Earthquakes

Why do earthquakes form?

Forces causing earthquakes were not close to the earthquake source but very distant. these distant forces cause a gradual build up of stress in the earth over tens or hundreds or thousands of years, slowly distorting the earth underneath our feet. Vibrations called seismic waves are generated and travel both through the earth and along its surface. Eventually, a pre-existing weakness in the earth--called a fault or a fault zone--can not resist the strain any longer. This is something like pulling a rubber band gradually until the band snaps. This theory is known as the "elastic rebound theory."

Intraplate earthquakes are not as frequent or as large as those along plate boundaries. The largest intraplate earthquakes are about 100 times smaller than the largest interplate earthquakes.

Intraplate earthquakes tend to occur in soft, weak areas of plate interiors. Scientists believe intraplate quakes may be caused by strains put on plate interiors by changes of temperature or pressure in the rock. Or the source of the strain may be a long distance away, at a plate boundary. These strains may produce quakes along normal, reverse, or strike-slip faults

The earth can be divided into four main layers: the solid inner core, the liquid outer core, the mantle, and the crust. An earthquake is caused by a sudden rupture in a fault. A fault is the fracture within the rocky mass inside the earth's crust. The depth and length of faults vary greatly.

Scientists have developed a theory, called plate tectonics, that explains why most earthquakes occur. According to this theory, Earth's outer shell consists of about 10 large, rigid plates and about 20 smaller ones. Each plate consists of a section of Earth's crust and a portion of the mantle, the thick layer of hot rock below the crust. Scientists call this layer of crust and upper mantle the lithosphere. The plates move slowly and continuously on the asthenosphere, a layer of hot, soft rock in the mantle. As the plates move, they collide, move apart, or slide past one another.

The movement of the plates strains the rock at and near plate boundaries and produces zones of faults around these boundaries. Along segments of some faults, the rock becomes locked in place and cannot slide as the plates move. Stress builds up in the rock on both sides of the fault and causes the rock to break and shift in an earthquake.

The world's deepest earthquakes occur in subduction zones down to a depth of about 450 miles (700 kilometers). Below that depth, the rock is too warm and soft to break suddenly and cause earthquakes.

How do earthquakes form?

Stress in the earth's outer layer cause a pushing effect against the sides of the fault. Due to this motion, rocks slip or collide against each other releasing energy. This released energy travels in waves through the earth's crust and causes the shaking that we feel during an earthquake.

Under the surface of the earth, the two sides of a fault are constantly moving, relative to one another. This movement is known as a fault slip. The movement of these two sides is not smooth and is accompanied by a gradual build-up of elastic strain energy within the rocks along the fault.

Eventually, the strain along the fault becomes too much.The fault then ruptures with a sudden movement releasing all the energy it has built up. This energy is released in the form of vibrations called 'seismic waves'.

These waves travel along the surface and through the earth at varying speeds depending on the material through which they move. It is actually these seismic waves that cause most of the destructive effects, which we associate with earthquakes.

The location on a fault where the slip first occurs is called the hypocentre, whereas the position directly above it on the ground surface is called the epicentre.

When these seismic waves reach the surface of the earth, they give rise to strong ground motion causing building and other man-made structures to shake or collapse or develop cracks and fissures.

Earthquakes can also cause landslides, sudden eruptions as in the case of a hot lava flow from a volcano or giant waves called tsunamis. Sometimes new land mass are also formed.

Where do earthquakes occur?

Where Are Earthquakes Likely to Occur?

Within areas of the crust are fractures, known as faults, along which two crustal blocks have slipped or moved against each other. One block may move up while the other moves down, or one may move horizontally in one direction and the other in the opposite direction. earthquakes occur repeatedly at faults, which are zones of weakness in the earth's crust.

How Deep Do Earthquakes Occur in the World?

Earthquakes occur In the crust or upper mantle which ranges from the surface to about 800 kilometers deep (about 500 miles).

95% of all the world's earthquakes occur at active plate boundaries . California, Alaska, Japan, South America, and the Philippines are all on plate boundaries. Only 5% are in areas of the plates far away from the boundaries.

