HR: 1340h
AN: G13A-0790    [Abstracts]
TI: Transient uplift since the 1960 Chilean earthquake
AU: * Shishikura, M
EM: m.shishikura@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan/AIST, SiteC7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Youlton, C
EM: cristian.youlton.m@mail.ucv.cl
AF: Facultad de Agronomy, Universidad Catorica de Valparaiso, San Francisco s/n, La Palma, Quillota, Casilla4-D Chile
AU: Sawai, Y
EM: yuki.sawai@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan/AIST, SiteC7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: Postseismic uplift midway along the 1960 Chile rupture appears to be slowing down. We studied the uplift near Puerto Montt, 70 km east of the Pacific coast. Puerto Montt is located on the boundary between the 1960 earthquake's coseismic and a coseismic and/or postseismic upwarp farther inland. This deformation was mapped by George Plafker in 1968, and the postseismic uplift was checked by him and others in 1989. The postseismic uplift has been attributed to afterslip on the plate interface, stress relaxation in the forearc mantle, or both. We obtained further information about shoreline change before and after the earthquake from the Pacific coast to 120 km eastward inland, about midway along the length of the 1960 rupture. Following in Plafker's footsteps, we used several criteria for distinguishing between coseismic and postseismic movement. We asked eyewitnesses whether sea level changed at the time of the earthquake, and whether it has changed in the decades since. We also looked for emerged landforms and for environmental changes in vegetation. Where reliable testimony or natural features provided exact reference points, we measured differences in height by means of total station. Twenty-seven places yielded numerical estimates of land-level changes since 1960. The largest total amount of displacement during past 44 years was estimated to be 2.1 m in Chamiza, 10 km east from Puerto Montt. According to the interviews, most of the uplift occurred during or in the first month the earthquake. The total amount of uplift tends to decrease eastward from Chamiza. The easternmost point (60 km east from Puerto Montt) did not rise or fall during the 1960 earthquake, but gradually rose nearly 1 m in the first 40 years thereafter. The postseismic uplift has probably been canceled by subsidence later in earthquake deformation cycles. The Puerto Montt tide gauge record shows little or no uplift in the 1990s. Similarly, some eyewitnesses in and near Chamiza say that in the past decade, the sea has begun to rise. Although historical (1575) and prehistoric earthquakes perhaps comparable to the 1960 event produced stratigraphic evidence for tsunami and coseismic subsidence 55 km to the west, near Maullin, we have not found marine terraces that would indicate net late Holocene uplift in the Puerto Montt area. George Plafker, through Brian Atwater, kindly gave us copies of his field notes from 1968 and 1989. Marco Cisternas organized the project. Youlton's work supported by Fondecyt 1020224.
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting