HR: 16:00h
AN: G22E-01 INVITED     [PDF]
TI: Incubation of Chile's 1960 Earthquake
AU: * Atwater, B F
EM: atwater@u.washington.edu
AF: U.S. Geological Survey, University of Washington Box 351310, Seattle, WA 98195 United States
AU: Cisternas, M
EM: marco.cisternas@ucv.cl
AF: Facultad de Agronomia, Universidad Catolica de Valparaiso, Valparaiso, V Chile
AU: Salgado, I
EM: nachosalgado@123.cl
AF: Facultad de Agronomia, Universidad Catolica de Valparaiso, Valparaiso, V Chile
AU: Machuca, G
EM: gmachuca@udec.cl
AF: Departmento de Cs. Historicas y Sociales, Universidad de Concepci˘n, Concepcion, VII Chile
AU: Lagos, M
EM: malagoslo@puc.cl
AF: Instituto de Geografia, Universidad Catolica de Chile, Santiago, Metro Chile
AU: Eipert, A
EM: eiperta@carleton.edu
AF: Department of Geology, Carleton College, Northfield, MN 55057 United States
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: Infrequent occurrence of giant events may help explain how the 1960 Chile earthquake attained M 9.5. Although old documents imply that this earthquake followed great earthquakes of 1575, 1737 and 1837, only three earthquakes of the past 1000 years produced geologic records like those for 1960. These earlier earthquakes include the 1575 event but not 1737 or 1837. Because the 1960 earthquake had nearly twice the seismic slip expected from plate convergence since 1837, much of the strain released in 1960 may have been accumulating since 1575. Geologic evidence for such incubation comes from new paleoseismic findings at the Rˇo Maullin estuary, which indents the Pacific coast at 41.5§ S midway along the 1960 rupture. The 1960 earthquake lowered the area by 1.5 m, and the ensuing tsunami spread sand across lowland soils. The subsidence killed forests and changed pastures into sandy tidal flats. Guided by these 1960 analogs, we inferred tsunami and earthquake history from sand sheets, tree rings, and old maps. At Chuyaquen, 10 km upriver from the sea, we studied sand sheets in 31 backhoe pits on a geologic transect 1 km long. Each sheet overlies the buried soil of a former marsh or meadow. The sand sheet from 1960 extends the entire length of the transect. Three earlier sheets can be correlated at least half that far. The oldest one, probably a tsunami deposit, surrounds herbaceous plants that date to AD 990-1160. Next comes a sandy tidal-flat deposit dated by stratigraphic position to about 1000-1500. The penultimate sheet is a tsunami deposit younger than twigs from 1410-1630. It probably represents the 1575 earthquake, whose accounts of shaking, tsunami, and landslides rival those of 1960. In that case, the record excludes the 1737 and 1837 events. The 1737 and 1837 events also appear missing in tree-ring evidence from islands of Misquihue, 30 km upriver from the sea. Here the subsidence in 1960 admitted brackish tidal water that defoliated tens of thousands of trees. We sampled 45 such trees, some of them completely dead and the rest surviving only from shoots near the ground. One-third of these trees lived through the 1837 earthquake; they contain over 180 annual rings. Five of the trees also contain rings earlier than 1737. From this evidence, we tentatively infer that the islands underwent more subsidence in 1960 than they did in 1737 or 1837. Comparisons with old Chilean documents for the estuary further suggest that subsidence in 1837 did not approach that of 1960. In their depiction and description of the Misquihue islands in 1874, surveyor Francisco Vidal and botanist Carlos Juliet show nothing like the ghost forests seen today. Twice in the first 37 years after the 1837 earthquake, surveyors mapped as emergent several islands that the 1960 earthquake would lower into tidal water. Today, 43 years after they subsided in 1960, these islands remain submerged as barren intertidal flats. Research supported by Fondecyt 1020224.
DE: 4564 Tsunamis and storm surges
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Geodesy [G]
MN: 2003 Fall Meeting