Sunday, March 21, 2010

Earthquakes, urbanization, and fatalities

Roger Bilham is a fellow of CIRES,the Cooperative Institute for Research in Environmental Sciences, at the University of Colorado, Boulder. His research includes archival research into historical earthquakes, the statistics of urban earthquakes, and global seismic hazards. His web page on the Haiti earthquake includes a link to a news piece he wrote for Nature, where he argues that the destructiveness of the Haiti quake was due not only to its large (although not enormous) magnitude and energy release, but also the amplification of ground shaking by softer sediments, the proximity of Port-au-Prince to the epicenter, and especially to poor building construction practices in a large urban center.

He presents the rather fascinating diagram below, plotting fatalities against earthquake magnitudes:
Note that the vertical scale is logarithmic. There is a general correlation that larger magnitudes cause more deaths, but this is far from a linear plot. Notice that may of the most deadly quakes have been in fairly recent years, and don't highly correlate with magnitude. Why? Urbanization and poor construction. Better earthquake engineering is one of the reasons that the damage in Chile, although bad enough, was less than that in Haiti, although the Chilean quake was much larger.

Dr. Bilham comments:
With a Richter magnitude of M=7 the Haiti earthquake is classed as a major earthquake, however, no previous M=7 earthquake has resulted in this many fatalities (the Messina, Italy, earthquake of 1908 killed 82,000). At 230,000 the death toll is close to being the most lethal earthquake since 1900, and the second most lethal earthquake ever. The large death-toll was caused by the almost complete absence of earthquake resistance in most of the structures in Port-au-Prince and the surrounding towns and villages.

A recent article (2009) discussing the seismic future of cities may be downloaded here. It forecasts a rising death toll from earthquakes, especially in the developing nations, where earthquake resistance building codes are either absent or not enforced.

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