A sufficiently large earthquake is bound to cause some degree of damage, but an earthquake measuring 9.0 can almost level cities completely, depending on the earthquake preparedness of the area. Glass from building can shatter and fall, threatening pedestrians walking by. Roofs can sometimes cave in, and in some circumstances buildings can take enough structural foundation to collapse entirely. Most people who die in earthquakes do not die as a direct result of the earthquake, but rather as a result of damaged and collapsing buildings.
Any earthquake measuring 9.0 would likely result in significant tremors. Tremors are the small aftershocks that usually accompany earthquakes. In the case of a 9.0 earthquake, the tremors can be quite large earthquakes in themselves. Earthquakes occur when there is a movement in the tectonic plates that all land rest on. Because these plates are connected, tremors often take place quite far from the location of the original earthquake.
Tsunamis are one of the most devastating aspects of a large earthquake. When tectonic plates shift, the resulting earthquake displaces water. In the case of an earthquake measuring 9.0, the amount of water displaced can be immense. These waves shoot across the water, causing destruction thousands of miles away from the earthquake. These waves devastate seaside towns, wash people out to sea and completely submerge low-lying islands.
One curious aspect of large earthquakes is soil liquefaction, or simply liquefaction. When immense earthquakes occur, the shaking of the ground creates immense stress on soil. The pressure looks for an escape and finds it in the pores of soil. Water shoots up through these pores, creating a kind of sludge. In turn, this sludge acts like quicksand swallowing whatever lies on top of it. If larger buildings are resting on liquefied soil, they may topple completely.