Earthquake, shaking of the earths surface caused by rapid movement of the earths rocky outer layer. Earthquakes occur when energy stored within the earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the earth by earthquake waves. The study of earthquakes and the waves they create is called seismology. Scientists who study earthquakes are called seismologists. (Websters p. 423) The destruction an earthquake causes, depends on its magnitude or the amount of shaking that occurs. The size varies from small imperceptible shaking, to large shocks felt miles around.
Earthquakes can tear up the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Many Lives can be lost because of this destruction. (The Road to Jaramillo p. 211) Several hundred earthquakes, or seismic tremors, occur per day around the world. A worldwide network of seismographs detect about one million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8. 6 on the Richter scale and caused millions of dollars in damage, occur worldwide once every few years.
Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California (magnitude 7. 0), and the 1995 tremor in Kbe, Japan (magnitude 6. 8), occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people. (The Road to Jaramillo p. 213-215) In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 Tang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage.
Good precautions, such as education, emergency planning, and constructing stronger, more flexible structures, can limit the loss of life and decrease the damage caused by earthquakes. (The Road to Jaramillo p. 213-215,263) Seismologists examine the parts of an earthquake, like what happens to the earths surface during an earthquake, how the energy of an earthquake moves from inside the earth to the surface, and how this energy causes damage. By studying the different parts and actions of earthquakes, seismologists learn more about their effects and how to predict ground shaking in order to reduce damage.
On Shifting Ground p. 109-110) Focus and Epicenter The point within the earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter. The point on the earths surface directly above the focus is called the epicenter. Earthquake waves begin to radiate out from the focus and follow along the fault rupture. If the focus is near the surface between 0 and 70 km (0 and 40 mi. ) deep shallow focus earthquakes are produced. If it is deep below the crust between 70 and 700 km (40 and 400 mi. ) deep a deep focus earthquake will occur.
Shallow-focus earthquakes tend to be larger, and therefore more damaging, earthquakes. This is because they are closer to the surface where the rocks are stronger and build up more strain. (The Ocean of Truth p. 76 & The road to Jaramillo p. 94-97) Seismologists know from observations that most earthquakes originate as shallow-focus earthquakes and most of them occur near plate boundaries areas where the earths crustal plates move against each other. Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subducts, or moves under another plate.