1.) EFFECTS OF EARTHQUAKE

2.)GLOBAL PLATE TECTONIC MOVEMENT

MEANING:

An earthquake (also known as a tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.

At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake

epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event — whether a natural phenomenon or an event caused by humans — that generates seismic waves.

Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.

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.

Size and frequency of occurrence:

Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.

Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years. This is an example of the Gutenberg-Richter law.

The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The

USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.

n recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.

Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.

CAUSES OF EARTHQUAKE:

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 (see lithosphere). 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.

Effects of earthquakes include:

Ground shaking :

The severity of this depends on many things, but most commonly the distance from the epicentre and focus of the earthquake. The ground shaking can cause massive damage to structure (buildings etc) on the land that is shaking.

Land rupture :

In some cases the ground near the epicentre can literally “open up” and fissures can also be seen at some distance from the epicentre.

Land slippage:

You can sometimes see how the ground has moved by looking at things that used to be in straight lines eg fences, and seeing how they now seem to have a “kink” in them - this is indicative of ground slippage.

Landslides:

These can happen at any time, though the shaking of the earth from and earthquake makes them possible as the hillsides can become unstable.

Fires:

These are caused by gas mains and water pipes rupturing in the main earthquake. More deaths and damage were caused in the 1906 San Francisco earthquake by fires than by the initial earthquake.

EARTHQUAKE PREVENTION:

Scientists are continuously thinking of ways to try and reduce earthquake power. Some are trying to lessen the friction between colliding plates. They poured water down a fault where two plates were grinding together. The water “lubricated” the fault, letting one piece jerk free with a number of little earthquakes and preventing a large tremor.

Special instruments are now in existence to give warnings in California, Japan, and New Zealand. They show if land has begun to shift dangerously on a certain side of a fault. Other instruments are used to detect certain gases collecting in groundwater

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Architects are also designing earthquake-proof buildings, constructing on rock instead of gravel, or on soft sand or clay. Large structures are made with strong frameworks of steel or reinforced concrete, so that the frame stands firm even if the ground is shaking. The pyramid-shaped Transamerica building in San Francisco was designed in this fashion.

Researchers are always trying to reduce the impact of earthquakes. They continue to study and experiment with ways to tame the Earth. However, we all still have much more to learn before we can control the power of one of nature’s most amazing phenomena.