Give geographical reasons.

Buildings collapsed at the foothills of the Himalayas because of an earthquake. Before collapsing they were moving forward and backward.

An earthquake occurs when the tension in the normal movements within the earth’s crust increases leading to formation of massive waves creating trembling on the earth’s surface. Initially when the energy is emitted from the earth’s surface, it comes out in the form of primary seismic waves that cause the particles it passes through to move to and fro. This causes forward and backward movements or swaying in the earth’s surface. This would cause the swaying in any structure present in the area where such a phenomenon occurs. The collapse would be triggered by the secondary seismic waves which causes the crust to move up and down. Thus, the forward and backward movement caused by the primary waves would be observed first.