Ground motions during the Haiti earthquake were significantly amplified along a mountain ridge, causing substantial structural damage, reports a study published online this week in Nature Geoscience. The paper is published this month as part of Nature Geoscience’s special issue on the Haiti earthquake.

Microzonation maps use local geological conditions to characterize seismic hazard, but do not generally consider topography — the shape of the Earth’s surface. Susan Hough and colleagues measured aftershocks following the Haiti quake to assess how ground motion varied in different parts of Port-au-Prince. They compared sites located on soft sediments, hard ground and rocky, mountainous ridges. As expected, structures built on soft sediments experienced enhanced shaking but, surprisingly, the strongest ground motions occurred on a mountainous ridge and contributed to the collapse of a hotel.

These findings indicate that topography, as well as the local rock type, plays an important role in seismic hazard.