This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, October 21, 2010

Hotel Montana, Haiti, and amplified seismic waves

Hotel Montana, Haiti, after the January 12, 2010 earthquake
Photo credit

The 2010 Haiti earthquake killed over 230,000 people, and caused extensive damage in the capital, Port-au-Prince.  Three factors have generally been cited as causes of the extensive damage: (1) the proximity of the city to the earthquake; (2) poor construction; and (3) liquifaction and soft-sediment amplification. These factors do not, however, explain why the relatively well-constructed buildings, such as the Hotel Montana, two United Nations buildings, and a number of substantial private residences sitting on a relatively hard bedrock ridge also suffered extensive damage. In a Nature article published on-line recently, a fourth factor has been recognized. Hough et al. (Nature Geoscience, October 17, 2010 on-inline publication) installed portable seismometers to monitor after-shocks and found that the topographic shape of the ridge amplified the ground motions were strongly amplified at frequencies between ~0.5-20 Hz, a frequency range that corresponds to the fundamental periods of 1-5 story buildings.  By modeling the ridge as a wedge with an internal angle of 135 degrees, and a width of 400 m, they were able to provide an analytic solution that an amplification of 2.7 for frequencies of ~7 Hz, in good agreement with the observations.  This work suggests that topographic effects need to be incorporated into microzonation maps that characterize seismic hazards.

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