Laurie Dingler | Measuring the magnitude of an earthquake is not so simple – Standard Times

Janet asked after last week’s column, “People have reported earthquake intensity and depth in Richter but not length in time. I lived in San Mateo during the 1989 earthquake. It seems to have lasted forever, and if it had lasted longer, the damage would have been worse. So why not a report?” How long did the earthquake last?”

Thank you Janet. First, what is the volume of Richter? Scientific agencies have not reported on Richter’s size for decades. However, some media outlets still misuse the term. Yes, we still use the concept of Richter’s volume and number range roughly as he defined it. But the way it is measured and what it means is very different now from the original definition in 1935.

Charles Richter is remembered primarily as the first seismometer although he made many other important contributions to seismology. Caltech installed a network of Wood Anderson seismographs in Southern California in the 1920s, and Richter began studying automated earthquakes in 1928. These instruments were the standard for regional studies, and we had two working at Humboldt from 1948 to 1992 .

Prior to Richter, earthquakes were measured by their intensity, qualitative measures based on damage and human perception of the strength of the vibration. People at the time likely knew the Rossi-Forel scale used to map the shaking strength of the 1906 San Francisco earthquake or the Mercalli scale that was modified and adapted as a standard by the USGS in the 20th century.

With its new seismic network, Caltech wanted to publish catalogs of earthquakes and needed something more quantitative than their intensity. Richter was influenced by Kiyo Wadati in Japan who proposed the displacement of the Earth as a measure of the magnitude of an earthquake. Richter measured the largest seismic signal recorded on Wood-Anderson instruments and, by adding/subtracting a factor based on distance, converted it to a unitless number by taking the logarithm of the value.

Richter borrowed the term “magnitude” from astronomy for his new treatment, though to the contrary. In astronomy, the brighter a star appears, the lower the size. Richter calibrated the scale so that earthquakes fall between zero and 10. After its publication in 1935, the idea was accepted by other seismologists and quickly became the global standard. When I entered graduate school in 1969, we basically used two forces, the Richter magnitude for regional earthquakes and the surface-wave magnitude (same idea but using different seismographs) for earthquakes from far away.

Janet’s question indicates that the length of the vibration has nothing to do with determining volume. When it comes to Richter wave and surface wave sizes, she is right. You are measuring the largest signal. It is assumed that the signal increases in size with the increase in the size of the source. The great earthquakes of the 1960s demonstrated the limitations of this methodology. The surface wave magnitude of Chile (1960) was 8.5 and Alaska (1964) 8.4, roughly the same as in 1906 San Francisco’s 8.3. However, the Chile earthquake lasted for at least 5 minutes and ruptured a 40,000 square mile fault zone, much larger than the 40-second San Francisco earthquake and ruptured 2,000 square miles.

We now know that the seismic amplitude saturates around magnitude 7. Instead, the signal increases when larger earthquakes occur. That’s logical. Larger earthquakes last longer because the rupture takes longer. In 1979, Tom Hanks and Hiro Kanamori proposed a torque meter. The magnitude of the moment depends on the size of the entire fault rupture and uses most of the seismic signals.

The magnitude of the moment is calibrated to roughly match the original Richter scale – moderate earthquakes have values ​​in the M4-5 range and major earthquakes are M7 and above. But recalculation makes a difference for large earthquakes. On the moment-size scale, 1964 Alaska weighs in at 9.2 and in 1960 Chile, a colossal 9.5; 1906 San Francisco downgraded to 7.9. Moment intensity is now the standard for nearly all medium to large earthquakes.

The duration of an earthquake is a fraction of the magnitude number as defined today. The M5 earthquake will last for 10 seconds or so and the M8 about a minute. The US Geological Survey’s recent earthquakes page includes information on the duration of some large earthquakes. Go to quake.usgs.gov and click on the latest earthquake map. For major earthquakes such as Alaska 8.2 or the recent Haiti 7.0, you will find a link in the left-hand menu of the finite fault with a map of the rupture area and, at the bottom, a graph of the duration of the rupture. 8.2 lasted nearly two minutes; The duration for Haiti was about 25 seconds.

Duration does not tell the whole story. The nature and speed of the rupture also affects the potential for damage. Many fault cracks propagate at the speed of sound, creating high-frequency vibrations that you will feel and vigorously shake structures. But some ruptures proceed more slowly and produce less high-frequency energy. The magnitude of the moment may still be large but the impact on the built environment is small and people may not feel much vibration.

There is still a lot of confusion about the size. As in Richter’s original definition, the magnitude of the day doesn’t tell you anything about the damage or the strength of the vibration. Rather, it is a measure of the potential for harm based on its location. Both M8.2 28 July in Alaska and 12 August M8.1 in the South Atlantic produced about 30 times more energy and lasted two to three times longer than 14 August M7.2 in Haiti. However, the eight attacks did no damage and the death toll in Haiti is now nearly 2,200.

Thank you Janet. More questions are welcomed – see below on how to contact me.

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