Earthquake
An earthquake is the sudden release of strain energy in the Earth's crust resulting in waves of shaking that radiate outwards from the earthquake source. When stresses in the crust exceed the strength of the rock, it breaks along lines of weakness, either a pre-existing or new fault plane. The point where an earthquake starts is termed the focus or hypocentre and may be many kilometres deep within the earth. The point at the surface directly above the focus is called the earthquake epicentre.
- There are large earthquakes and small earthquakes. Big earthquakes can take down buildings and cause death and injury.
- The study of earthquakes is called seismology.
- When the earth moves in an earthquake, it can cause waves in the ocean, and if a wave grows large enough, it's called a "tsunami".
- A tsunami can cause just as much death and destruction as an earthquake. Landslides can happen, too. This is a very important part of the Earth's cycle.
- Earthquakes are measured with a seismometer. The magnitude of an earthquake, and the intensity of shaking, is measured on a numerical scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 (or more) causes damage over a wide area.
Cause
Earthquakes are caused by tectonic movements in the Earth's crust. The main cause is that when tectonic plates collide, one rides over the other, causing orogeny (mountain building), earthquakes and volcanoes.
An Earthquake is a sudden tremor or movement of the earth's crust, which originates naturally at or below the surface. The word natural is important here, since it excludes shock waves caused by French nuclear tests, man made explosions and landslides caused by building work.
There are two main causes of earthquakes
Firstly, they can be linked to explosive volcanic eruptions; they are in fact very common in areas of volcanic activity where they either proceed or accompany eruptions.
Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type.
The boundaries between moving plates form the largest fault surfaces on Earth.
When they stick, relative motion between the plates leads to increasing stress. This continues until the stress rises and breaks, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy.
Terminology
An earthquake can be likened to the effect observed when a stone is thrown into water. After the stone hits the water a series of concentric waves will move outwards from the center. The same events occur in an earthquake. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. Geologists and Geographers call the origin of the earthquake the focus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called the epicentre.
The strength, or magnitude, of the shockwaves determines the extent of the damage caused. Two main scales exist for defining the strength, the Mercalli Scale and the Richter Scale.
Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This produces three different types of waves which have their own distinct characteristics and can only move through certain layers within the Earth. Lets take a look at these three forms of shock waves.
Types of shockwaves
P-Waves
Primary Waves (P-Waves) are identical in character to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through both solids and liquids. The ground is forced to move forwards and backwards as it is compressed and decompressed. This produces relatively small displacements of the ground.
P Waves can be reflected and refracted, and under certain circumstances can change into S-Waves.
Particles are compressed and expanded in the wave's direction.
S-Waves
Secondary Waves (S-Waves) travel more slowly than P-Waves and arrive at any given point after the P-Waves. Like P-Waves they are high frequency, short-wavelength waves, but instead of being longitudinal they are transverse. They move in all directions away from their source, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them 'S' shaped.
S-waves move particles at 90° to the wave's direction.
L-Waves
Surface Waves (L-Waves) are low frequency transverse vibrations with a long wavelength. They are created close to the epicentre and can only travel through the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes. This is because L Waves have a motion similar to that of waves in the sea. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground. Together with secondary effects such as landslides, fires and tsunami these waves account for the loss of approximately 10,000 lives and over $100 million per year.
L-waves move particles in a circular path.
Tectonic Earthquakes
Tectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves. The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.
The process which explains these earthquakes, known as Elastic Rebound Theory can be demonstrated with a green twig or branch. Holding both ends, the twig can be slowly bent. As it is bent, energy is built up within it. A point will be reached where the twig suddenly snaps. At this moment the energy within the twig has exceeded the Elastic Limit of the twig. As it snaps the energy is released, causing the twig to vibrate and to produce sound waves.
Perhaps the most famous example of plates sliding past each other is the San Andreas Fault in California. Here, two plates, the Pacific Plate and the North American Plate, are both moving in a roughly northwesterly direction, but one is moving faster than the other. The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other. Occasionally, as in 1989, a much larger movement occurs, triggering a far more violent 'quake'.
Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks , or a period of less frequent shocks as the two rock masses temporarily 'stick' and become locked together. Detailed surveys in San Francisco have shown that railway lines, fences and other longitudinal features very slowly become deformed as the pressure builds up in the rocks, then become noticeably offset when a movement occurs along the fault. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses 'settle down' in their new positions. Such aftershocks cause problems for rescue services, bringing down buildings already weakened by the main earthquake.
Volcanic Earthquakes
Volcanic earthquakes are far less common than Tectonic ones. They are triggered by the explosive eruption of a volcano. Given that not all volcanoes are prone to violent eruption, and that most are 'quiet' for the majority of the time, it is not surprising to find that they are comparatively rare.
When a volcano explodes, it is likely that the associated earthquake effects will be confined to an area 10 to 20 miles around its base, where as a tectonic earthquake may be felt around the globe.
The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly upon contact with the air. This tends to chock the volcanic vent and block the further escape of pressure. For example, in the case of Mt Pelee, the lava solidified before it could flow down the sides of the volcano. Instead it formed a spine of solid rock within the volcano vent. The only way in which such a blockage can be removed is by the build up of pressure to the point at which the blockage is literally exploded out of the way. In reality, the weakest part of the volcano will be the part which gives way, sometimes leading to a sideways explosion as in the Mt St.Helens eruption.
When extraordinary levels of pressure develop, the resultant explosion can be devastating, producing an earthquake of considerable magnitude. When Krakatoa ( Indonesia, between Java and Sumatra ) exploded in 1883, the explosion was heard over 5000 km away in Australia. The shockwaves produced a series of tsunami ( large sea waves ), one of which was over 36m high; that's the same as four, two story houses stacked on top of each other. These swept over the coastal areas of Java and Sumatra killing over 36,000 people.
By contrast, volcanoes producing free flowing basic lava rarely cause earthquakes. The lava flows freely out of the vent and down the sides of the volcano, releasing pressure evenly and constantly. Since pressure doesn't build up, violent explosions do not occur.
Earthquake Fault Types
There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip.
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Normal faults occur mainly in areas where the crust is being extended.
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Reverse faults occur in areas where the crust is being shortened.
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Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other.
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.
- Aftershocks are formed as the crust 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
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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.
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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 occurred in Turkey in the 20th century.
Tsunami
Tsunami or a chain of fast moving waves in the ocean caused by powerful earthquakes is a very serious challenge for people's safety and for earthquake engineering. Those waves can inundate coastal areas, destroy houses and even swipe away whole towns.
Unfortunately, tsunami can not be prevented. However, there are warning systems which may warn the population before the big waves reach the land to let them enough time to rush to safety.
Earthquake-proofing
Some countries, such as Japan or parts of a country like California in the United States, have a lot of earthquakes. In these places it is a good practice to build houses and other buildings so they will not collapse when there is an earthquake. This is called seismic design or "earthquake-proofing".
For many years earthquakes have occurred. That is why there are earthquake proof buildings. The ability of a building to withstand the stress of an earthquake depends upon its type of construction, shape, mass distribution, and rigidity. Different combinations are used. Different shapes of buildings such as square, rectangular, and shell buildings can withstand earthquakes far better than skyscrapers. To reduce stress, a building's ground floor can be supported by extremely rigid, hollow columns, while the rest of the building is supported by flexible columns located inside the hollow columns. A different method is to use rollers or rubber pads to separate the base columns from the ground, allowing the columns to shake parallel during an earthquake.
To help prevent a roof from collapsing you can make your roof out of light-weight materials. Outdoor walls can be made with stronger and more reinforced materials such as steel or reinforced concrete. During an earthquake flexible windows may help hold the windows together so they don’t break.
Important Facts
Where do earthquakes occur?
Anywhere! However, they are unevenly distributed over the earth, with the majority occurring at the boundaries of the major crustal plates. These plate boundaries are of three types: destructive, where the plates collide; constructive, where the plates move apart; and conservative plate boundaries, like the San Andreas Fault, where the plates slide past each other. Earthquakes also occur, less frequently, within the plates and far from the plate boundaries, as in eastern USA, Australia and the United Kingdom.
Which countries have the largest and most frequent earthquakes?
