HR: 1340h
AN: G13A-0788    [Abstracts]
TI: Estuarine Evidence Of Postseismic Transients In 17th-century Hokkaido And 20th-century Chile
AU: * Sawai, Y
EM: yuki.sawai@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Kamataki, T
EM: t.kamataki@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Nasu, H
EM: nhiroo@nichibun.ac.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Nasu, H
EM: nhiroo@nichibun.ac.jp
AF: International Research Center for Japanese Studies, Oeyama-cho, Goryo, Nishikyo-ku, Kyoto, 610-1192 Japan
AU: Satake, K
EM: kenji.satake@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Shishikura, M
EM: m.shishikura@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: We studied postseismic transient of two giant interplate earthquakes; one is a 17th-century earthquake along the southern Kuril trench, eastern Hokkaido, and the other is the 1960 Chilean earthquake. The 17th-century earthquake along the southern Kuril trench induced postseismic uplift that probably lasted for decades, as judged from stratigraphy and paleoecology at two estuaries along the Pacific coast of eastern Hokkaido. The estuaries, at Mochirippu and Kiritappu, were invaded by an unusually large tsunami before their tidal flats emerged and became lowland forests. The tsunami is marked by a sand sheet mainly 2-5 cm thick. The sand is both underlain and overlain by tidal-flat mud. However, the mud above the sand grades upward into peat that contains volcanic ash layers from late in the 17th century. The upward sequence of mud, sand, mud, and peat implies that an earthquake (marked by the tsunami deposits) predates gradual coastal uplift (marked by the transition from mud to peat). We quantified the land-level history by means of fossil diatom assemblages in the mud and peat. The reconstructed land-levels show several decimeters of preseismic subsidence, no coseismic change, and at least 1 m of postseismic uplift. The volcanic ash layers, erupted from southwest Hokkaido and dated with the local historical records, show that the uplift started before 1667 and ended by 1694. Postseismic uplift of the 1960 Chilean earthquake provided stratigraphic and paleoecological evidence. The delta of the Rio Coihuin is in middle along the 1960 rupture zone and 10 km east of Puerto Montt, which was at the inland limit of the coseismic downwarp in 1960. The delta contains three terraces, each underlain by a different stratigraphic sequence. The_@highest terrace, now above all tides, is underlain by lahar-runout sand_@and silt derived from Mt. Calbuco, 20 km to the northeast. Inset into_@this terrace are at least lower ones that form tidal marshes. The marshes_@are underlain by intertidal deposits dominated by mud and peaty mud. A family that has farmed a remnant of the lahar-runout terrace provided_@detailed accounts of changes in land level since the 1960 earthquake._@The highest terrace had been inundated by tidal water during the monthly high tide and dominated by a tidal marsh plant, Salicornia sp., before the 1960 event. The middle terrace had been also dominated by other tidal marsh plant, Juncus balticus and Scripus americanus that the family used for their craft. However, after the event, the terraces emerged and the tidal vegetation changed. The highest terrace became freshwater upland forest, and the middle terrace increased Agrostis alba and Salicornia sp. Tidal flat changed to the present lowest terrace dominated by Puccinellia sp. Guided by this testimony, we checked deposits beneath the middle terrace beside the family's house. These deposits consist of peat, mud, sand, and volcanic ash layers. _@The uppermost peat-over-mud contact probably represents emergence that the family has been watching after the 1960 earthquake, though we cannot yet date the contact. Above the peat-over-mud contact, the peat contains rhizomes of Spartina densiflora and plant macrofossils of salt-tolerant vascular plants. Judging from the present vegetation around the delta, this change in vascular plants shows at least 1m of emergence.
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting