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An earthquake (also known as a quake, or tremor) is a violent movement of the rocks in the Earth's crust and is the result of a sudden release of energy in the Earth's crust that creates seismic waves.Technically, earthquake may be defined as “vibrations induced in the earth’s crust due to internal or external causes that virtually shake up a part of the crust and all the structures and living and non-living things existing on it”.

Causes Of EARTHQUAKE

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 mountain building, earthquakes and volcanoes.

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. Volcanic eruptions, rock falls, 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.

There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip.

Normal faults occur mainly in areas where the crust is being extended. Earthquakes associated with normal faults are generally less than magnitude 7.

FAULTS

Aerial photo of the San Andreas Fault

Reverse faults occur in areas where the crust is being shortened. They are associated with the most powerful earthquakes, including almost all of those of magnitude 8 or more.

Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other. They can produce major earthquakes up to about magnitude 8.

FOCUS AND EPICENTER

An earthquake's focus also known as hypocenter is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture. This occurs directly beneath the epicenter, at a distance known as the focal.

In the case of earthquakes, the epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone.

For example, in the magnitude 7.9, 2002 Denali earthquake in Alaska, the epicenter was at the western end of the rupture, but the greatest damage occurred about 330 km away at the eastern end of the rupture zone.

seismic waves

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of an earthquake, explosion, or a volcano.

PRIMARY WAVES

P-waves are longitudinal in nature. P waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel at nearly twice the speed of S waves, they travel at the speed of sound.

SECONDARY WAVES

S-waves are shear waves and are transverse in nature. These waves arrive at seismograph stations after the P waves during an earthquake and displace the ground perpendicular to the direction of propagation S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S waves are about 60% slower than P waves.

Effects of earthquakes

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures

Landslides and avalanchesEarthquake can produce slope instability leading to landslides, a major geological hazard.

Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water.Tsunamis travel 600-800 kilometers per hour, depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.

RICHTER SCALE

The Richter magnitude scale is a scale of numbers used to tell the size of earthquakes. The scale is logarithmic, with a base of 10.Developed in 1935 by Charles Francis Richter in partnership with Beno Gutenberg,

Kinemetrics seismograph

DescriptorRichter

Magnitude number

Damage caused by the earthquake Frequency of occurrence

MicroLess than

2.0Micro earthquakes, people cannot

feel these.About 8,000 each day

Very minor 2.0-2.9People do not feel these, but

seismographs are able to detect them.

About 1,000 per day

Minor 3.0-3.9People often feel these, but they

rarely cause damage.About 49,000 each year

Light 4.0-4.9Objects inside houses are

disturbed, causing noise. Nothing is damaged.

About 6,200 each year

Moderate 5.0-5.9Buildings that are not built well may be damaged. Light objects inside a house may be moved.

About 800 per year

Strong 6.0-6.9Moderately powerful. May cause a

lot of damage in a larger area.About 120 per year

Major 7.0-7.9Can damage things seriously over

larger areas.About 18 per year

Great 8.0-9.9

Massive damage is caused. Heavy objects are thrown into the air and

cracks appear on the ground, as well as visible shockwaves. Overhead highways may be

destroyed, and buildings are toppled.

About 1 per 20 years

EARTHQUAKE PROOFING

To be earthquake proof, buildings, structures and their foundations need to be built to be resistant to sideways loads. The lighter the building is, the less the loads.Buildings are basically designed to support a vertical load in order to support the walls, roof and all the stuff inside to keep them standingAs the buildings get bigger and taller other techniques are employed such as “base isolation.” During the past 30 years, engineers have constructed skyscrapers that float on systems of ball bearings, springs and padded cylinders. Acting like shock absorbers in a car, these systems allow the building to be decoupled from the shaking of the ground. These buildings don’t sit directly on the ground, so they’re protected from some earthquake shocks. In the event of a major earthquake, they can sway up to a few feet.

730 ton motion damper inside the

Taipei 101 skyscraper

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