Around 75% of the world's seismic energy is released at the edge of the Pacific, where the thinner Pacific plate is forced beneath thicker continental crust along 'subduction zones'. This 40,000 km band of seismicity stretches up the west coasts of South and Central America and from the Northern USA to Alaska, the Aleutians, Japan, China, the Philippines, Indonesia and Australasia.
Around 15% of the total seismic energy is released where the Eurasian and African plates are colliding, forming a band of seismicity which stretches from Burma, westwards to the Himalayas to the Caucasus and the Mediterranean.
What is the biggest earthquake that has ever happened?
One of the largest earthquakes ever was the Chile event of 22 May 1960 with moment magnitude of 9.5 Mw. Other large earthquakes include Lisbon, 1 November 1755, magnitude 8.7 Ms; Assam, 12 June 1897, magnitude 8.7 Ms; Alaska, 28 March 1964, moment magnitude 9.2 Mw. Although the magnitude scale is open ended, the strength of the crustal rocks prior to fracturing limits the upper magnitude of earthquakes.
What is the difference between magnitude and intensity?
Magnitude is a measure of earthquake size and remains unchanged with distance from the earthquake. Intensity, however, describes the degree of shaking caused by an earthquake at a given place and decreases with distance from the earthquake epicentre. We can, therefore talk about a magnitude 5.4 ML event with intensity of 6 EMS in the epicentral area, on the Lleyn Peninsula, but intensity 3 EMS at Carlisle. Magnitude measurement requires instrumental monitoring for its calculation, however, assigning an intensity requires a sample of the felt responses of the population. This is then graded according to the EMS intensity scale. For example, Intensity 1, Not felt, 2, Scarcely perceptible, 3, weak, felt by a few, up to 12 assigned for total devastation. Study of intensity and the production of isoseismal maps, contouring areas of equal intensity, is particularly important for the study of earthquakes which occurred prior to instrumental monitoring.
For comparison purposes, a magnitude 5 ML earthquake is equivalent to the explosion of 1,000 tons of TNT whereas a magnitude 6 ML earthquake is the energy equivalent of 30,000 tons of TNT or a 30 kilotonne nuclear explosion.
Common Terms Used in Seismology
The study of earthquakes
| Aftershock | An earthquake which follows a larger earthquake or main shock and originates at or near the focus of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time. |
| Amplitude | The maximum height of a wave crest or depth of a trough. |
| Array | An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver. |
| Arrival | The appearance of a seismic wave on the seismic record. |
| Arrival time | The time at which a particular wave phase arrives at a detector. |
| Aseismic area | An area that is almost free of earthquakes. |
| Body wave | A seismic wave that travels through the interior of the earth and is not related to a boundary surface. |
| Crust | The outer layer of the Earth's surface. |
| Earthquake | Shaking of the earth caused by a sudden movement of rock beneath its surface. |
| Earthquake swarm | A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time. |
| Elastic wave | Rock is an elastic material that when strained by normal external forces can return to its original state. When the strength of the rock is exceeded, the rock ruptures, generating elastic seismic or earthquake waves. |
| Epicentre | That point on the Earth's surface directly above the hypocentre of an earthquake. |
| Fault | A weak area in the Earth's crust where two sides of a fracture or fracture zone move relative to each other. |
| First arrival | The first recorded signal on a seismogram is the direction of the first P-wave, where upward ground motion is compressional and downward motion is dilatational. |
| Focus | The point where earthquake rupture or fault movement originates. |
| Foreshock | A small earthquake that may precede a larger earthquake or main shock and that originates at or near the focus of the larger event. |
| Frequency | The frequency of a wave (Hz) is the number of wave cycles per second. |
| Hypocentre | The calculated location of the focus of an earthquake. |
| Induced seismicity | Non-natural events induced by man's activity. These include mining induced events, events caused by loading of dams or pumping of water in geothermal areas. |
| Intensity | A measure of the effects of an earthquake at a particular place on humans and (or) structures. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the epicentre and the local geology at that point. |
| Isoseismal line | A line enclosing points on the Earth's surface at which earthquake intensity is the same. It is usually elliptical in shape |
| Love wave | A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation. It is named after A.E.H. Love, the English mathematician who discovered it. |
