Acceleration due to seismic waves - Chile poster

One of the primary hazards intrinsically associated with an earthquake is groundshaking.  This is a vector, with both horizontal ad vertical components.  The second derivative of the displacement is the acceleration.   Buildings are made to withstand vertical acceleration - after all, that is what gravity is.  However, by default, buildings are not engineered to withstand horizontal accelerations, although lateral bracing can be used, and should be within seismic zones.

The figure below, from our USGS poster on the Chile quake, plots ground motion accelerations from the Chile quake, and shows a couple of very interesting features. Note that the maximum acceleration caused by the Chilean quake is around 4.8 m/s², which is half the acceleration of gravity (9.8 m/s²). That's pretty impressive. Also, note that the contours of equal acceleration run in a north-south direction, parallel to the plate boundary between the Nazca and South American plates.