oa The Use of Microtremors to Map Subsurface Structure of Shear Wave Seismic Impedance Boundaries in Unconsolidated Overburden: Examples from Eastern Canada

In overburden areas of the Eastern Canada, shear wave seismic impedance boundaries between soft and firm soils can exceed 20:1; such boundaries are commonly associated with Holocene post-glacial sediments overlying Pleistocene glacial sediments. in some locations, where post-glacial sediments overlie firm bedrock the impedance contrasts can be even larger. in all such cases horizontal-to-vertical spectral ratios (HVSRs) of microtremor ambient seismic noise yield narrow well-defined spectral peaks associated with the fundamental resonant frequency of these sites as governed by the thickness and shear wave velocity-depth function of the soft upper layer. Microtremor instruments using broad-band (0.2 – 60 Hz) 3-component velocity sensors have been utilized in a reconnaissance mode to estimate thethickness of soft soil at survey areas in Eastern Canada where groundwater Investigations or earthquake seismic vulnerability studies are being conducted. Many of these studies have been “ground-truthed” with multi-component Landstreamer reflection seismic lines or with shear wave refraction/reflection/downhole site Investigations. Regional site period-versus-thickness curves have been developed for isopach mapping and cross-section construction. Examples of microtremor applications include detection and delineation of buried valleys in soft Champlain Sea sediments of the Ottawa Valley-St. Lawrence Lowlands. these include: -areas where the subsurface topography of firm ground may have influenced earthquake shaking amplification with subsequent landsliding and other forms of ground deformation, -delineation of buried valleys for positioning of long-term earthquake monitoring facilities, and -mapping the presence of buried valleys for groundwater aquifer determination. Other similar studies have been conducted in the St. John River valley near Fredericton N.B. as an aid in the assessment of earthquake hazards.