We still often hear about “an earthquake measuring so many degrees on the Richter scale” in the news or when reading about an earthquake. This is incorrect for one or more reasons. To understand why, we must delve into what the Richter scale is, when it is used, and, above all, when it is not.
What is the Richter Scale?
The Richter scale is a scale used to measure the magnitude of an earthquake. According to the National Geological Institute (IGN), the magnitude of the earthquake is defined as “a measure of the energy released by an earthquake and is determined from the signal recorded in a seismogram.”
Several magnitude scales exist for earthquakes, but the one most familiar to the public is the local Richter magnitude (ML). This scale is specifically used for measuring earthquakes captured from close range, typically less than 600 kilometers from the seismograph as specified by the IGN.
Who was Charles Francis Richter?
Named after American seismologist Charles Francis Richter, who was born in 1900 in Ohio, this scale represents a systematic approach to measuring earthquake strength. While seismographs had been in use for decades, it was Richter who, in 1935, proposed establishing a magnitude scale to quantify these seismic events, with assistance from German-American seismologist Beno Gutenberg.
How are Magnitudes Calculated?
The scale calculates magnitudes using the logarithm of the amplitude of seismic waves. In simpler terms, the magnitude of an earthquake is logarithmically proportional to the height of the waves recorded by seismographs. However, calculations must be adjusted based on the type of seismograph used.
What We Measure with the Richter Scale, and What We Don’t
While the Richter scale (ML) is valuable for local earthquakes, it is primarily suited for measuring small to moderate magnitudes, typically in the range of approximately 2 to 6.5. As experts have noted, the Richter scale can occasionally overestimate or underestimate an earthquake’s true strength, particularly for larger events.
As such, even if two earthquakes are captured within the 600-kilometer range, the Richter scale may not always provide an accurate measure of their magnitude. This uncertainty has led geologists to develop alternative scales, such as body wave magnitude (Mb) and surface wave magnitude (Ms), with none being universally applicable. This has necessitated the creation of the moment magnitude scale, Mw.
Magnitude and Intensity
It’s essential to distinguish between magnitude and intensity. Unlike magnitude, which quantifies the earthquake’s strength, intensity refers to the impacts or damage caused by the quake. Various scales, such as the European Macroseismic Scale, measure this aspect on a scale from I to XII.
The ML Scale and the Mw Scale
Recognizing the need for a more universal scale, the United States Geological Survey (USGS) introduced the moment magnitude scale (Mw). Unlike the Richter scale, which relies on seismic wave measurements, the Mw scale considers the geological properties of tectonic movement.
This scale is based on the seismic moment, calculated using factors such as the area of fault movement and the stiffness of surrounding rocks. The Mw scale is now widely regarded as the most accurate and universal method for measuring earthquakes, although we may still encounter references to the Richter scale in media reports.
Common Misconceptions: Degrees vs. Magnitudes
Another prevalent misunderstanding involves referring to earthquake measurements as degrees, such as saying “an earthquake measuring 5.5 on the Richter scale.” This confusion likely stems from other systems that use degrees for measuring intensity.
By understanding the distinctions between these scales and the specific contexts in which they apply, we can move toward a more precise conversation about seismic events and their impacts.