The scale developed by Charles Richter in 1935 was the first standardized method for quantifying earthquake size, marking a fundamental shift in seismology. His creation, the Richter Local Magnitude Scale (\(M_L\)), provided scientists with a consistent, objective measure to compare seismic events. Though revolutionary and a household name, seismologists have largely abandoned the Richter scale for more sophisticated techniques. The original method was incapable of accurately measuring the largest global earthquakes due to technical flaws.
Defining the Original Scale
The Richter scale operates on a base-10 logarithmic system to assign a single number to an earthquake’s size. Each whole number increase represents a tenfold increase in the measured amplitude of the seismic waves recorded by a seismograph. For example, a magnitude 6 earthquake produces wave amplitudes ten times larger than a magnitude 5 earthquake. This logarithmic relationship corresponds to a significant increase in energy released, with each step up representing approximately 32 times more energy than the previous whole number. The calculation was initially based on the maximum wave amplitude detected by the Wood-Anderson torsion seismometer.
Technical Limitations That Led to Obsolescence
The primary reason for the Richter scale’s obsolescence is “saturation.” This occurs because the scale relies only on the peak amplitude of relatively short-period seismic waves. For earthquakes above magnitude 6.5 or 7, the rupture size exceeds the wavelength being measured, causing the scale to lose its ability to distinguish between a very large and a truly massive event. The seismograph’s recording of ground motion simply maxes out, or “clips,” and the resulting magnitude calculation clusters around the same value, failing to reflect the actual increase in energy released.
An additional limitation was the scale’s original, highly localized calibration. Richter developed his formula specifically for earthquakes in Southern California, using data from a limited network of seismometers. The formula included corrections tuned to the specific geological structure of that region. Applying this local magnitude scale globally introduced significant inaccuracies due to vastly different crustal structures. To accurately compare events worldwide, a measurement method independent of regional geology and short-period wave amplitude was required.
The Modern Method of Earthquake Measurement
The scientific community relies on the Moment Magnitude Scale (\(M_w\)) for consistently measuring the size of all earthquakes, especially the largest ones. Developed in the 1970s, \(M_w\) avoids the saturation problem entirely by not measuring the peak amplitude of a seismic wave. Instead, it measures the “seismic moment” (\(M_0\)), which is a physical property of the fault rupture. The seismic moment represents the total energy released by the earthquake.
This calculation involves three distinct physical factors related to the fault rupture: the area of the fault that slipped, the average distance the fault slipped, and the rigidity of the rock material involved in the rupture. By focusing on these physical dimensions of the earthquake source, the \(M_w\) scale provides a much more accurate and absolute measure of the energy released.
The Moment Magnitude Scale is applicable globally because the physical parameters it measures are inherent to the fault mechanics, not dependent on local crustal structure or distance from the seismograph. For small to medium earthquakes (typically below magnitude 6.5), the numerical values of the Moment Magnitude Scale are intentionally designed to be virtually identical to the original Richter scale values. This design ensures continuity with historical records while providing superior accuracy for larger events.
Why the Term Still Persists
Despite the universal adoption of the Moment Magnitude Scale, the term “Richter scale” remains embedded in public consciousness and media reporting. This persistence is largely due to the scale’s historical significance as the first widely publicized measurement of earthquake size. For decades, it was the only term the public knew.
For the vast majority of small to moderate earthquakes that occur globally, the calculated magnitude on the Moment Magnitude Scale is nearly the same as it would have been on the Richter scale. This numerical similarity contributes to the confusion, making it easy for the public to use the familiar name. The term has simply become a generic, inaccurate shorthand for “earthquake magnitude,” even when the reported number is technically a Moment Magnitude. Seismologists, however, only report the Moment Magnitude as the official measure of an earthquake’s size.