An earthquake is an abrupt, rapid vibration of the earth, caused by the sudden release of accumulated potential energy into the kinetic energy of motion. Most earthquakes are produced along faults, tectonic plates, boundary zones, or along the mid-oceanic ridges. More than 150,000 shakes strong enough to be felt by humans arise each year worldwide. An earthquake is usually followed by a series of aftershocks because of the continued release of frictional stress. Most aftershocks are smaller than the main earthquake, but they can still cause considerable damage to already weakened natural and human constructed features. Approximately 13 million people have died as the result of earthquakes during the past 4,000 years.
How an earthquake happens?
A release of energy in the earth's crust, followed by vibration of the earth's surface is the process in which an earthquake occurs. This energy is either created by an impulsive disarticulation of segments of the crust - a volcanic eruption, or by tectonic plates. The forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface, slowly move over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free generating spherical "seismic waves" which travel outward in all directions.
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Geologists divided seismic waves into two main types;
Body waves - Speed decrease with increasing density of rocks and increases with increasing elasticity of rocks. There are two forms of body waves:
P Waves - Vibrate parallel to wave movement, travel through solid, liquid and gas, the fastest.
S Waves (Shear Waves) - Vibrate perpendicular to wave movement, travel through solids only.
Surface waves - Travel along the outer layer of Earth. Rolling, shaking motion causes most of an earthquake's damage. Slowest seismic waves. There are two forms of surface waves:
R Waves - Indirect motion of material in wave.
L Waves - Shear motion in a horizontal plane, most destructive, generally faster than R waves.
A fault is a huge fracture in rocks in which the rocks have slipped. Faults are divided into three main groups depending on the way they move:
Normal faults - the overlying rock moves downward under the other.
Thrust (reverse) faults - the overlying rock moves upward higher than the other.
Strike-slip (lateral) faults - the rocks move horizontally past one another in the opposite direction.
Most faulting along spreading zones is normal, along seduction zones is thrust, and along transform faults is strike-slip.
The earthquake focus is the region where the sudden energy originates. The focal depth of an earthquake is the deepness from the earth's surface to the focus. The depth of the earthquakes is categorized as follows:
Shallow - Until 70 Kilometers.
Intermediate - Ranges between 70 kilometers and 300 kilometers.
Deep - May reach 700 kilometers.
The depth to the center of the Earth is about 6,370 kilometers, so even the deepest earthquakes start in rather shallow parts of the Earth's core.
The epicenter of an earthquake is the position on the Earth's surface directly above the focus. The focal depth and the epicenter are the geographic position of an earthquake that universally illustrates its location.
Seismograph is an instrument that perceives, records, and measures any vibrations generated by earthquakes by reacting to any activity or movement of the ground surface underneath it and then converts the wave energy into a standard unit of measurement. The changing intensity of the vibrations is represented through a zig-zag line called a seismogram. Scientists are able to establish an earthquake's time, epicenter, focal depth, fault, and amount of energy discharged from a seismogram.
The brutality of an earthquake can be articulated in a number of ways. The magnitude of an earthquake, usually expressed by the Richter scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings as previously stated. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location.
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"The Richter scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale. The Modified Mercalli Scale expresses the intensity of an earthquake's effects in a given locality in values ranging from I to XII. The most commonly used adaptation covers the range of intensity from the condition of "I -- Not felt except by a very few under especially favorable conditions," to "XII -- Damage total. Lines of sight and level are distorted. Objects thrown upward into the air." Evaluation of earthquake intensity can be made only after eyewitness reports and results of field investigations are studied and interpreted. The maximum intensity experienced in the Alaska earthquake of 1964 was X; damage from the San Francisco and New Madrid earthquakes reached a maximum intensity of XI." Maintained by John Watson and Kathie Watson. Last modified 10-23-97. URL:http://pubs.usgs.gov/gip/earthq1/measure.html
Table 10m-1 describes the relationship between Richter scale magnitude and energy released. The following equation can be used to approximate the amount of energy released from an earthquake in joules when Richter magnitude (M) is known:
Energy in joules = 1.74 x 10(5 + 1.44*M)
1.3 x 108
Smallest earthquake detectable by people.
2.8 x 1012
Energy released by the Hiroshima atomic bomb.
6.0 - 6.9
7.6 x 1013 to 1.5 x 1015
About 120 shallow earthquakes of this magnitude occur each year on the Earth.
7.7 x 1014
Northridge, California earthquake January 17, 1994.
2.1 x 1015
Major earthquake threshold.
7.9 x 1015
Turkey earthquake August 17, 1999. More than 12,000 people killed.
1.5 x 1016
Deadliest earthquake in the last 100 years. Tangshan, China, July 28, 1976. Approximately 255,000 people perished.
1.6 x 1017
San Francisco earthquake of April 18, 1906.
4.3 x 1018
December 26, 2004 Sumatra earthquake.
