Synoptic Description of Earthquakes
An earthquake is a sudden shaking or trembling vibrations of the ground produced in the earth's crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can range from barely noticeable to catastrophically destructive where the surface of the Earth can be altered by thrusting up cliffs or opening great cracks in the ground.

Focus, Epicenter, and Shock Wave of an Earthquake
The point at which layers of rock shift and reposition in relation to one another is called the focus; this is the effective center of the earthquake. Directly above the focus, a second point called the epicenter marks the corresponding point of highest-intensity shock on the surface. Shock waves propagate like ripples from the focus and epicenter, decreasing in intensity as they travel outward. Scientist rate earthquakes on the Richter scale and the Mercalli Scale.

Causes of a Earthquake
Earthquakes are caused by various conditions: 
  • Sudden earth movements along geological faults. A fault is a crack or rupture in the earth along which rocks on each of it's two side move past each other. There are two kinds of quakes, tectonic(plates that make up the earth's crust cause a fault) and subduction-zone quakes( one plate slides beneath the other plate). Subduction-zone quakes account for nearly half of the world's destructive seismic events and 75 percent of the earth's seismic energy.

When a fault breaks it releases a tremendous amount of energy as waves which is felt as an earthquake (© JCP Geologists, Inc. 1995).
  • Volcanic activity when lava builds up to high pressures and energy is release in the form of an eruption. 
  • Other causes include meteorite impacts, undersea landslides and explosion of nuclear bombs 
According to current theory, Earth's surface is made up of many large slabs of crust called plates, which ride like giant rafts on semifluid rock below. Geologists believe that the plates are driven by large convection currents created by heat generated deep within Earth by the radioactive decay of certain elements. While most earthquakes happen at plate boundaries, some occur in the middle of a plate. Just as the continents have moved, plate boundaries have also changed. Over the years, Earth's lithosphere has been split up and put together many times, leaving millions of scars or faults. Many of these old faults are static, but every so often stresses build up because of rock movement in the mantle, causing a fault to rupture and an earthquake to occur.

Occurrences of a Earthquake
Geologists and seismologists have found that earthquakes occur repeatedly at faults, which are zones of weakness in the earth's crust. Most tectonic quakes occur at the boundaries of plates, in zones where one plate slides past another, as at the San Andreas Fault(a transform plate boundary) in California, North America's most quake-prone area. Subduction-zone quakes are concentrated along the so- called Ring of Fire, a narrow band about 38,600 km long, that coincides with the margins of the Pacific Ocean. Alaska's disastrous Good Friday earthquake of 1964 is an example of such an event. It was rated 9.2 on the Richter scale. This powerful earthquake claimed 131 lives and caused destruction to parts of Anchorage and Valdez. The image map on the right (for frame browsers only) shows some of the destruction.(Microsoft: Anchorage Museum of Hist. & Art). Other affected areas which account for 70 percent of the world's earthquakes are the Japanese Islands, the Kuril Islands, the west coast of North and South America as well as the entire Central America.

Hyogoken-Nanbu Earthquake[Kobe].
Right: Leaning Building
Left: Broken Highway
Many earthquakes occur each year, on average greater than 800,000, but most are small and not felt by humans. A severe earthquake, with a magnitude of greater than 8.0, can be expected every 8 to 10 years. But a significant number of smaller earthquakes, which are still capable of destruction, occur each year. For a list of the most destructive earthquakes click here.

Social and Economical Impact of a Earthquake
Earthquakes produce such tragic and dramatic effects as destroyed cities, broken dams, earth slides, giant sea waves called tsunamis, and volcanic eruptions. For example, a quake near Hebgen, Montana, in 1959 caused a slide that killed several people and temporarily blocked the Madison River, thereby creating a lake and threatening the town of Ennis with a catastrophic flood. In September 1985 a devastating earthquake shook Mexico City, Mexico, causing severe damage and destroying many of the city's buildings. 

September 1985, Mexico City, Mexico.
(George Chan,Photo Researchers, Inc.)
It left almost 30,000 people homeless and 7000 dead. On the average about 10,000 people die each year as a result of earthquakes. Yet the most famous earthqake was the 1906 San Fransico Earthquake, which caused extensive damage and 700 lives were lost.

California Earthquake, 1906 
According to a study carried out by the United Nations and covering the years 1926 to 1950, there were 350,000 deaths, and property damage losses exceeded 10 billion dollars due to earthquakes. Loss of life, damage, and societal disruption are reminders of what earthquakes can do when they strike in close proximity to highly urbanized areas. Earthquake events are natural occurrences, but earthquake-related losses are in large measure the result of social processes and activities that affect the extent to which people and property are placed at risk. Combating the earthquake problem requires in-depth understanding of those social processes and the translation of this understanding into effective action programs. 

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