Measuring an earthquake

There are several ways that earthquake magnitude is expressed. The most famous was devised in 1934 by the late Dr. Charles F. Richter. On the Richter scale, and other magnitude scales, each whole number step represents a tenfold increase in the size of seismic waves measured on a seismograph--a machine that measures how much the ground moves in an earthquake. However, a single step on the Richter scale corresponds to a thirtyfold increase in the amount of energy released in an earthquake.

The news media usually refer to the size of an earthquake as having been measured on the Richter scale. In fact, a number of different scales are in common use. For example the "surface-wave" magnitude is now the standard for measuring moderate-size quakes. During the past 15 years, scientists have determined that both Richter magnitudes and surface wave magnitudes underestimate the energy released by the largest earthquakes. As a result, the magnitude of the 1964 earthquake in Alaska, which was initially assigned a surface-wave magnitude of 8.3, is now considered to have had a magnitude of 9.2.

The intensity of an earthquake is not measured using seismographs like Richter-scale magnitudes are, but is based upon earthquake's effects on man-made structures. The intensity of an earthquake can be very different in places only a hundred feet apart, because the amount of shaking, and therefore the damage, depends upon the kind of soil or rock beneath a particular place.

A modified version of the Mercalli intensity scale, developed in 1902 by an Italian geologist, is often used to measure earthquake intensity. This scale ranges between Roman numerals "I," which is rarely felt, to "XII," which results in damage to nearly all structures. The scale is outlined in the table on the next page.

Most residents in earthquake-prone parts of Alaska have experienced intensities up to IV. In the 1964 Alaskan earthquake, there were Mercalli intensities of X near the epicenter of the quake in Prince William Sound; there were intensity VII effects in Kodiak, Homer, Seward, Valdez, Cordova, and Anchorage; there were intensity V to VI effects in Fairbanks, Fort Yukon, Yakutat, and Sitka. Regardless of how the magnitude or intensity of an earthquake is measured, any earthquake is important if it impacts you, your family, or your community.

Here is additional information on earthquake magnitudes.