Thang Richter được giải thích: Lịch sử, Công thức và Giới hạn

Charles Richter and the Birth of Earthquake Measurement

In 1935, a young seismologist at the California Institute of Technology named Charles Francis Richter had a practical problem. The seismological laboratory received reports of dozens of earthquakes each week from stations across Southern California, but there was no standard way to compare their sizes. Some quakes were clearly larger than others, but no one had a number for "how much larger."

Richter, working with his colleague Beno Gutenberg, devised an elegant solution. He would use the maximum amplitude recorded on a SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. of a standard type — the Wood-Anderson torsion seismometer — and adjust for the distance between the station and the earthquake. The result was a dimensionless number he initially called the "magnitude" scale but which the world came to call the Richter scale. His 1935 paper established the framework that all subsequent magnitude scales would build upon, and SeismogramThe recorded output of a seismograph, showing ground motion as a function of time. Seismologists analyze seismograms to determine earthquake magnitude, depth, and location. analysis became the foundation of earthquake measurement worldwide.

How the Richter Scale Works: The Logarithmic Formula

The original Richter ScaleThe original logarithmic magnitude scale developed by Charles Richter in 1935 to measure local earthquake magnitude. Largely replaced by moment magnitude but still commonly referenced in media., technically known as local magnitude (ML), is defined by a deceptively simple formula. Richter took the logarithm (base 10) of the maximum wave amplitude measured in micrometres on a standard seismograph at 100 kilometres distance and called that the magnitude. For stations at different distances, he applied correction factors derived from tables of how amplitude decreases with distance in Southern California.

The logarithmic nature has a critical practical implication: each whole number increase in magnitude corresponds to a tenfold increase in the amplitude of shaking recorded on the seismograph. In terms of actual energy released, the relationship is steeper — each magnitude unit represents approximately 31.6 times more energy. A MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. 7.0 earthquake therefore releases about 31.6 times more energy than a 6.0, and about 1,000 times more than a 5.0.

Limitations of the Richter Scale for Large Earthquakes

The original Richter scale worked well for moderate earthquakes in Southern California, the region for which it was calibrated. When seismologists tried to apply it to large earthquakes elsewhere, problems emerged. The most serious was "saturation" — for very large earthquakes, different magnitude scales give inconsistent readings that fail to grow appropriately with earthquake size.

The Local Magnitude (ML)The original Richter magnitude, calculated from the maximum amplitude recorded on a Wood-Anderson seismograph within 600 km of the epicenter. Only valid for local, shallow earthquakes. scale saturates because it is based on high-frequency waves recorded on a particular type of instrument. For great earthquakes with magnitude above about 6.5 to 7.0, the energy is mostly carried at very long periods — slow, sweeping motions of the ground that the original Wood-Anderson seismometer could not capture well. This is why scientists noticed that the Richter scale gave similar values for the 1960 Chile earthquake and the 1964 Alaska earthquake, even though both events were clearly "off the charts" in terms of destruction. Those two events are now known to have had moments equivalent to approximately magnitude 9.5 and 9.2, respectively.

Why Scientists Switched to Moment Magnitude

The Moment Magnitude ScaleThe modern standard for measuring earthquake size (Mw), based on the seismic moment — the product of fault area, average slip, and rock rigidity. Accurate for all earthquake sizes. scale (Mw), developed by Thomas Hanks and Hiroo Kanamori in 1979, solved the saturation problem by measuring something physically meaningful: the seismic moment of the earthquake. Seismic moment is the product of three quantities — the rigidity of the rock, the area of the fault that ruptured, and the average amount of slip. This can be calculated from SeismogramThe recorded output of a seismograph, showing ground motion as a function of time. Seismologists analyze seismograms to determine earthquake magnitude, depth, and location. analysis at very long periods, using Broadband SeismometerA seismometer capable of recording seismic waves across a wide frequency range (0.001-50 Hz). The primary instrument in modern global seismograph networks.s that record the full spectrum of ground motion.

Because seismic moment scales with the true physical size of the earthquake without saturating, Mw gives consistent and physically meaningful values for all earthquake sizes. The formula relating Mw to seismic moment is logarithmic, and Richter himself calibrated the original scale so that for moderate earthquakes the values agree closely. This means you can often see ML and Mw values that are nearly identical for moderate quakes, which eases the transition.

Common Misconceptions About the Richter Scale

Several persistent misconceptions surround the Richter scale. The most common is that it is the scale scientists use today — it is not. The USGS (United States Geological Survey)The primary US government agency responsible for monitoring earthquakes, operating the National Earthquake Information Center, and publishing real-time earthquake data worldwide. and virtually all major seismological agencies worldwide now report moment magnitude (Mw), yet news media continue to say "on the Richter scale" as if that were the current standard. The mistake is understandable because for most newsworthy earthquakes in the magnitude 5–7 range, Mw and ML give very similar numbers.

Another misconception is that there is a maximum value for the scale. There is no theoretical ceiling. Richter himself noted that the scale was open-ended at both ends. Similarly, there is no minimum — seismologists routinely detect and catalogue micro-earthquakes at negative magnitudes. The practical limits are set by what instruments can detect and what physics can produce.

A third misconception is that each magnitude step is "10 times more destructive." This confuses amplitude (which does increase tenfold per step) with energy (which increases 31.6-fold per step) with actual damage (which depends on depth, distance, soil, and construction quality in complex ways).

Richter Scale in Media: Why It Persists

Despite being scientifically superseded, the "Richter scale" name continues to dominate media coverage of earthquakes for several reasons. It is short, simple, and familiar — a single word that any headline can carry. The phrase entered popular culture deeply during the mid-20th century when Richter himself was a prominent public scientist who gave numerous interviews and was telegenic in explaining earthquakes to the public.

The persistence also reflects the close numerical agreement between scales for the moderate earthquakes that most news stories cover. When a reporter writes "magnitude 5.8 on the Richter scale," the actual moment magnitude is likely 5.8 or very close to it. The scientific inaccuracy rarely produces a misleading number in those cases. Seismologists have largely accepted that the battle to retire the term from popular usage is unwinnable, even as they continue to use Moment Magnitude ScaleThe modern standard for measuring earthquake size (Mw), based on the seismic moment — the product of fault area, average slip, and rock rigidity. Accurate for all earthquake sizes. exclusively in research and official communications.