SEISMIC HAZARD ASSESSMENT OF THE BURLINGTON AND COLCHESTER QUADRANGLES, NORTHWESTERN VERMONT

Northwestern Vermont has a greater earthquake hazard potential than elsewhere in the northeast United States. The hazard derives from proximity to seismic source areas in northern New York and the Montreal-Ottawa seismic zone in Canada and could lead to significant ground shaking, amplification of bedrock motions within overlying soil profiles, and the potential for soils to liquefy and/or spread laterally. In order to understand and plan for these seismic hazards, the Vermont Geological Survey has undertaken this study in western Chittenden County, predominantly in the form of a Seismic Site Class (SSC) map, which will be discussed.

Seismic Site Classifications are based on criteria of the International Building Code, 2006 edition (IBC) and the National Earthquake Hazards Reduction Program. SSCs correspond to the relative stiffness of the soil and rock profile conditions within the top 30 meters preferably as measured directly in terms of soil and rock shear wave velocity (V_s), or indirectly by subsurface explorations providing profile data and penetration resistances or through laboratory/field strength testing of soft soils. SSCs in this study are based on: 1) Site-specific subsurface data, including soil borings, a limited number of Cone Penetration Test soundings (some with V_s measurements), and V_s measurements using the Multichannel Analysis of Surface Waves and Microtremor Array Methods; 2) water well logs; 3) a compilation of bedrock outcrop and shallow soil data, and 4) existing surficial geologic maps.