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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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