HR: 14:10h
AN: G13C-03    [Abstracts]
TI: Master Cycles of Land-Level Change and Great-Earthquake Recurrence in Cascadia, Northeast Japan, and Southern Chile
AU: * Atwater, B F
EM: atwater@u.washington.edu
AF: U.S. Geological Survey at University of Washington, Box 351310, Seattle, WA 98195 United States
AB: In examples from three subduction zones, events of the past few thousand years settle apparent conflicts between rates averaged across shorter and longer periods of time. At these zones the major plate-boundary earthquakes recur in cycles too long to measure with historical and instrumental records alone. CASCADIA. The Pacific coast of Washington State rose faster in the past 100 years than it did, on average, in the past 125,000 years. This difference became apparent a quarter century ago, when coastal uplift rates were estimated geodetically by leveling and tide gauging. The short-term rates were found to exceed, by an order of magnitude, long-term rates shown by Pleistocene marine terraces. Ensuing work by many investigators revealed a Holocene history of coseismic subsidence, not only in Washington but also in British Columbia, Oregon, and California. The subsidence, at irregular intervals averaging about 500 years, mostly negates gradual uplift like that measured geodetically. The Pleistocene terraces thus follow a sawtoothed trajectory, ten steps gradually up and nine steps abruptly down. This resolution of a geodesy-vs-geology conflict laid groundwork for today's consensus that Cascadia produces great plate-boundary earthquakes. JAPAN. Short-term subsidence seemingly conflicts with long-term uplift along the southern Kuril trench. Eastern Hokkaido subsided 1 m in the past 100 years (tide gauges, bench marks) even though it rose 20-50 m in the past 125,000 years (marine terraces). Japanese geophysicists recognized the conflict over three decades ago. Some predicted that a moment-magnitude 7.8 earthquake in 1973 would resolve it. But no onshore uplift occurred during the earthquake and less than 0.1 m of uplift followed. A M 8.0 earthquake in 2003 similarly left the region's long-term uplift unexplained. However, eastern Hokkaido has a Holocene history of episodic uplift. The episodes, identified by Yuki Sawai and his coworkers, are recorded by forest peat that overlies estuarine mud. The region rose about 1 m during individual episodes. The cause of uplift is being investigated. The prime suspect is postseismic deformation induced by plate-boundary earthquakes of M ~8.5 or larger. Such events are unknown in the region's 200 years of written history.. CHILE. The M 9.5 1960 earthquake seemingly overspent its plate-tectonic budget. Its slip, 20 m on average, expended 250 years of plate motion at the NUVEL-1 convergence rate and over 500 years of plate motion at convergence rates recently estimated by campaign GPS. Yet Chile's written history contains three predecessors to the 1960 event -- in 1837, 1737, and 1575 -- for recurrence intervals averaging just 128 years. When did the 1960 slip accumulate? Marco Cisternas and his coworkers are finding clues at an estuary midway along the 1960 rupture. Sand sheets on its margins record subsidence, a tsunami, or both from 1960, 1575, and six preceding events of the past 2000 years. Consistently absent are signs of the 1737 and 1837 events; these apparently failed to break much if any of the central part of the 1960 rupture area. In that case, infrequent giants like the 1960 earthquake expend most of the plate convergence in south-central Chile.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 4564 Tsunamis and storm surges
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting