CGRG Bibliography of Canadian Geomorphology
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Author : Aylsworth, J.M.;and Hunter, J.A.
Date : 2006.
Title : Evidence of two large paleoseismic events in the lower Ottawa Valley, Canada.
Publication : Joint Annual Meeting of the Geological Association of Canada and the Mineralogical Association of Canada. University of Quebec in Montreal (UQAM) May 14-17, 2006.
Issue :
Page(s) :
Abstract
The lower Ottawa Valley has experienced two of the most geologically destructive paleo-earthquakes known to have occurred in eastern Canada. Estimates of magnitude range from a minimum of 6.5, with the epicentre occurring within a distance of 40- 60 km, to a low 7 magnitude. Immediately east of Ottawa, radiometric age dating of 14 massive, retrogressive earthflows indicates that an event, ca. 4550 yr B.P., triggered widespread landsliding in sensitive marine clays over an region of 1250 km2. Overburden thickness indicated a mean value of 43 m near the headscarps, compared to a regional average of 21 m, which suggests that overburden thickness may be a critical factor in the earthquake-induced landsliding. Slightly further east, several areas of severely disturbed marine clay terrain overlie small deep bedrock basins. The largest area, near Alfred, is 50 km2 of severe ground deformation, characterized by irregular, hummocky topography and severely deformed sediments. Local relief varies from 3 to 8 m and individual hollows are 100-300 m in diameter. Sections and borehole cores reveal disturbances ranging from brittle shear to liquefaction to a depth of 50 m. Sand dykes and blows occur at surface. The disturbance is attributed to a massive earthquake, ca. 7060 yr BP, which produced strong ground motion amplification effects in thick (150+ m) soft soils, resulting in severe near-surface sediment deformation, irregular subsidence, and possibly some lateral spreading in an otherwise-flat erosional plain in marine sediments. As well, disturbances to a depth of 20 m were observed on sub-bottom acoustic profiles of the adjacent Ottawa River. Liquefaction of thick bodies of fine sand at depth in the marine sequence likely triggered much of the deformation. P wave seismic surveys established basin shape and depth of soil disturbance. The “Nakamura” method of measuring ambient seismic noise showed a close correlation between spectral peaks and fundamental site periods predicted from shear wave reflection soundings. Thick soft sediments (low Vs) promote broad-band amplification of ground motion due to large near-surface shear wave velocity gradients within the soil column. Although attenuation of high frequency spectral components may occur due to the high attenuation properties of soft soil, significant low frequency amplification may result (during a brief monitoring program, > ×6 amplification has been recorded for very small quakes). Significant resonance amplification of earthquake ground motion in the 0.3 to 1 Hz range is probable (> ×8 amplification at the fundamental site period).
Bibliography of Canadian Geomorphology