Ever wonder about all these different measurements of earthquakes? Most of us have heard of the Richter Scale, but it’s no longer used. Today, we use the Moment magnitude — which reports energy released for earthquakes larger than 3.5 magnitude — and Intensity scale which varies by where you are compared to the earthquake. Read on…
The well known, but not well understood, Richter scale was developed by Charles Richter in 1935. Simply put, it measures the amplitude (height of wave) of a waveform recorded on a seismograph.
Since magnitude scales are logarithmic, the size represented ramps very very quickly. Each point higher represents an order of magnitude higher — e.g., a 7.0 is 10x larger than a 6.0. That same one point increase also represents a release of 30x more energy.
The problem with the Richter scale (ML) is that it’s only valid for certain frequencies and distances of earthquakes (e.g., it’s capped at a magnitude of 7.0 and most effective up to magnitude 5). At first, as more seismograph stations were added around the world, Richter’s work was extended with additional scales including body wave magnitude (Mb) and surface wave magnitude (Ms).
Because of the limitations of ML, Mb, and Ms, the Moment magnitude (Mw) was introduced in 1979. The Richter scale has been widely replaced with the Moment Magnitude Scale. With that in mind, if you hear someone talking about earthquake size using “Richter scale,” it’s likely an error — it’s simply no longer used when reporting to the public, and hasn’t been for decades.
Moment is looking to put a number to the energy released in an earthquake. Rather than looking at size of waves, it looks at the amount of slip on a fault line multiplied by the area of the fault surface that slips. It’s then converted in a number that makes it similar to Richter and other scales.
Magnitude vs. Intensity
The moment magnitude of an earthquake is a single number that describes the event. How the earthquake feels is a different type of number … and that very much depends on where you are in relation to the earthquake, the type of soil you are on, and of course, the type and height of building you are in.
Today, we measure intensity with the Modified Mercalli Intensity Scale — a scale from I – X. This scale will vary by area and distance for the same earthquake. The following is an abbreviated description of the levels of Modified Mercalli intensity (from https://earthquake.usgs.gov/learn/topics/mercalli.php).
|I||Not felt||Not felt except by a very few under especially favorable conditions.|
|II||Weak||Felt only by a few persons at rest,especially on upper floors of buildings.|
|III||Weak||Felt quite noticeably by persons indoors, especially on upper floors of
buildings. Many people do not recognize it as an earthquake. Standing motor cars
may rock slightly. Vibrations similar to the passing of a truck. Duration
|IV||Light||Felt indoors by many, outdoors by few during the day. At night, some
awakened. Dishes, windows, doors disturbed; walls make cracking sound.
Sensation like heavy truck striking building. Standing motor cars rocked
|V||Moderate||Felt by nearly everyone; many awakened. Some dishes, windows broken.
Unstable objects overturned. Pendulum clocks may stop.
|VI||Strong||Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.|
|VII||Very strong||Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly
built or badly designed structures; some chimneys broken.
|VIII||Severe||Damage slight in specially designed structures; considerable damage in
ordinary substantial buildings with partial collapse. Damage great in poorly
built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
|IX||Violent||Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.|
|X||Extreme||Some well-built wooden structures destroyed; most masonry and frame
structures destroyed with foundations. Rails bent.
Abridged from The Severity of an Earthquake, USGS General Interest Publication 1989-288-913
How Does Richter compare to Moment?
This table, from “Comparing the Richter and Moment Magnitude Scales” by Pearson Education, demonstrates how some of the most famous earthquakes rated on each scale by comparison.
|Date||Location||Richter scale||Moment scale|
|1811–1812||New Madrid, midwestern U.S.||8.7||8.1|
|1906||San Francisco, California||8.3||7.8|
|1971||San Fernando, California||6.4||6.7|
|1985||Mexico City, Mexico||8.1||8.1|
|1989||San Francisco, California||7.1||6.9|
Prior to Richter: Looking at Earthquake History
Seismographs started being used in around 1890, and as a result, for earthquakes between 1890 and 1935 (when the Richter scale was introduced), scientists can go back to the historical seismograph records and determine the Richter scale.
But, prior to seismographs, magnitudes have to be estimated. USGS explains “For earthquakes that occurred between about 1890 (when modern seismographs came into use) and 1935 when Charles Richter developed the magnitude scale, people went back to the old records and compared the seismograms from those days with similar records for later earthquakes. For earthquakes prior to about 1890, magnitudes have been estimated by looking at the physical effects (such as amount of faulting, landslides, sandblows or river channel changes) plus the human effects (such as the area of damage or felt reports or how strongly a quake was felt) and comparing them to modern earthquakes.”