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Seismic Microzonation
by Rania Bedair, Ph.D. Eng
Seismic microzonation is the process of subdividing a potential earthquake-prone area into zones with respect to the site’s characteristics so that seismic hazards at different locations within a given area can be correctly identified and site classification maps can be developed. The process is mainly based on estimating the response of the soil layers under earthquake excitations and, thus, the variation of earthquake characteristics on the ground surface. Microzonation provides the basis for site-specific risk analysis which can assist in the mitigation of earthquake damage as well as the evaluation of the vulnerability of existing buildings. In fact, the development of site classification maps is now required for performing advanced seismic hazards and seismic risk assessments in urban areas. Site classification maps can be used by structural engineers for guidance in designing earthquake-resistant structures.
The most widely used microzonation system classifies site response in soil categories based on estimating the mean shear wave velocity of the uppermost 30m of the subsurface layers (VS30). A seismic site’s class is a relative description of the stiffness of the soil or rock column within 30 m of the ground surface and is used to calculate ground surface shaking design levels according to Building Codes. Thus, maps depicting earthquake spectral acceleration levels, corresponding to a 2% probability of occurrence within a 50-year exposure period, can be easily developed. These values represent expected levels of bedrock shaking and are used to calculate design ground acceleration.
Microzonation maps for Greater Montreal (Luc Chouinard, 2013)
Estimates of VS30 from active seismic surveys tend to be expensive and time-consuming to perform over large regions. Alternative methods using ambient noise records have instead been developed to obtain this parameter and, hence, to classify the seismic sites (e.g. Chouinard 2013). Over the last decade, among all of the seismic passive survey methods, the horizontal to vertical (H/V) spectral ratio method has been widely used owing to its low cost and time effectiveness. The technique is based on the measurement of microtremors or environmental seismic noise which are characterized by low energy and amplitude levels (Okada, 2003). It has proven to be one of the most suitable procedures for estimating the fundamental or resonance frequency of soft deposits and thus microzonation maps can be rapidly and inexpensively developed.
Based on field studies and numerical models, Luc Chouinard and his team have applied the H/V technique to develop global microzonation maps for the Island of Montreal. The technique can also be applied in other locations in order to obtain an accurate soil classification for current and future building projects.
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