Effects of earthquake on building frame?

Buildings are designed to resist loads in specific orientations. The largest load a building will normally experience and be designed to resist is it's own self weight. This will normally act vertically downwards through the building.

Seismic waves introduce a significant non vertical component to this loading (whether that be shaking side to side as a result of Love Waves or the rolling motion caused by Rayligh waves) which creates torsional (twisting), shearing and bending stresses that the building would never normally experience. This can exceed the strength of the structure and cause portions of it to fail or even collapse.