What is an Earthquake?
An earthquake (know also as quake, tremor, temblor or seismic activity) 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 results.  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 of damage. To find earthquake preparedness kits , click here! 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 year(estimated)
- 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
What is the Worst Earthquake in history? 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. Earthquake fault types There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. Earthquakes away from plate boundaries Where plate boundaries occur within continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g. the “Big bend” region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms. All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes. Shallow-focus and deep-focus earthquakes The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers). These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure. Earthquakes and volcanic activity Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helens eruption of 1980. Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltimeters (a device which measures the ground slope) and used as sensors to predict imminent or upcoming eruptions. Earthquake clusters Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern. Aftershocks An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock. Earthquake swarms Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park. Earthquake storms Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.
More Resources:
The U.S. National Earthquake Information Center |
