What exactly causes the damage? (continued: surface faulting. Structures on the fault will be disrupted by the tearing motion. But people do (eg, san andreas fault): ground shaking (this is generally the greatest threat to buildings and people). In a big earthquake, the shaking can be severe even 100"s kms away from the epicenter. Most ground shaking damage is generally a direct consequence of surface wave motion. If you are close to the epicentre, the p- and s- wave arrivals can cause damage too. It is hard to design buildings to withstand earthquakes because they are unpredictable. Seismic waves slow down as they pass from one material into another. Energy is transferred into greater shaking to compensate for the lost energy. The amount of shaking also depends on the nature of the soil/rocks in the area: Shaking depends on the nature of soil: rock < sand soil (stiff) > clay soil (soft; mud).