What is an Earthquake?
An earthquake is a trembling or shaking
movement of the Earth's surface. Earthquakes normally result from the
movement of faults, quasi-planar zones of deformation within its
uppermost layers. Basically earths's tectonic plates shift every so
often, and the areas on the fault lines are the ones that feel it's
Earthquakes occur every day on Earth, but the vast
majority of them are minor and cause no damage. Large earthquakes can
cause serious destruction and massive loss of life through a variety of
agents of damage including fault rupture, vibratory ground motion
(i.e., shaking), inundation (e.g., tsunami,
seiche, dam failure), various kinds of permanent ground failure (e.g.
liquefaction, landslide), and fire or hazardous materials release. In a
particular earthquake, any of these agents of damage can dominate, and
historically each has caused major damage and great loss of life, but
for most earthquakes shaking is the dominant and most widespread cause
Where do most earthquakes occur?
The earth is made of layers, divided into the
crust, mantle, and core. The crust is the Earth’s surface, a thin, hard
layer of rock, broken into many pieces. Each of these pieces is known
as a crustal plate. Some form continents, others the ocean floor, but
they are always moving. According to the theory of plate tectonics, the
earth’s crust is made of six major plates and nine smaller ones that
lie on the mantle, a thicker, denser layer of hot, soft, molten rock.
These plates float around within the mantle, in a hot, soft zone known
as the asthenosphere.
The core is made up of even hotter rocks below the mantle, and currents
of burning rock rise up through the mantle. These currents spread out
once they hit the bottom surface of the crust. This behavior tends to
tear the crust, pulling the apart, grinding some plates against others,
colliding them into one another. Continental drift (when major plates
are slowly but steadily moved apart) also contribute, carrying plates
until they collide. It is through these collisions that mountain ranges
are also formed. This movement of our dynamic planet produces
earthquakes and volcanoes.
Earthquakes occur most frequently (about 95% of the time) at the point
where two plates scrape against one another. When these two plates move
against each other, the crack is known as a fault. A famous example is
the 700-mile-long San Andreas Fault running up the length of California
(United States). When plates jam against one another, stress builds up
in between. When the pressure becomes too great, the bends and snaps
free with a jerky motion. This sudden motion is an earthquake.
The "Ring of Fire"
is an arc stretching from
New Zealand, along the eastern edge of Asia, north across the Aleutian
Islands of Alaska, and south along the coast of North and South
America. It is composed over 75% of the world's active and dormant
volcanoes. (See the
full USGS MAP here)
- In South America the Nazca plate is colliding
with the South American plate. This has created the Andes and volcanoes
such as Cotopaxi and Azul.
- In Central America, the tiny Cocos plate is
crashing into the North American plate and is therefore responsible for
the Mexican volcanoes of Popocatepetl and Paricutun (which rose up from
a cornfield in 1943 and became a instant mountains).
- Between Northern California and British
Columbia, the Pacific, Juan de Fuca, and Gorda plates have built the
Cascades and the infamous Mount Saint Helens, which erupted in 1980.
- Alaska's Aleutian Islands are growing as the
Pacific plate hits the North American plate. The deep Aleutian Trench
has been created at the subduction zone with a maximum depth of 25,194
feet (7679 meters).
- From Russia's Kamchatka Peninsula to Japan, the
subduction of the Pacific plate under the Eurasian plate is responsible
for Japanese islands and volcanoes (such as Mt. Fuji).
- The final section of the Ring of Fire exists
where the Indo-Australian plate subducts under the Pacific plate and
has created volcanoes in the New Guinea and Micronesian areas. Near New
Zealand, the Pacific Plate slides under the Indo-Australian plate.
What kind of damage can earthquakes cause?
The method for measuring the shaking effect
and damage is what is known as the Richter Scale. The Richter magnitude
scale is a mathematical technique used to quantify the size of
earthquakes. Developed in 1935 by Charles Richter in collaboration with
Beno Gutenberg, both of the California Institute of Technology, the
Richter scale assigns a single number to quantify the size of an
earthquake. Here's what the scale looks like.
(Section of collapsed freeway after the 1989 Loma Prieta earthquake)
- Less than 2.0 - Microearthquakes, not felt.-
About 8,000 per year
- 2.0-2.9 - Generally not felt, but recorded - No
damages - About 1,000 per year
- 3.0-3.9 - Often felt, but rarely causes
damage.- 49,000 per year (estimated)
- 4.0-4.9 - Noticeable shaking of indoor items,
rattling noises. Significant damage unlikely. - 6,200 per
- 5.0-5.9 - Can cause major damage to poorly
constructed buildings over small regions. At most slight damage to
well-designed buildings.- 800 per year
- 6.0-6.9 - Can be destructive in areas up to
about 100 miles across in populated areas.- 120 per year
- 7.0-7.9 - Can cause serious damage over larger
areas. Skyscrapers as risk. 18 per year
- 8.0 or greater - Can cause serious damage in
areas several hundred miles across. Building structures collapse -
skyscrapers as SERIOUS risk. Average 1 per year
The Great China Earthquake of 1556
The worst natural disaster in recorded
history, at least in terms of lives lost, was caused by an earthquake
in Hausien in the Shensi Province of China in 1556. The earthquake
devastated 98 counties and eight provinces of Central China. The
destruction spanned an area of 500 miles, and in some counties the
average death toll was 60 percent of the population. An estimated total
of 830,000 people lost their lives, most of them from the collapse of
poorly constructed houses. The magnitude of this earthquake has been
estimated to be from 8.0 to 8.3 on the Richter scale.
The Great 1906 San Fransisco Earthquake
At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. In the public's mind, this earthquake is perhaps remembered most for the fire it spawned in San Francisco, giving it the somewhat misleading appellation of the "San Francisco earthquake". Shaking damage, however, was equally severe in many other places along the fault rupture. The frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to underestimate the total loss of life by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and 189 were reported elsewhere. Today An earthquake like this in California would set off chain reactions with more than just death and destruction. There would be wrongful deaths, muggings, robberies and possibly riots, from all over the state and neighboring states possibly. The people left to clean up the mess for them it gets even messier. Lets say a wrongful death did occur during an earthquake and lets say this happened in San Diego. A San Diego wrongful death attorney would need to be contacted right away. During a natural disaster it is a lot easier for people just to cover things up and just lie about the truth. The earthquakes are just the beginning of the destruction that can occur in California because the population of California today is astronomically different than the California of 1906. The way of life is a lot different today, leaving the out come of an earthquake unpredictable.
December 26, 2003 - Bam Iran
Tragedy hit Iran on 26 December 2003 when a
major earthquake registering 6.5 on the Richter scale hit its
south-eastern province of Kerman at 05:28 (local time). The area most
affected was the ancient city of Bam where more than 43,000 people were
killed, an estimated 30,000 injured and up to 75,000 left homeless,
according to official estimates.
The U.S. National
Earthquake Information Center
Extended Information and support AO at the same time