How Earthquakes Happen
An earthquake is the movement of the earth, which is a natural disaster and is often violent and devastating. An earthquake is usually caused by the sudden release of stress and pressure that may have gathered up over a very long period of time.
Waves of disturbance, or energetic waves known specifically as seismic waves, spread out from the origin of the focus, of the earthquake which is highly likely to be movement along a fault. Although and however, some earthquakes of the world are associated with activities from volcanoes.
In general, earthquakes are classified by their depth of focus which are:
1. Shallow - Earthquake between 0 to 70 kilometers (0-43miles) deep.
2. Intermediate - Earthquake between 70 to 300 kilometers (43-186miles) deep.
3. Deep - Earthquake between 300 to 700 kilometers (186-434miles) deep.
Over 75% of earthquake energy is concentrated in a belt around the Pacific Ocean and this is because most of the seismic activity occurs at margins of tectonic plates (see below for more information). This means that particular areas of the world are more probable to suffer from earthquakes. In other words, for example, the west coast of North and South America, Southeast Asia, Philippines, New Zealand and Japan are the most vulnerable to experience earthquakes.
The effects of earthquakes are naturally very disturbing, dangerous and fearful and also can be quite catastrophic. Near the focus, ground waves actually throw about the land surface. The surface effects may include the opening of large cracks (known as fissures), the breaking of roads and pipelines, bending and twisting of railway tracks and the falling down of bridges and buildings. Secondary effects, compared to surface effects, can be the same and very destructive if the vibrations of the ground initiate and trigger landslides, avalanches, tsunami or may cause large fires.
There are many systems of measuring the intensity of earthquakes. The common system in use is the Richter Scale which was created by an American seismologist and physicist Charles Francis Richter (1900-1985).
Table of the Richter Scale
Features Observed & Identified
This is detected by the use of seismographs
Sensitive people can notice this
Vibrations caused by large vehiccles when passing by
Light and loose objects are rocked and also felt by people walking
People are awakened from this and also felt in general
Light and loose objects fall and trees also sway
Walls of buildings starts to crack
Buildings get some damaged and chimneys fall
Houses collapse, pipes break and ground starts to crack
Buildings destroyed, ground cracks badly and sometimes triggers landslides
Most buildings destroyed, bridges collapse and break, railways destroyed and landslides occur
The most violent destruction and the ground moves in wave
This is a logarithmic scale, an amplitude of waves recorded by a seismograph, and because of this, the magnitude of one level is very much higher than the previous level. The principle study of earthquakes is called seismology, and so therefore in this field, seismographs are used to record the seismic waves (or shock waves as the waves of disturbance) as they spread out from the source. The seismograph has some means of conducting the ground vibrations through a device that transforms movement into a signal that can be recorded. The are several seismic stations around the world that record ground movements. Plus each one contains several seismographs with many seismometers which is the primary detector in connection with the seismograph.
Furthermore, at destructive plate boundaries one tectonic plate dips beneath the other at an oceanic trench in a process called subduction and in so doing old lithosphere is returned to the Earth's mantle. For example, island arcs are in connection with volcanic activity and associated with subduction at an ocean trench, where earthquakes are more often to occur.
When two continental plates converge, the continents collide to produce mountains such as the Alps, Pyrenees and Himalayas of today. The transform faults of conservative plate boundaries are produced by the relative motion of two tectonic plates alongside each other and one good example is the one in California known as the San Andreas fault. The San Andreas fault is the region which encounters earthquakes along a major fracture.
Basically, tectonic plates are massive pieces of solid rock irregularly shaped and are of the Earth's crust and uppermost mantle. This is a general notion that brings together the variety of features and processes of the Earth's crust and is responsible for sea-floor spreading, continental drift, volcanic activity, earthquakes and the structure of the crust.
The coastlines on opposite sides of the world oceans seems to fit together, such as the Atlantic Ocean for example, and this has been acknowledged long time ago. With new discovery and evidence gathered, the crust and upper mantle of the Earth is scientifically named the lithosphere. This is believed to be consisted of seven large and over twelve smaller plates composed of continental crust or oceanic crust.
There are three types of plate boundary which are:
1. Constructive (Divergent) - This is relating to the ocean ridges where the two tectonic plates are moving apart. Such example of this occurrence is the plate in North America and Eurasia.
2. Destructive (Subduction Zones) - This is relating to the ocean trenches where plates are moving together, especially when an oceanic plate is forced under a continental plate. The Pacific and Eurasian plate is an example of this occurrence.
3. Conservative - This is relating to the transform faults where plates move and slide sideways past each other without a collision. The North American and Pacific plate is known for this type of activity.
4. Collision (Plate Margin) - This occurs when two continental plates moves towards each other, and the continental crust does not sink or nor destroy. Earthquakes are very common along these collision margins. One example is the Eurasian and Indo-Australian plate.