Earthquakes usually occur where two of the earth's tectonic plates meet. These boundary regions, therefore, are particularly prone to earthquakes (and also to volcanic eruptions). The edges of the giant Pacific plate, which lies beneath the Pacific Ocean, form a particularly intense ring of tectonic activity, known as the "ring of fire." This activity causes frequent earthquakes along the west coast of North America and in Chile, Alaska, and Japan, and is also responsible for volcanic eruptions in the northwest United States, the Andes Mountains, and the Philippines.

Types of earthquakes

Interplate earthquakes occur along the three types of plate boundaries: (1) mid-ocean spreading ridges, (2) subduction zones, and (3) transform faults.

How are earthquakes measured?

A seismometer is an instrument that senses the earth's motion; a seismograph combines a seismometer with recording equipment to obtain a permanent record of the motion. From this record scientists can calculate how much energy was released in an earthquake, which is one way to decide its magnitude. Calculations are made from several different seismograms, both close to and far from an earthquake source to determine its magnitude. Calculations from various seismic stations and seismographs should give the same magnitude, with only one magnitude for any given earthquake.

To determine the strength and location of earthquakes, scientists use a recording instrument known as a seismograph. A seismograph is equipped with sensors called seismometers that can detect ground motions caused by seismic waves from both near and distant earthquakes. Some seismometers are capable of detecting ground motion as small as 1 billionth of a meter, or about 40 billionth of an inch.


A seismograph produces wavy lines that reflect the size of seismic waves passing beneath it. The record of the wave, called a seismogram, is imprinted on paper, film, or recording tape or is stored and displayed by computers

The Richter scale is a standard scale used to compare earthquakes. It is a logarithmic scale, meaning that the numbers on the scale measure factors of 10. So, for example, an earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0. On the Richter scale, anything below 2.0 is undetectable to a normal person and is called a microquake. Microquakes occur constantly. Moderate earthquakes measure less than 6.0 or so on the Richter scale. Earthquakes measuring more than 6.0 can cause significant damage. The maximum quake rating ever measured is about 8.9.

The Modified Mercalli Intensity Scale uses Roman Numerals from I to XII to describe different earthquake effects is commonly used.

Safety first for earthquakes

Can Earthquakes Be Prevented?

There is no known way to prevent earthquakes, but it is possible to lessen the impact. The amount of devastation from an earthquake can be greatly diminished by building structures using earthquake resistant design, making the interiors of buildings safe from falling objects, and educating people about earthquake safety.

China cabinet (Should be attached to wall studs)

Tall knickknack shelves (Should be attached to wall studs)

Bookshelves (Should be attached to wall studs)

Heavy hanging plant over a place where people sit (should be light, unbreakable pot and make sure all plants hang from ceiling studs)

A mirror on the wall (Make sure it is well fastened to the wall)

Heavy objects on wall shelves (should be moved to bottom shelves or secured)

Unsecured TV on a rolling cart (Make sure cart wheels are blocked so TV can’t roll)

Bed by a big window (Bed should be moved away)

Heavy picture above a bed (Bed or picture should be moved)

A hanging light above a bed (Light should be secured with extra wire or chain, or the bed should be moved.)

Cabinet doors not fastened to stay closed (Install latches)

Unattached water heater (Attach water heater to the wall studs)

Gas stove with rigid feed line (Replace gas line with flexible connectors)

Heavy wall clock (Attach to wall studs)

Chimney (Brace outside chimney to the house)

House not bolted to the foundation. (Foundation should be bolted)

The most important thing to remember during an earthquake is to DROP, COVER and HOLD ON. So remember to DROP to the floor and get under something for COVER and HOLD ON during the shaking.

Many people are injured by falling debris just outside buildings. Usually it's better to stay inside if you are already inside, and to stay outside if you are already outside. Inside, try taking cover underneath something sturdy like a desk, and avoid windows, mirrors, and heavy furniture that might fall such as overloaded bookshelves. Outside, avoid walls, chimneys, and electrical power lines. It is best to protect your head.