Earthquakes

INTRODUCTION 1
HOW EARTHQUAKES OCCUR 2
Fault types 3
Earthquake fault types 3
Different types of Faults 3
Tidal forces 5
Earthquake clusters 6
Aftershock 6
Earthquake swarm 6
Earthquake storm 6
METHODS USED IN MEASURING EARTHQUAKES 6
The Richter Scale 6
The Mercalli Scale 7
HAZARDS CONNECTED TO EARTHQUAKES 8
MEASURES TAKEN TO MINIMIZE EARTHQUAKES 9
BIBLIOGRAPHY 11

INTRODUCTION
Earthquakes are caused by the movement of the earth's tectonic plates. Earthquakes occur where the earth's plates meet along plate boundaries.For example as two plates move towards each other, one can be pushed down under the other one into the mantle. If this plate gets stuck it causes a lot of pressure on surrounding rocks. When this pressure is released it produces shock waves. These are called seismic waves. This is an earthquake. The waves spread out from the point where the earthquake started - the focus. More damage is done near the focus. The point on the earth's surface directly above the focus is the epicentre.
Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The subterranean point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is determined by the use of scales, e.g., the moment magnitude scale, Richter scale, and the modified Mercalli scale.
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!
An earthquake can be described as a sudden release of energy in the earth's crust or upper mantle, usually caused by movement along a fault plane or by volcanic activity and resulting in the generation of seismic waves which can be destructive. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity
HOW EARTHQUAKES OCCUR
Most earthquakes are causally related to compressional or tensional stresses built up at the margins of the huge moving lithospheric plates that make up the earth's surface. The immediate cause of most shallow earthquakes is the sudden release of stress along a fault, or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another. These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable.
Fault types
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.
Earthquake fault types
There are three main types of fault, all of which may cause an earthquake:
Different types of Faults
A close look at faults helps geologists to understand how the tectonic plates have moved relative to one another.
Types of movement of crustal blocks that can occur along faults during an earthquake:

©Redrawn from University of Otago (Richard Sibson)
1. Where the crust is being pulled apart, normal faulting occurs, in which the overlying (hanging-wall) block moves down with respect to the lower (foot wall) block.
2. Where the crust is being compressed, reverse faulting occurs, in which the hanging-wall block moves up and over the footwall block – reverse slip on a gently inclined plane is referred to as thrust faulting.
3. Crustal blocks may also move sideways past each other, usually along nearly-vertical faults. This ‘strike-slip’ movement is described as sinistral when the far side moves to the left, and dextral, when the far side moves to the right.
4. An oblique slip involves various combinations of these basic movements, as in the 1855 Wairarapa Fault rupture, which included both reverse and dextral movement.
Faults can be as short as a few metres and as long as 1000km. The fault rupture from an earthquake isn’t always a straight or continuous line. Sometimes there can be short offsets between parts of the fault, and even major faults can have large bends in them.

. 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.
Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults, particularly continental transforms can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are generally less than magnitude 7.

This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth's crust, and the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in response to stress; they do not rupture in earthquakes. The maximum observed lengths of ruptures and mapped faults, which may break in one go are approximately 1000 km. Examples are the earthquakes in Chile, 1960; Alaska, 1957; Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali Fault in Alaska (2002), are about half to one third as long as the lengths along subducting plate margins, and those along normal faults are even shorter.

The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees. Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Japan, 2011; Alaska, 1964), making the most powerful earthquakes possible.

Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust, thus earthquakes with magnitudes much larger than 8 are not possible. Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, as in Iceland, where the thickness of the brittle layer is only about 6 km.

In addition, there exists a hierarchy of stress level in the three fault types. Thrust faults are generated by the highest, strike slip by intermediate, and normal faults by the lowest stress levels. This can easily be understood by considering the direction of the greatest principal stress, the direction of the force that 'pushes' the rock mass during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction, thus the pushing force (greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass 'escapes' in the direction of the least principal stress, namely upward, lifting the rock mass up, thus the overburden equals the least principal stress. Strike-slip faulting is intermediate between the other two types described above. This difference in stress regime in the three faulting environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions.
Earthquakes away from plate boundaries
Tidal forces
Research work has shown a robust correlation between small tidally induced forces and non-volcanic tremor activity.
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 that cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern. Aftershock
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 swarm
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.In August 2012, a swarm of earthquakes shook Southern California's Imperial Valley, showing the most recorded activity in the area since the 1970s.
Earthquake storm
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

METHODS USED IN MEASURING EARTHQUAKES
The Richter Scale
The magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in 1934. The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the earthquake, no matter what type of wave was the strongest.

The Richter magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people.

The Richter magnitude scale can be used to desribe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude 10.0 and beyond.

Although Richter originally proposed this way of measuring an earthquake's "size," he only used a certain type of seismograph and measured shallow earthquakes in Southern California. Scientists have now made other "magnitude" scales, all calibrated to Richter's original method, to use a variety of seismographs and measure the depths of earthquakes of all sizes.

Here's a table describing the magnitudes of earthquakes, their effects, and the estimated number of those earthquakes that occur each year.
The Mercalli Scale

Another way to measure the strength of an earthquake is to use the Mercalli scale. Invented by Giuseppe Mercalli in 1902, this scale uses the observations of the people who experienced the earthquake to estimate its intensity.

