Strong ground motion is also the primary cause of damages to the ground and soil upon which, or in which, people must build. These damages to the soil and ground can take a variety of forms: cracking and fissuring and weakening, sinking, settlement and surface fault displacement.
Sometimes, due to earthquake, there is tilting action in the ground. This causes plain land to tilt, causing excessive stresses on buildings, resulting in damage to buildings.
If a structure is built upon soil which is not homogeneous, then there is differential settlement, with some part of the structure sinking more than other. This induces excessive stresses and causes cracking.
During an earthquake, significant damage can result due to instability of the soil in the area affected by internal seismic waves. The soil response depends on the mechanical characteristics of the soil layers, the depth of the water table and the intensities and duration of the ground shaking. If the soil consists of deposits of loose granular materials it may be compacted by the ground vibrations induced by the earthquake, resulting in large settlement and differential settlements of the ground surface. This compaction of the soil may result in the development of excess hydrostatic pore water pressures of sufficient magnitude to cause liquefaction of the soil, resulting in settlement, tilting and rupture of structures.
Indirect Effects of Earthquakes
A tsunami is a very large sea wave that is generated by a disturbance along the ocean floor. This disturbance can be an earthquake, a landslide, or a volcanic eruption. A tsunami is undetectable far out in the ocean, but once it reaches shallow water, this fast-traveling wave grows very large. Tsunamis are very destructive, as this wall of water can destroy everything in its path.
Landslide means descent of a mass of earth and rock down a mountain slope. Landslides may occur when water from rain and melting snow sinks through the earth on top of a slope, seeps through cracks and pore spaces in underlying sandstone, and encounters a layer of slippery material, such as shale or clay, inclined toward the valley. Earthquakes and volcanic eruptions can also cause severe, fast-moving landslides.
Landslides that suddenly rush down a steep slope can cause great destruction across a wide area of habitable land and sometimes cause floods by damming up bodies of water.
Floods & Fires
The amount of damage caused by post-earthquake fire depends on the types of building materials used, whether water lines are intact, and whether natural gas mains have been broken. Ruptured gas mains may lead to numerous fires, and fire fighting cannot be effective if the water mains are not intact to transport water to the fires.
Earthquakes may also give rise to floods. Many times, large earthquakes can cause cracking in Dams. So, to contain the increased pressure, the authorities have to immediately release a lot of water to reduce the reservoir pressure. This gives rise to very heavy flooding in the region, causing great destruction.