ABSTRACT: Shake, rattle, and roll! – Evidence of large earthquakes in the geological record.

CGRG Bibliography of Canadian Geomorphology
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Large earthquakes leave telltale geological evidence that can be identified and used to provide a better understanding of local and regional seismic risk. This evidence complements the instrumented record of earthquakes. In regions such as western Canada, which have only a brief history of settlement, the geological record may provide evidence of larger or different types of earthquakes than have occurred historically. The geological signature of even strong earthquakes, however, is subtle. Few obvious signs remain of notable recent large earthquakes, for example those near San Francisco in 1989 (M7.1), Los Angeles in 1994 (M6.7), and Kobe in 1995 (M6.9). Of these three examples, only the Hyogo-ken Nanbu (Kobe)earthquake ruptured the surface, with as much as 2 m of displacement of the ground along the Nojima fault; even in this case, however, little can be seen at the surface only five years after the earthquake. What then is the telltale geological evidence of past earthquakes? The evidence is of two general kinds – some is a direct consequence of displacement on faults and is here termed primary; other evidence is produced by nontectonic surface or near-surface processes related to earthquake shaking or to tsunamis, and is termed secondary.Primary evidence of past earthquakes is geomorphic or stratigraphic in nature and is produced directly by ground rupture or coseismic land-level change. Secondary evidence is a product of ground shaking or tsunamis. Ground ruptureScarps, shutter ridges, linear ridges, sag ponds, offset stream courses, and other related landforms delineate the surface traces of faults and may provide evidence of one or more earthquakes on these structures in the recent geological past . These features have been used successfully to identifyand map active and potentially active faults in many areas, for example California and Japan. However, caution is required in inferring a seismic origin for scarps in areas of moderate and high relief, as they may be produced by gravitational movements rather than earthquakes. Features produced by surface rupture are uncommon in areas that were glaciated in late Pleistocene time because glaciers modified the landscape over which they flowed and left a relatively young (i.e. Holocene), “reset” surface upon which faulting could operate. Faults that experience only rare earthquakes may not have a particularly strong surface expression due to surface weathering and erosion. Some of these limitations are overcome by searching for offset strata in sediment fills that extend across faults. The common approach is todig trenches at sites where faults are draped by geologically young, stratified sediments. Displaced strata exposed in the trench walls are radiocarbon dated and an earthquake chronology constructed. Particularly telling are faults that displace strata up to, but not above, a particular level – dating of the highest displaced beds and lowest intact strata may closely delimit the age of the quake. Coseismic land level changeA large earthquake may raise or lower land over large areas, leaving a geomorphic or stratigraphic imprint. Coseismic uplift may be manifested in elevated terraces – wave-cut platforms in coastal areas and strath or alluvial terraces in valleys. Other independent evidence is generally required to demonstratethat uplift occurred suddenly during an earthquake rather than more gradually due to other processes.

Bibliography of Canadian Geomorphology