Further Reading

skinner, Brian J., and stephen C. Porter. The Dynamic

Earth, an Introduction to Physical Geology. 5th ed.

New York: John Wiley & sons, 2004.

earthquakes An earthquake occurs when a sudden release of energy causes the ground to shake and vibrate, associated with the passage of waves of energy released at the source. Earthquakes can be extremely devastating and costly events, sometimes killing tens or hundreds of thousands and leveling entire cities in a few seconds. A single earthquake may release the energy equivalent to hundreds or thousands of nuclear blasts and may cost billions of dollars in damage, not to mention the toll in human suffering. Earthquakes are also associated with secondary hazards, such as tsunami, landslides, fire, famine, and disease that also exert their toll on humans and other animals.

Most earthquakes occur along plate boundaries. The lithosphere (or outer rigid shell) of the Earth is broken into about 12 large tectonic plates, each moving relative to the others. There are many other smaller plates. Most earthquakes happen where two of these plates meet and are moving past each other, such as in southern California. Recent earthquakes in China, Turkey, sumatra (indonesia), and mexico have also been located along plate boundaries. A map of plate boundaries of the Earth and earthquakes shows where significant earthquakes have occurred in the past 50 years. most really big earthquakes occur at boundaries where the plates are moving toward each other (as in Alaska and Japan), or sliding past each other (as in southern California and Turkey). smaller earthquakes occur where the plates are moving apart, such as along midoceanic ridges where new magma rises and forms oceanic spreading centers.

The area that gets the most earthquakes in the continental united stated is southern California along the san Andreas Fault. The reason for this high number of earthquakes is that the Pacific plate is sliding north relative to the North American plate along the track of the san Andreas fault. The motion in this

Locations of significant earthquakes and plate boundaries. Shallow-focus earthquakes (in pink) are found at all types of plate boundaries; medium- and deep-focus quakes are found along subduction zones.

Shallow Subduction

area is characterized as a "stick-slip" type of sliding, where the two plates stick to each other along the plate boundary as the two plates slowly move past each other, and stresses rise over tens or hundreds of years. Eventually the stresses along the boundary rise so high that the strength of the rocks is exceeded, and the rocks suddenly break, causing the two plates to move dramatically (slip) up to 20-30 feet (5-7 m) in a few seconds. This sudden motion of previously stuck segments along a fault plane is an earthquake. The severity of the earthquake is determined by how large an area breaks in the earthquake, how far it moves, how deep within the Earth the break occurs, and the length of time that the broken or slipped area along the fault takes to move. The elastic-rebound theory states that recoverable (also known as elastic) stresses build up in a material until a specific level or breaking point is reached. When the breaking point or level is attained, the material suddenly breaks, releasing energy and stresses in an earthquake. In the case of earthquakes, rows of fruit trees, fences, roads, and railroad lines that became gradually bent across an active fault line as the stresses built up are typically noticeably offset across faults that have experienced an earthquake. When the earthquake occurs, the rocks snap along the fault, and the bent rows of trees, fences, or roads/rail line become straight again, but displaced across the fault.

Some areas away from active plate boundaries are also occasionally prone to earthquakes. Even though earthquakes in these areas are uncommon, they can be very destructive. Places including Boston, Massachusetts; Charleston, South Carolina; and New Madrid, Missouri (near St. Louis) have been sites of particularly bad earthquakes. In 1811 and 1812 three large earthquakes with magnitudes of 7.3, 7.5, and 7.8 were centered in New Madrid and shook nearly the entire United States, causing widespread destruction. Most buildings were toppled near the origin of the earthquake, and several deaths were reported (the region had a population of only 1,000 at the time, but is now densely populated). Damage to buildings was reported from as far away as Boston and Canada, where chimneys toppled, plaster cracked, and church bells were set to ringing by the shaking of the ground.

Many earthquakes in the past have been incredibly destructive, killing hundreds of thousands, like the ones in Wenchuan, China; Iran; Sumatra, Indonesia, and Mexico City in recent years (see the table below). Some earthquakes have killed nearly a million people, such as one in 1556 in China that killed 800,000-900,000, another in China in 1976 that killed an estimated 242,000 to 800,000 people, one in Calcutta, India, in 1737 that killed about 300,000 people, and the earthquake-related Indian Ocean tsunami that killed an estimated 286,000 people in 2004. The 2008 magnitude 8 Wenchuan earthquake in China has an official death toll of about 90,000, but unofficial estimates reach up to 1,000,000.

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