Shear wave seismic test locations and boreholes have outlined a buried bedrock valley in the Ottawa, Ontario, suburb of Orleans. This region is in a significant high seismic hazard zone, and the surficial materials are primarily high water-content, poorly compacted Holocene-age Champlain Sea sediments. Since the in-filled sediments exhibit an extremely low average shear wave velocity (~200 m/s) and the bedrock beneath exhibits shear wave velocities on the order of 2500 m/s, it is suspected that the buried valley may give rise to three-dimensional ground motion amplification phenomena in the event of significant earthquakes. In order to better define the buried valley and to prepare for future three-dimensional shake modeling, shear-wave reflection landstreamer lines were acquired, to confirm the depth to bedrock, and to detail its shape. Despite the large broad-frequency band of ambient traffic noise, it was possible to observe reflections from the bedrock as well as additional infra-overburden reflectors. The valley shape and its internal structure, as determined by these surveys, will form a vital contribution to the three-dimensional interpretation for soil response to earthquake shaking in the Ottawa area.


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