Possible hazards from earthquakes can be classified as follows:
1. Ground Motion
Shaking of the ground caused by the passage of seismic waves, especially surface waves, near the epicenter of the earthquake are responsible for the most damage during an earthquake and is thus a primary effect of an earthquake. The intensity of ground shaking depends on:
Local geologic conditions in the area. In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock.
Size of the Earthquake. In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking.
Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the epicenter. The distance factor depends on the type of material underlying the area. There are, however, strange exceptions. For example, the 1985 Mexico City Earthquake (magnitude 8.1) had an epicenter on the coast of Mexico, more than 350 km to the south, yet damage in Mexico City was substantial because Mexico City is built on soft unconsolidated sediments that fill a former lake.
Damage to structures from shaking depends on the type of construction. Concrete and masonry structures are brittle and thus more susceptible to damage wood and steel structures are more flexible and thus less susceptible to damage.
2. Faulting and Ground Rupture
Ground rupture generally occurs only along the fault zone that moves during the earthquake, and are thus a primary effect. Thus structures that are built across fault zones may collapse, whereas structures built adjacent to, but not crossing the fault may survive.
These are smaller earthquakes that occur after a main earthquake, and in most cases there are many of these (1260 were measured after the 1964 Alaskan Earthquake). Aftershocks occur because the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes. Aftershocks are very dangerous because they cause further collapse of structures damaged by the main shock. Aftershocks are a secondary effect of earthquakes.
Fire is a secondary effect of earthquakes. Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started. In the 1906 earthquake in San Francisco more than 90% of the damage to buildings was caused by fire.
In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps, and debris avalanches. These are secondary effects.
Liquefaction is a processes that occurs in water-saturated unconsolidated sediment due to shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material tends to flow. Liquefaction, because it is a direct result of ground shaking, is a primary effect. You can demonstrate this process to yourself next time your go the beach. Stand on the sand just after an incoming wave has passed. The sand will easily support your weight and you will not sink very deeply into the sand if you stand still. But, if you start to shake your body while standing on this wet sand, you will notice that the sand begins to flow as a result of liquefaction, and your feet will sink deeper into the sand.
7. Changes in Ground Level
A secondary or tertiary effect that is caused by faulting. Earthquakes may cause both uplift and subsidence of the land surface. During the 1964 Alaskan Earthquake, some areas were uplifted up to 11.5 meters, while other areas subsided up to 2.3 meters.
Tsunami a secondary effect that are giant ocean waves that can rapidly travel across oceans, as will be discussed in more detail later. Earthquakes that occur beneath sea level and along coastal areas can generate tsunami, which can cause damage thousands of kilometers away on the other side of the ocean.
Flooding is a secondary effect that may occur due to rupture of human made dams and levees, due to tsunami, and as a result of ground subsidence after an earthquake.