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EARTHQUAKES.

WHAT IS AN EARTHQUAKE?

An earthquake is the result of a sudden release of energy in the Earth’s crust that creates seismic waves.

The point of initial rupture of an earthquake is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.

At the Earth’s surface, earthquakes manifest themselves as a sudden shaking and sometimes displacement of 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.

WHY EARTHQUAKES HAPPEN?

Along the boundary separating any two tectonic plates, there can be movement. Depending on the boundary, as you must know, this movement can be of crushing, pulling apart, or sliding past each other.

The movement between plates and along faults is not smooth. They move in jerks, crushing, sliding and compressing, giving rise to earthquakes. The locations of earthquakes throughout the world, not surprisingly, delineate the major tectonic boundaries. Its because earthquakes normally take place in this boundaries and faults.

Because faults have friction, they resist the forces trying to move the pieces apart. As the forces build, the fault remains locked and the blocks get deformed because of the increasing stress. Eventually the stresses get so high that the fault breaks. This releases the built up stress and allows the sides of the fault to slide past one another. This is what we call an earthquake.

SEISMIC WAVES

Earthquakes create seismic waves which shake the ground as they pass.

Seismic waves are waves of force that travel through the Earth or other elastic body. This waves are created in different ways, one of them, are earthquakes.

There are two main types of seismic waves, body waves and surface waves.

Body waves travel through the interior of the Earth, while surface waves are analogous to water waves and travel just under the Earth’s surface. Body waves are generally faster, but surface waves can be far more destructive. The main types of body waves are P-waves and S-waves, and the two main types of surface waves are Rayleigh waves and Love waves.

Body waves:

– P waves (primary waves) are longitudinal waves, which means that the ground is alternately compressed and dilated in the direction of propagation. In solids, these waves generally travel almost twice as fast as S waves and can travel through any type of material, solids, liquids and gases. When generated by an earthquake they are less destructive than the S waves and surface waves that follow them, due to their smaller amplitudes.

The movement of the ground is someway similar to that of a slinky.

– S waves (secondary waves) are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation. S waves can travel only through solids, so they cannot spread throught the core of the earth.

The movement of the ground is someway similar to that of a shaked rope, perpendicular to the direction of the wave.

Surface waves:

– Rayleigh waves are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water. They are slower than body waves, but they are much more destructives.

– Love waves are surface waves that cause horizontal shearing of the ground. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity.

The seismic waves that an eartquake causes can be detected with a sensitive instrument called a seismograph.

Seismograph sketch (old model).

The record of ground shaking recorded by the seismograph is called a seismogram. The type of seismograph represented in the sketch above is one of the many invented in the 19th century. Most of them were electromagnetic and operated by suspending a magnetic mass, or pendulum, within an electric coil.

Seismogram example.

Nowadays, mainly digital seismographs are used.

HOW DO WE MEASURE EARTHQUAKES?

There are many ways to measure the size of an earthquake. Some depend on the amount of damage caused by the earthquake, which is a subjetive measure, while others depend on the amount of seismic energy emitted by the earthquake. The two main scales used are the Mercalli Intensity Scale and the Ritcher Magnitude Scale.

The Mercalli Intensity Scale

The Mercalli Intensity Scale assigns an intensity or rating to measure the effects of an earthquake at a particular location.

The Mercalli Intensity of any one earthquake can be very different from place to place. This is because the amount of damage caused by an earthquake at a particular location depends on the geology of the location. The population density, the methods used to construct buildings near the location, and the distance to the epicenter are also important in the Mercalli scale.

Although it is an opinionated measure of earthquake size, so it is not objective, seismologists still mail questionnaires to local residents after an earthquake asking them to rate the effects of the earthquake at their home.

The Richter magnitude scale

The Richter magnitude scale assigns a single number to measure the amount of seismic energy released by an earthquake.

It’s related to the maximum amplitude of the S wave measured from the seismogram. Because there is a great range in the sizes of different earthquakes, the Richter scale uses logarithms. This means a magnitude 7 earthquake is 10 times as large as a magnitude 6 earthquake, and releases over 30 times more energy.

This scale is an objective scale, giving exact data about the earthquake.

ADITIONAL DATA

Mercalli Intensity Scale (whole)

I. Instrumental Not felt by many people unless in favourable conditions.
II. Feeble Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.
III. Slight Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
IV. Moderate Felt indoors by many people, outdoors by few people during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rock noticeably. Dishes and windows rattle alarmingly.
V. Rather Strong Felt outside by most, may not be felt by some outside in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house.
VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
VII. Very Strong Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars.
VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Ruinous General panic; damage considerable in specially designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Disastrous Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.
XI. Very Disastrous Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Catastrophic Total damage – Almost everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move position.

Ritcher Magnitude Scale (whole)

Richter magnitudes Description Earthquake effects Frequency of occurrence
Less than 2.0 Micro Microearthquakes, not felt. About 8,000 per day
2.0-2.9 Minor Generally not felt, but recorded. About 1,000 per day
3.0-3.9 Often felt, but rarely causes damage. 49,000 per year (est.)
4.0-4.9 Light Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 6,200 per year (est.)
5.0-5.9 Moderate Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800 per year
6.0-6.9 Strong Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas. 120 per year
7.0-7.9 Major Can cause serious damage over larger areas. 18 per year
8.0-8.9 Great Can cause serious damage in areas several hundred miles across. 1 per year
9.0-9.9 Devastating in areas several thousand miles across. 1 per 20 years
10.0+ Epic Never recorded; see below for equivalent seismic energy yield. Extremely rare (Unknown)

Earthquake location map

Note: There are some images, but i don’t know how to upload them. The text is mainly that.

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