Urban seismic risk

Urban seismic risk is a subset of the general term seismic risk which describes the problems specific to centers of population when they are subjected to earthquakes. Many risks can be minimized with good earthquake construction, and seismic analysis. One of the best ways to deal with the issue is through an earthquake scenario analysis.

Toronto, Ontario, Canada is used as an example city in this article. The general issues are common to all cities subject to earthquakes.

Cities are a mixture of old and new construction as in this picture. Note the old brick building mixed in with the new highrises, and the famous Toronto CN Tower. Similar to methodologies used in nuclear reactors, a seismic walk down of the city is the best way to identify vulnerabilities and possible places for improvement.

Toronto is located on the shores of Lake Ontario, the site of much microseismicity. Historic records indicate that energetic earthquakes occur infrequently in the region, as with many other cities in the world, though Toronto was struck by a 5.0 magnitude earthquake on June 23, 2010, and a 5.1 magnitude earthquake on May 17th, 2013.

Most new construction complies with strict building codes, and buildings designed for loads that go beyond seismic. For example, Toronto's highrises are, for the most part, firmly situated on bedrock (for settlement reasons), and are designed for hurricane wind loads. They are not expected to have any problems structurally during an earthquake, although they may sway noticeably.

There are many places where the risk of seismic damage to older buildings is quite high. Old brick buildings on poor soils are highly vulnerable to earthquake damage, particularly when the mortar holding the bricks together has decayed. This picture shows an elevated expressway on filled land. Although there may not be total collapse, there will be sufficient damage to incapacitate the structure. Problems increase if there is the possibility for soil or soil liquefaction.

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