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floor stressing the overlaying coastal soils in compression. Periodically a slip will occur which causes the coastal portion to reduce in elevation and thrust toward the west, leading to tsunamis in the central and eastern north
Pacific Ocean (with several hours of warning) and a reflux of water toward the coastal shore, with little time for residents to escape.
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zone under
British Columbia, Washington, Oregon, and far northern California, is perfectly normal, being extremely hazardous in the long term, with the capability of generating coastal tsunamis of several hundred feet in height at the coast. These are caused by the interface between the subducted sea
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Paleoseismic investigations are commonly performed through trenching studies in which a trench is dug and a geologist logs the geological attributes of the rock layers. Trenching studies are especially relevant to seismically active regions, such as many parts of
California.
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Many notable discoveries have been made using the techniques of paleoseismology. For example, there is a common misconception that having many smaller earthquakes can somehow 'relieve' a major fault such as the
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of the
Pacific Northwest. It was thought for some time that there was low seismic hazard in the region because relatively few modern earthquakes have been recorded. It was thought that the
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can be seen in the walls of the trench. It becomes a matter of deducting the relative age of each fault, by cross-cutting patterns. The faults can be dated in absolute terms, if there is
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All of these comforting notions were shattered by paleoseismology studies showing evidence of extremely large earthquakes (the most recent being in
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INQUA Paleoseismology/ web site of the
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formed by liquefaction of sediments during a Late
Ordovician earthquake (northern Kentucky, USA)
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In this typical example, a trench is dug in an active sedimentation regime. Evidence of
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a pit has been dug for public educational purposes. Click image for more information
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James P. McCalpin (2009) Paleoseismology (2nd
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The fault has been marked with cordage and various features labeled.
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of over 8), leave some sort of trace in the sedimentation record.
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Paleoseismicity.org - Online platform for paleoseismologists
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James P. McCalpin (1996) Paleoseismology, Elsevier,
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351:"Caltech Press Release, 7/21/2004, Dr. Kerry Sieh"
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52:Please help
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337:References
255:subduction
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178:sediments
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377:usgs.gov
331:Seismite
310:See also
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