# Earthquakes

Seismologists estimate that there are 500,000 earthquakes worldwide each year. Although only one-fifth of these are strong enough to be felt and only 100 cause damage, the tremendous devastation caused by seismic activity costs billions of dollars of damage and hundreds of lives. Nearly 80% of all earthquakes occur along the densely populated, volcanically-active Pacific Rim.

## The San Andreas Fault Zone

The most famous seismic area in the U.S. is the San Andreas Fault Zone where the North American and Pacific tectonic plates touch as seen in this illustration of the fault near Los Angeles. If rocks stuck between the plates break or slip, the released energy produces seismic waves felt along the surface of the ground. The point directly above ground from where the rock breaks is the epicenter of the earthquake.

## Measuring the Severity of an Earthquake

## Richter Scale

Charles Richter, a seismologist at Caltech, published his earthquake measurement scale in 1935. The scale measures the strength or magnitude of an earthquake using a logarithmic formula based on seismographic readings and the seismograph’s distance from an earthquake. The Richter Scale ranks earthquake magnitude on a scale of whole numbers and fractions from 1 to 10.

## Moment Magnitude Scale

Introduced in 1979 by Caltech seismologists Thomas Hanks and Hiroo Kanamori, the Moment Magnitude Scale measures the “seismic moment” of an earthquake: the amount of energy released, size and amount of movement, and type of rock. It uses a numeric scale similar to the Richter Scale. Since 2002, the Moment Magnitude Scale has been the official scale used by the U.S. Geological Survey to measure large earthquakes. If media reports give a magnitude number for an earthquake larger than 5.0, it will likely be based on the Moment Magnitude Scale, even though a reporter may use the name “Richter.”

## Modified Mercalli Intensity Scale

In 1931, Harry Wood and Frank Neumann introduced the Modified Mercalli Intensity Scale to measure the severity (rather than the magnitude) of an earthquake. It was an update of a scale developed by Italian seismologist Giuseppe Mercalli earlier in the century. An earthquake’s intensity is ranked on a Roman numeral scale based on observations of damage rather than on mathematical formulas.