8.3 x 1018
Most powerful earthquake recorded in the last 100 years. Southern Chile on May 22, 1960. Claimed 3,000 lives.
There are a lot of factors affecting an earthquake's viciousness - damages, injuries, and deaths. First, the magnitude, Second, the earthquake's focal depth, the further the vibrations are from the focus the weaker they are, Third, the geological area effected and its soil type, unconsolidated rocks and sediments have a propensity to increase the amplitude of seismic waves raising the possibility of damage, while buildings on solid bedrock tend to receive fewer damage, Fourth, the building construction category, where different building materials and designs are more disposed to damage than others, Fifth, the population density where the earthquake occurred and its time of day.
The greatest loss of life because of an earthquake this century occurred in Tangshan, China in 1976 when an estimated 250,000 people died.
Significant Earthquakes of magnitude 6.5 or greater or ones that caused fatalities, injuries or substantial damage in middle east and north Africa for years 2005 - 2010.
Date time coordinates depth magnitude place
MAR 08 02 32 34.7 38.867 N 39.987 E 12 G 6.1 EASTERN TURKEY
At least 51 people killed, 100 injured and 5,000 displaced, 287 buildings destroyed and 700 heavily damaged.
MAY 19 17 35 01.0 25.291 N 37.740 E 5 G 5.7 WESTERN SAUDI ARABIA.
At least seven people injured Several large ground cracks and landslides Were observed in Al Madinah
NOV 03 23 26 52.0& 27.334 N 56.202 E 14 5.1 SOUTHERN IRAN.
At least 269 people injured and several houses damaged.
JAN 09 22 24 03.8 35.616 N 0.570 W 10 G 4.6 NORTHERN ALGERIA.
One person killed and several buildings damaged
FEB 01 07 33 40.5 36.825 N 3.473 E 10 G 4.6 NORTHERN ALGERIA.
FEB 13 20 55 31.5& 31.730 N 51.200 E 14 4.5 CENTRAL IRAN.
FEB 15 10 36 19.0 33.327 N 35.305 E 10 G 5.1 LEBANON - SYRIA REGION.
MAY 01 00 15 27.4& 33.860 N 48.590 E 16 4.5 WESTERN IRAN.
JUN 06 20 02 56.8 35.883 N 0.658 W 4 G 5.5 NORTHERN ALGERIA.
OCT 25 20 17 20.2& 26.533 N 54.985 E 29 5.4 SOUTHERN IRAN.
DEC 07 13 36 21.3& 26.990 N 55.800 E 15 5.4 SOUTHERN IRAN.
JAN 21 07 38 57.0& 39.592 N 42.863 E 3 5.2 EASTERN TURKEY.
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MAR 06 22 32 06.8& 33.490 N 48.930 E 16 4.7 WESTERN IRAN.
AUG 25 04 24 21.9 28.154 N 56.652 E 10 G 5.0 SOUTHERN IRAN.
NOV 20 05 20 03.4& 31.680 N 49.930 E 7 4.8 WESTERN IRAN.
FEB 28 07 31 02.6 28.120 N 56.865 E 18 G 6.0 SOUTHERN IRAN.
MAR 20 19 44 25.1 36.623 N 5.328 E 10 G 5.2 NORTHERN ALGERIA.
MAR 25 07 28 57.6 27.574 N 55.685 E 18 G 5.9 SOUTHERN IRAN.
MAR 31 01 17 00.9 33.500 N 48.780 E 7 G 6.1 WESTERN IRAN.
MAY 07 06 20 53.7& 30.790 N 56.700 E 14 G 5.0 CENTRAL IRAN.
JUN 03 07 15 35.6 26.759 N 55.843 E 12 D 5.1 SOUTHERN IRAN.
JUN 28 21 02 09.9 26.925 N 55.866 E 11 G 5.8 SOUTHERN IRAN.
JAN 10 18 47 30.1 37.103 N 54.574 E 32 D 5.4 NORTHERN IRAN.
JAN 10 23 48 50.0& 37.017 N 27.804 E 16 5.5 WESTERN TURKEY.
JAN 25 16 44 16.1 37.622 N 43.703 E 41 5.9 TURKEY-IRAQ BORDER REGION.
FEB 22 02 25 22.9 30.754 N 56.816 E 14 G 6.4 CENTRAL IRAN.
MAR 12 07 36 12.1 39.440 N 40.978 E 11 5.6 EASTERN TURKEY.
MAR 14 01 55 55.6& 39.354 N 40.890 E 5 5.8 EASTERN TURKEY.
MAY 03 07 21 10.4 33.711 N 48.685 E 12 4.9 WESTERN IRAN.
JUN 06 07 41 28.7& 39.220 N 41.080 E 10 5.6 EASTERN TURKEY.
NOV 27 10 22 19.1 26.774 N 55.858 E 10 G 5.9 SOUTHERN IRAN.