So far, earthquakes have been described as small enough to go unnoticed or large enough to cause damage. However, there are more technical ways to describe an earthquake's size and impact on society. Seismology is the study of earthquakes, and seismologists are scientists who study earthquakes. They use specialized instruments to help them predict, record, monitor, and measure earthquakes.

Seismographs

Seismologists use specialized instruments called seismographs to detect waves that transfer an earthquake's energy. Some are so sensitive that they can detect even the smallest vibrations in Earth's crust caused by waves, or even traffic.

Today, most seismographs are computerized. They collect data from earthquakes and almost instantly analyze this data. Early seismographs worked by graphing the P, S, and surface waves generated by an earthquake on paper using a pen and a weight suspended from a frame. These drawings were called seismograms. Large earthquakes would move the pen more dramatically, creating a seismogram with a large amplitude. Smaller earthquakes would not move the pen much and would create seismograms with a smaller amplitude.

The different speeds at which surface waves travel make it possible for seismologists to calculate the distance between the location of the seismograph and epicenter of the earthquake. The epicenter can be established using the time interval between the initial P wave and the S wave from seismograph readings. Charts correlating P and S wave time intervals with epicenter distances can be used to determine the distance the seismograph is from the epicenter.

Seismographs are placed all over the world to continually record and monitor earthquake activity on the entire planet. Data collected from these stations help scientists monitor changes in Earth's crust so that they can better predict the occurrence and magnitude of the next major earthquake.

Waves carry the energy released by an earthquake outwards in all directions. Seismographs placed on Earth's surface will detect the quicker P waves first, while the detection of slower moving S waves and surface waves will follow. Because only P and S waves are able to travel below the surface of Earth, they are used to determine the focus of an earthquake using triangulation.

Earthquake Magnitude

You may have heard news reports referring to earthquake magnitude readings, such as the magnitude 7 earthquake in Haiti, the magnitude 8.8 earthquake in Chile, and the magnitude 7.2 earthquake in Baja California, Mexico, in 2010. Magnitude is the measure of an earthquake's relative size. There are several ways that seismologists can measure magnitude, based upon the type of seismograph used and seismic wave measured. Some measures of magnitude are based upon P and S waves that travel within Earth, while others are based on Love and Rayleigh waves that travel along or near Earth's surface.

The original earthquake magnitude scale was developed in 1935 by Dr. Charles Richter. The Richter scale combined mathematics with seismograph data so that scientists could compare earthquakes around the world. Due to technological advances, the Richter scale has been replaced by the moment-magnitude scale we use today. Both are scales based on a logarithm of the wave amplitude recorded by seismographs. This means that, while the shaking of a magnitude 7 earthquake is 10 times greater than a magnitude 6 earthquake, the energy released by a magnitude 7 earthquake is approximately 32 times greater than the magnitude 6 earthquake.

Earthquakes that make news reports are usually greater than magnitude 4.5, because tremors below magnitude 4.5 can go unnoticed by many people. There are several thousand of these earthquakes recorded each year! Earthquakes above magnitude 4.5 garner more attention because of the damage they can cause. Scientists hypothesize that earthquakes surpassing magnitudes of 9 are very rare, because the lithosphere is rigid and does not usually have the capacity to store the energy necessary to produce such a large earthquake. This is reassuring news as events like the December 26, 2004 magnitude 9.1 earthquake, and the tsunami it caused near the island of Sumatra, illustrate how devastating they can be.

MODIFIED MERCALLI INTENSITY SCALE MOMENT-MAGNITUDE SCALE

I. Not felt except by a very few. Less than 3 Micro Earthquake II. Felt only by a few people at rest. Delicately suspended objects may swing. 3 - 3.9 Minor Earthquake III. Felt quite noticeably by people indoors. Vibration similar to the passing of a truck.     IV. Felt indoors by many, outdoors by few. Sensation like heavy truck striking building. 4 - 4.9 Light Earthquake V. Felt by nearly everyone; many awakened. Unstable objects overturned.     VI. Felt by all, many frightened. Some heavy furniture moved. Damage slight.     VII. Damage negligible in buildings of good design and construction; slight to moderate damage in well-built ordinary structures; considerable damage in poorly built or badly designed structures. 5 - 5.9 Moderate Earthquake VIII. Damage slight in specially designed structures; considerable damage in ordinary buildings with partial collapse. Damage great in poorly built structures.     IX. Damage considerable in specially designed structures. Damage great in ordinary buildings, with partial collapse. Buildings shifted off foundations. 6 - 6.9 Strong Earthquake X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations.     XI. Few, if any (masonry) structures remain standing. Bridges destroyed. 7 and higher Major Earthquake XII. Total Damage. Lines of sight and level are distorted. Greater than 8 Great Earthquake

The Modified Mercalli Intensity Scale

The magnitude of an earthquake doesn't always translate to how much damage it causes. Sometimes, an earthquake with a lower magnitude can cause even more damage than one with a greater magnitude.

The Modified Mercalli Intensity scale measures the effect that an earthquake has on populated areas. This scale is based upon observation and reports of shaking and damage done to structures. Therefore, it is only applicable to locations where there are people. Earthquakes in remote locations or in the ocean cannot be measured using this scale.

It is designated using 12 increasing levels of intensity represented by the Roman numerals I – XII. A location where people do not feel any ground movement would have an intensity of I. At an intensity of VI, everyone would feel the ground move, and there would be some damage. An area where almost everything is destroyed would have an intensity reading of XII.