Richter Scale

Whenever a major earthquake is in the news, you'll probably hear about its Richter Scale rating. You might also hear about its Mercalli Scale rating, though this isn't discussed as often. These two ratings describe the power of the earthquake from two different perspectives.

Photo courtesy NGDC Destruction caused by a (Richter) magnitude 6.6 earthquake in Caracas, Venezuela. The 1967 earthquake took 240 lives and caused more than $50 million worth of property damage.

The Richter Scale is used to rate the magnitude of an earthquake -- the amount of energy it released. This is calculated using information gathered by a seismograph. The Richter Scale is logarithmic, meaning that whole-number jumps indicate a tenfold increase. In this case, the increase is in wave amplitude. That is, the wave amplitude in a level 6 earthquake is 10 times greater than in a level 5 earthquake, and the amplitude increases 100 times between a level 7 earthquake and a level 9 earthquake. The amount of energy released increases 31.7 times between whole number values.

The largest earthquake on record registered an 9.5 on the currently used Richter Scale, though there have certainly been stronger quakes in Earth's history. The majority of earthquakes register less than 3 on the Richter Scale. These tremors, which aren't usually felt by humans, are called microquakes. Generally, you won't see much damage from earthquakes that rate below 4 on the Richter Scale. Major earthquakes generally register at 7 or above.

Photo courtesy NGDC Damage to a school in Anchorage, Alaska, caused by the 1964 Prince William Sound earthquake. The earthquake, which killed 131 people and caused $538 million of property damage, registered an 9.2 on the Richter Scale.

Richter ratings only give you a rough idea of the actual impact of an earthquake. As we've seen, an earthquake's destructive power varies depending on the composition of the ground in an area and the design and placement of manmade structures. The extent of damage is rated on the Mercalli Scale. Mercalli ratings, which are given as Roman numerals, are based on largely subjective interpretations. A low intensity earthquake, one in which only some people feel the vibration and there is no significant property damage, is rated as a II. The highest rating, a XII, is applied only to earthquakes in which structures are destroyed, the ground is cracked and other natural disasters, such as landslides or Tsunamis, are initiated.

Photo courtesy NGDC Damage from a magnitude 7.4 earthquake that hit Niigata, Japan, in 1964.

Richter Scale ratings are determined soon after an earthquake, once scientists can compare the data from different seismograph stations. Mercalli ratings, on the other hand, can't be determined until investigators have had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have a good idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating.

In some areas, severe earthquake damage is the result of liquefaction of soil. In the right conditions, the violent shaking from an earthquake will make loosely packed sediments and soil behave like a liquid. When a building or house is built on this type of sediment, liquefaction will cause the structure to collapse more easily. Highly developed areas built on loose ground material can suffer severe damage from even a relatively mild earthquake. Liquefaction can also cause severe mudslides, like the ones that took so many lives in the recent earthquake that shook Central America. In this case, in fact, mudslides were the most significant destructive force, claiming hundreds of lives.