The Severity of an Earthquake
Source: National Earthquake Information Center, U.S. Geological Survey
Earthquakes are the result of forces deep within Earth's interior that continuously affect its surface. The energy from these forces is stored in a variety of ways within the rocks. When this energy is released suddenly—by shearing movements along faults in the crust of Earth, for example—an earthquake results. The area of the fault where the sudden rupture takes place is called the focus or hypocenter of the earthquake. The point on Earth's surface directly above the focus is called the epicenter of the earthquake.
The severity of an earthquake can be expressed in terms of both intensity and magnitude. The two terms are quite different, however, and they are often confused. Intensity is based on the observed effects of ground shaking on people, buildings, and natural features. It varies from place to place within the disturbed region depending on the location of the observer with respect to the earthquake epicenter. Magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake. It is based on the amplitude of the earthquake waves recorded on instruments, which have a common calibration. Magnitude is thus represented by a single, instrumentally determined value.
The Richter Magnitude Scale
Seismic waves are the vibrations from earthquakes that travel through Earth; they are recorded on instruments called seismographs. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.
The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8.8 to 8.9 range.
Why Are There So Many Earthquake Magnitude Scales?
Earthquake size, as measured by the Richter Scale, is a well-known, but not well understood, concept. What is even less well understood is the proliferation of magnitude scales and their relation to Richter's original magnitude scale. Richter's magnitude scale was first created for measuring the size of earthquakes occurring in southern California, using relatively high-frequency data from nearby seismograph stations. This magnitude scale was referred to as ML, with the L standing for local.
As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges. In order to take advantage of the growing number of globally distributed seismograph stations, new magnitude scales that are an extension of Richter's original idea were developed. These include body-wave magnitude, “mb,” and surface-wave magnitude, “MS.” Each is valid for a particular frequency range and type of seismic signal. In its range of validity each is equivalent to the Richter magnitude.
Because of the limitations of all three magnitude scales—ML, mb, and MS—a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, or “MW,” was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. New techniques that take advantage of modern telecommunications have recently been implemented, allowing reporting agencies to obtain rapid estimates of moment magnitude for significant earthquakes. So nowadays, when most seismologists announce a magnitude number, they are rarely referring to the Richter Scale.
The Modified Mercalli Intensity Scale
The effect of an earthquake on Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses, such as people awakening, movement of furniture, damage to chimneys, and finally—total destruction. Although numerous intensity scales have been developed over the past several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects. The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.