What is an Earthquake?

Earthquakeis the shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks is suddenly released and transmitted to the surface of the Earth by earthquake waves. The effect is like the sudden breaking and snapping back of a stretched elastic band where waves radiate out from the rupturing fault starting at the focus of the earthquake. Most earthquakes are caused by the sudden slip along geologic faults or when pressure within the crust increases slowly over hundreds of years and finally exceeds the yield point of the rocks.

The size of an earthquake is normally indicated in units of magnitude. There are many different ways that magnitude is measured from seismograms because each method only works over a limited range of magnitudes and with different types of seismometers. Some methods are based on body waves (which travel deep within the structure of the earth), some based on surface waves (which primarily travel along the uppermost layers of the earth), and some based on completely different methodologies. However, all of the methods are designed to agree well over the range of magnitudes where they are reliable. Earthquake magnitude is a logarithmic measure of earthquake size.
Earthquake waves make the ground move, shaking buildings and causing poorly designed or weak structures to partially or totally collapse. The ground shaking weakens soils and foundation materials under structures and causes dramatic changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil beneath structures and buildings weakens. Shaking may also dislodge large earth and rock masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or property damage.

The destruction that can be caused by an earthquake depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage to the structure. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.

To date it has not been possible to predict the occurrence of an earthquake with much precision. However, one way of estimating the likelihood of future earthquakes is to study how fast strain accumulates. When plate movements build the strain in rocks to a critical level, like pulling a rubber band too tight, the rocks will suddenly break and slip to a new position. Scientists can measure how much strain accumulates along a fault segment each year, how much time has passed since the last earthquake along the segment, and how much strain was released in the last earthquake. This information is then used to calculate the time required for the accumulating strain to build to the levels that result in an earthquake. This simple model is complicated by the fact that such detailed information about faults is rarely available. Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs detects about 1 million small earthquakes per year. Very large earthquakes which cause millions of money in damage occur worldwide once every few years while moderate earthquakes occur about 20 times a year. Moderate earthquakes can also cause significant economic damage and loss of lives.Since earthquakes are not certainly predictable, it implies that earthquakes of any size may occur depending on the amount of strain energy that shall have been accumulated at that particular time and the nature of rock material through which seismic waves propagates.