1887

Abstract

The National Building Code of Canada has been amended to improve predicted earthquake accelerations on soil and rock sites. As a basis, such estimations utilize the description of soils and rocks as given by the zones defined by the National Earthquake Hazard Reduction Program (NEHRP) of the U.S.A.; these zones are primarily based on the measured travel-time-weighted average shear wave velocity of a site from surface to a depth of 30 m (Vs30). NEHRP zone maps are being considered for major cities in high earthquake hazard zones in Canada. As a demonstration project, a combined research team from Carleton University and Geological Survey of Canada are surveying the Ottawa, Ontario area. Within the city boundaries, NEHRP zones are estimated to vary between NEHRP A (firm bedrock) and NEHRP E (> 30 m of soft low Vs soil). Although a substantial database of subsurface geological information has been previously compiled, based on water-wells, geotechnical boreholes and shallow refraction seismic studies, very few subsurface Vs measurements of soil or rock properties exist. For this work, surface refraction/reflection methods are currently being applied in a reconnaissance manner throughout the study area to obtain regional estimates of average Vs profiles versus depth. The technique requires a 24-channel seismograph, a polarized shear wave surface source, and horizontal axis geophones oriented in SH mode. To date, the eastern half of the city has been surveyed with 200 surface seismic sites situated at an approximate 2 km spacing. Many of the sites were positioned in city parks in densely urbanized areas, and along roadsides in suburban regions. As expected, high Vs30 values (> 2000 m/s) have been obtained in areas of thin overburden overlying Paleozoic bedrock, and very low Vs30 values (<180 m/s) have been obtained in areas of thick Holocene clay. In addition, buried bedrock valleys filled with soft sediments have been identified and delineated for future 3-dimensional resonance studies.

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/content/papers/10.3997/2214-4609-pdb.181.129
2006-04-02
2024-03-28
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