(See endnotes 11, 12 for more details)
An earthquake occurs when two sides of a fault slip suddenly against each other and produce vibrations. A fault is a fracture in the rock or the crust of the Earth where the two sides are displaced against each other in a minor or major way. The slippage causes a shaking or vibration because of a sudden release of stored elastic energy from within the Earth and seismic waves (waves of energy) may be produced. Seismic shaking is not uniform. As the waves approach the ground surface and potentially travel through areas of loose soil, the amplitude of the waves will increase producing larger waves that may be far more damaging to structures than the original waves. Other earthquake hazards include ground failure and/or rupturing of the fault resulting in the creation of cracks, settlement of the soil, vertical shifting of the ground after large earthquakes, soil liquefaction, where the soil, in an area of sand and silt as well landfill areas, takes on the characteristics of a liquid during the shaking of an earthquake and flows like a liquid; this can cause enormous damage to any structures that are within this area. Aftershocks, which may occur for periods of time after the initial major shock, can be extremely damaging to structures and can cause additional injuries and deaths. (See endnotes 6, 8, 10.) Thousands of faults are currently present in California, the United States, and the rest of the world. There are thousands of earthquakes each year throughout the world.
The amount and severity of structural hazards caused by earthquakes are typically determined by: the amount of energy released and the intensity of the energy at a given place; the distance from the epicenter to the structure; the design of the structure; the condition and type of surface material at the site of the damage; the nature and condition of the various structures; and the quality and quantity of any earthquake resistant provisions. The epicenter is on the surface of the Earth directly above the hypocenter of an earthquake, the location where the eruptions within the Earth start.
Damage can typically occur to: buildings of all types from single homes to large factories, schools, and hospitals; ports and harbors; highways; railways; bridges; dams; electric transmission systems; power plants; water treatment systems and supply lines; wastewater treatment systems and supply lines; natural gas and oil systems and supply lines; communication systems; etc. Landslides and fires may occur. Structural damage of any type, collapsing walls and falling interior ceilings, pipes and light fixtures can cause serious injury and loss of life, as well as loss of critical facilities. Roadways and other means of transportation may be severely damaged and impede rescue efforts.
The seismic waves, which are the vibrations coming from the earthquake and cause the damage, are recorded by seismographs, which help determine the time, locations, and magnitude of the event. Different scales may be used in determining the nature and size of the earthquake.
The Richter scale, a common measurement scale for earthquakes, has certain limitations for very large earthquakes. It is a mathematical technique used to compare the size of earthquakes. The magnitude of the earthquake is reported in whole numbers and decimal fractions. It is based on a logarithmic scale and therefore an increase of 1 is actually 10 times greater rather than the initial number (e.g., a reading of 6 would be 10 times greater than a reading of 5). The Moment Magnitude Scale is more precise for large earthquakes because it is related to the total energy released during the earthquake. Intensity measures the amount of shaking at a given location, while moment measures the size of the earthquake at its source. (See endnote 9.)