AGE AND SIGNIFICANCE OF EARTHQUAKE-INDUCED LIQUEFACTION NEAR VANCOUVER, BRITISH-COLUMBIA, CANADA

In late 1994, sand dykes, large sand blows, and deformed strata were e xposed in the walls of an excavation at Annacis Island on the Fraser R iver delta near Vancouver, British Columbia. The features record lique faction during a large earthquake about 1700 years ago; this was perha ps the largest earthquake to affect the Vancouver area in the last 350 0 years. Similar, less well-dated features have been reported from sev eral other sites on the Fraser delta and may be products of the same e arthquake. Three radiocarbon ages that closely delimit the time of liq uefaction on Annacis Island are similar to the most precise radiocarbo n ages on coseismically subsided marsh soils at estuaries in southern Washington and Oregon. Both the liquefaction and the subsidence may ha ve been produced by a single great plate-boundary earthquake at the Ca scadia subduction zone. Alternatively, liquefaction at Annacis Island may have been caused by a large crustal or subcrustal earthquake of ab out the same age as a plate-boundary earthquake farther west. The data from Annacis Island and other sites on the Fraser delta suggest that earthquakes capable of producing extensive liquefaction in this area a re rare events. Further, liquefaction analysis using historical seismi city suggests that current assessment procedures may overestimate liqu efaction risk.