One of the most frightening and destructive phenomena of nature is an earthquake. An earthquake is a series of violent vibrations on the earth’s surface caused by the generation of seismic waves due to sudden rupture within the earth during the release of accumulated strain energy. The location where the earthquake starts is called hypocenter. The location above is called the epicenter. The epicenter is the point on the surface where the earthquake is the strongest.

Causes

• Earthquakes occur due to sudden, violent shifting of massive rocks called tectonic plates, which are the earth’s outermost layer of crust and upper mantle. Due to the heating and cooling of the rock below these plates, convection occurs. This results in the movement in the overlying plates. This releases the stress that accumulates along faults. The brittle outer part of the Earth crust fractures along faults. A fault is a deep crack that marks the boundary between two of these plates. Most earthquakes happen near the boundaries of tectonic plates, both where the plates spread apart and grind together. In the process of breaking, vibrations called “seismic waves” are generated. These waves travel outward from the source of the earthquake along the surface and through the Earth at varying speeds. These vibrations cause the entire planet to quiver.
• Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake.
• Nuclear explosions under the ground can create waves that are very similar to natural seismic waves.

Where do earthquake occur?

The Earth’s major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. The most important earthquake belt is:

• The Circum-Pacific Belt, which affects many populated coastal regions around the Pacific Ocean—as, for example, those of New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America.
• The Alpide Belt, passes through the Mediterranean region eastward through Asia and joins the Circum-Pacific Belt in the East Indies. The energy released in earthquakes from this belt is about 15 percent of the world total.

There also are striking connected belts of seismic activity, mainly along oceanic ridges including those in the Arctic Ocean, the Atlantic Ocean, and the western Indian Ocean.

Classification of earthquakes

The magnitude of the earthquake is measured based on the ground motion recorded by an instrument and is related to the amount of energy released by an earthquake and expressed in the Richter scale.

The Richter scale is used to measure the amount of energy released by the earthquake. The severity of an earthquake runs from 0 to 9 on this scale.

• Slight Magnitude up to 4.9 on the Richter scale.
• Moderate Magnitude 5.0 to 6.9
• Great Magnitude 7.0 to 7.9
• Violent Magnitude 8.0 and more

But Richter magnitude is only accurate for earthquakes up to about 500 kilometers. Seismologists have developed a system called “moment magnitude,” which takes into account the actual area of fault ruptured and gives a more consistent measure of earthquake size across the spectrum.

The study of earthquakes is called seismology. The earthquake generates seismic waves, which can be recorded on a sensitive instrument called a seismograph. The record of ground shaking recorded by the seismograph is called a seismogram.

Damage Caused by Earthquakes

1. The effects of an earthquake are strongest in a broad zone surrounding the epicenter.
2. Earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects.
3. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking.
4. The waves can produce destructive vibrations in buildings and breakwater and gas lines, starting uncontrollable fires.
5. An earthquake can trigger mudslides, which slip down mountain slopes and can bury habitations below.
6. A submarine earthquake can cause a tsunami, a series of damaging waves that ripple outward from the earthquake epicenter and inundate coastal cities.

Prevention Is Better Than Cure

The earthquake can cause massive damage and destruction. Earthquakes strike suddenly, violently, and without warning at any time of the day or night. If an earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage. Scientists are continuously thinking of ways to try and reduce earthquake power. Although there are no guarantees of safety during an earthquake, identifying potential hazards ahead of time and planning can save lives and significantly reduce injuries and property damage.

The risks that earthquakes pose to society, including death, injury, and economic loss, can be greatly reduced by

1. Better planning, construction, and mitigation practices before earthquakes happen.
2. Providing critical and timely information to improve response after they occur.

In earthquake-prone areas, populated areas need to take measures to protect themselves against the effects of earthquakes to reduce deaths and losses.

• Earthquake drills should be conducted frequently in earthquake-prone settlements, so that people would be familiarized with emergency procedures during an actual earthquake, reducing death tolls.
• Adequate shelters, medicine, and food should also be provided in the settlement to handle after-effects of the earthquake.
• Disaster plans and civil defense units should also be well maintained to ensure efficient rescue actions after a disaster strikes.
• Seismographs, machines that can detect earthquakes, should be utilized to predict potential earthquakes, alerting authorities to evacuate the people as soon as an earthquake threat is reported.
• Tsunamis warning systems are also important in coastal areas prone to the earthquake to reduce the great loss of life and damage to property when the waves roll in.
• The earthquake risk can be reduced by microzonation, which is the identification of separate individual areas having different potentials for hazardous earthquake effects.
• 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.