Introduction:
Earthquakes can be among the most devastating and terrifying of natural hazards. Although floods, tornadoes and hurricanes account for much greater annual loss in the United States, severe earthquakes pose the largest risk in terms of sudden loss of life and property.
Amount of seismic energy released:
The greater the vibrational energy, the greater the chance for destruction.
Duration of shaking:
This is one of the most important parameters of ground motion for causing damage.
Depth of focus, or hypocenter:
The shallower the focus (the point of an earthquake's origin within the earth), usually the greater the potential for destructive shock waves reaching the earth's surface. Even stronger events of much greater depth typically produce only moderate shaking at ground level.
Distance from epicenter:
The potential for damage tends to be greatest near the epicenter (the point on the ground directly above the focus), and decreases away from it.
Geologic setting:
A wide range of foundation materials exhibits a similarly wide range of responses to seismic vibrations. For example, in soft unconsolidated material, earthquake vibrations last longer and develop greater amplitudes, which produce more ground shaking, than in areas underlain by hard bedrock. Likewise, areas having active faults are at greater risk.
Geographic and topographic setting: This characteristic relates more to secondary effects of earthquakes than to primary effects such as ground shaking, ground rupture, and local uplift and subsidence. Secondary effects include landslides (generally in hilly or mountainous areas), seismic sea waves, or tsunamis (pretty much restricted to oceans and coastal areas), and fires (from ruptured gas lines and downed utility lines).
Population and building density:
In general, risk increases as population and building density increase. Types of buildings: Wooden frame structures tend to respond to earthquakes better than do more rigid brick or masonry buildings. Taller buildings are more vulnerable than one- or two-story buildings when located on soft, unconsolidated sediments, but taller buildings tend to be the more stable when on a hard bedrock foundation.
Time of day:
Experience shows there are fewer casualties if an earthquake occurs in late evening/early morning because most people are at home and awake and are in a good position to respond properly.
Origin of Earthquakes:
Earthquakes occur when great stresses building up within the earth are suddenly released this sudden release of this stored energy causes movement of the earth's crust along fractures, called faults, and generates shock waves.
The two basic types of seismic waves are body waves, or primary waves, which travel through the interior of the earth, and surface waves, which travel along the earth's surface and are believed to be responsible for most earthquake damage
There are two types of body waves: P waves, or primary waves, and S waves, or secondary waves.
Differences in P- and S-wave characteristics have provided much information about the structure and composition of the earth's interior.
Although most earthquakes are associated with movement along faults, they can also be triggered by volcanic activity, by large landslides, and by some types of human activity. However, in areas not known for frequent earthquakes, pinpointing the cause of the rare tremor can be very difficult.
The theory of plate tectonics explains most earthquake occurrences. Ninety percent or more of all earthquakes occur along boundaries between large, slowly moving slabs, or plates, of the earth's crust and upper mantle, collectively called the lithosphere.
Most earthquakes are shallow (0-40 miles to the focus), occurring in the lithosphere. The mechanism for most very shallow earthquakes probably involves fracturing of brittle rock in the crust or relief of internal stresses due to frictional resistance locking opposite sides of a fault.