The Mercalli scale isn't considered as scientific as the Richter scale, though. Some witnesses of the earthquake might exaggerate just how bad things were during the earthquake and you may not find two witnesses who agree on what happened; everybody will say something different. The amount of damage caused by the earthquake may not accurately record how strong it was either.

Some things that affect the amount of damage that occurs are:

1. the building designs 2. the distance from the epicenter, 3. the type of surface material (rock or dirt) the buildings rest on.
Different building designs hold up differently in an earthquake and the further you are from the earthquake, the less damage you'll usually see. Whether a building is built on solid rock or sand makes a big difference in how much damage it takes. Solid rock usually shakes less than sand, so a building built on top of solid rock shouldn't be as damaged as it might if it was sitting on a sandy lot.
HAZARDS CONNECTED TO EARTHQUAKES

An earthquake, one of the most destructive natural phenomena, consists of rapid vibrations of rock near the surface of the earth. It is the most terrifying of all natural phenomena and has brought fear since ancient times because of its sudden unpredictable occurrence and enormous capacity of destruction. A single shock usually last no more than a few seconds, although several quakes may last for as much as a couple of minutes. We have to know what earthquakes are in order for us to be prepared against them. Earthquakes are tremors that move the earth. They can create a lot of damage. The main damage is caused to human, animal life and damage to properties. Earthquakes cause a lot of devastation, thereby causing great loss of life and property.
Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. Earthquakes have happened around the world and may cause a lot of damage to homes. A 6.3 earthquake destroyed historic buildings in Italy. On January 12, 2010 a powerful 7.0 earthquake struck Haiti and left a powerful impact on people. It caused people to die and made people want to help the people of Haiti.
A final effect of an earthquake is producing various damaging effects to the areas they act upon. This includes damage to buildings and in worst cases the loss of human life. The effect of the rumbling produced by earthquakes usually leads to the destruction of structures such as buildings, bridges, and dams. They can also trigger landslides.
In conclusion earthquakes are not new to the world. It is a very common thing on many parts of the earth. Earthquakes are almost unpredictable and devastating acts of nature. There is still a lot more to be known about earthquakes that we still do not know about today
MEASURES TAKEN TO MINIMIZE EARTHQUAKES

Reducing earthquake damage

Earthquakes and avalanches share characteristics that suggest a new approach to reducing earthquake damage. Both are the sudden release of gradually accumulated energy in a chaotic fractal structure. Exact time predictions will always be difficult and approximate, no matter how good the equipment. Both release forces that are difficult or impossible to resist.

Avalanche damage is commonly minimized by deliberately triggering avalanches when the area in danger has been evacuated. A small explosive shot from a cannon or dropped from an aircraft can trigger deadly avalanches when nobody is in the path of destruction. Since resistance or accurate prediction of earthquakes will always be difficult or impossible perhaps we could minimize their harm by trying to trigger them like avalanches.

An announcement could be broadcast and published for weeks ahead of time that on a specified Sunday afternoon at a specified time there might be an earthquake. People should be outside their homes with their fire extinguishers handy and their utilities shut off. Drivers should pull off the road and boats should be moved into open water. Hospitals and fire departments should be prepared.

The damage and loss of life if an earthquake happened under these conditions would much less than if it happened by surprise. The energy would be released, greatly reducing the likelihood of another earthquake in the near future. Repeated on an annual basis this could release accumulated energy as a series of smaller earthquakes instead of a few larger ones, further reducing the destruction. It would also be an earthquake drill; people would know what to do if warning signs were detected at any other time.

A further advantage is that seismologists could make special preparations to observe the vibrations from the trigger and the resulting earthquake. If the explosives are arranged in long straight lines and set off simultaneously then interference patterns could be used to map the deep structure of the earth in the region of interest. The Soviet Union is believed to have used nuclear explosives for deep earth seismographic mapping in this way. This would improve our understanding of the fault zones and guide future placement of triggering explosives.
The engineering problems are daunting. Our knowledge of the deep structures is not detailed enough to tell us the best places for the explosives. The best places will often be deeper than we can drill. The amount of explosives needed is much more than we have. Several simultaneous nuclear explosions would be our best chance. They will be too far below the earth to leak radiation to the surface and there are surplus warheads available that must be disposed of. The only encouraging note is the smallness of the explosives needed to trigger avalanches.

The social problems are also daunting. People hurt by a triggered earthquake will blame the responsible agency even though the earthquake was inevitable. Since that earthquake might not have happened in their life times their complaints may have merit.

We may never be able to accurately predict or effectively resist avalanches but we routinely minimize their damage by triggering them. We may never be able to accurately predict or effectively resist earthquakes but we may be able to someday minimize their damage by triggering them.

BIBLIOGRAPHY 1. http://www.chaospark.com/science/quake.htm 2. Jennings.A (2005),world of geography : usa , britanica 3. 4. http://www.geo.mtu.edu/UPSeis/hazards.html 5. http://michaelgivens84.edublogs.org/files/2011/01/Strike-slip-fault-2ka1yb6.gif 6. http://www.earthquakecountry.info/roots/seven_steps.html 7. http://redsismica.uprm.edu/english/Info/sisnotas_med.php 8. http://skywalker.cochise.edu/wellerr/students/measure-quake/paper.htm 9. http://pubs.usgs.gov/fs/2006/3016/ 10. http://en.wikipedia.org/wiki/Types_of_earthquake 11. http://www.universetoday.com/82164/types-of-earthquakes/