-colored peat and light-colored mud layers, each ranges several tens of cm to 100 cm in thickness. Six couplets of peat and mud layers were recognized from the stratigraphic record formed in the last 1500 years. According to the micro-biological and sedimentological analyses, rapid water-level rise caused the facies change from peat layers to mud layers. Four kinds of criteria strongly suggest that the facies changes reflect the regional coastal subsidence due to the plate boundary earthquakes: 1) wide lateral extent of the facies boundary, 1) the suddenness of facies change, 3) Coincidence of tsunami deposits with the facies boundary, 4) Correspondence to historical earthquakes. Six couplets of peat and mud laterally extend over two km in the marsh. On one hand, the contact of peat layer and overriding mud layer is sharp and often shows scoured surface. On the other hand, the mud layers gradually change into peat layers. These facies changes indicate the rapid water-level rise demolishing the marsh vegetation and gradual recovery of marsh condition. Cross- or parallel laminated sand beds often found at the base of mud layers suggest water-level rise beginning with intrusion of strong sediment flows. Sand beds consisting of multiple fining-upward units, in some cases, suggest the deposition from successive water pulse such as tsunamis or river floods. On the basis of 14C ages and tephrochronology, four peat-mud contacts were correlated to historical earthquakes occurred along the Nankai Trough in AD 684, 1096, 1361 and 1707, respectively. Other two peat-mud contacts, formed in 6th and 8th Century, do not have historical counterparts. They probably indicate the occurrences of earthquakes along the Fujikawa-kako fault zone.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.T53A1123F
- Keywords:
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- 4564 Tsunamis and storm surges;
- 7223 Earthquake interaction;
- forecasting;
- and prediction (1217;
- 1242);
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)