Lori Dengler | Measuring the size of an earthquake isn’t that easy – Times-Standard
Janet asked after last week’s column: “People report the intensity and depth of Richter’s earthquakes, but not the duration. I lived in San Mateo during the 1989 earthquake. It seemed to go on for ages, and if it had lasted longer, the damage would have been even worse. So why not report how long the earthquake lasts? “
Thanks Janet. First, what is Richter’s magnitude? Scientific agencies have not reported Richter’s magnitude for decades. However, some media still abuse the term. Yes, we still use the concept of magnitude and Richter’s range of numbers roughly as he defined them. But how it is measured and what it means is very different today from the original definition of 1935.
Charles Richter is mainly remembered for the first magnitude scale of earthquakes, 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 instrumental seismicity in 1928. These instruments were the standard for area studies, and we had two operating at Humboldt. from 1948 to 1992.
Before Richter, earthquakes were measured by intensity, qualitative scales based on damage and human perception of the strength of the tremors. People of that time were probably familiar with the Rossi-Forel scale used to map the strength of tremors during the 1906 San Francisco earthquake or the Mercalli scale which was modified and adapted as a standard by the USGS to 20th century.
With its new seismic network, Caltech wanted to publish earthquake catalogs and needed something more quantitative than intensity. Richter was influenced by Kiyoo Wadati in Japan who suggested ground displacement as a measure of the size of the earthquake. Richter measured the largest seismic signal recorded on the Wood-Anderson instruments and, 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 parameter, albeit in reverse. In astronomy, the brighter a star appears, the lower its magnitude. Richter calibrated the scale so that earthquakes fell between zero and 10. After its publication in 1935, the idea was accepted by other seismologists and quickly became the world standard. When I entered graduate school in 1969, we mainly used two magnitudes, the Richter magnitude for regional earthquakes and the surface wave magnitude (same idea but using different seismographs) for distant earthquakes.
Janet’s question implies that the length of the shakes has nothing to do with determining the magnitude. When it comes to Richter and surface wave magnitudes, she’s right. You measure the greatest signal. It is assumed that the signal will become larger and larger as the size of the source increases. The great earthquakes of the 1960s showed the limits of this methodology. The surface wave magnitude for Chile (1960) was 8.5 and that for Alaska (1964) 8.4, almost the same as for 1906 in San Francisco 8.3. Still, the earthquake in Chile lasted at least 5 minutes and shattered a 40,000 square mile fault zone, far larger than San Francisco’s 40-second quake and 2,000 square mile rupture.
We now know that the seismic amplitude saturates around magnitude 7. Instead, the signal lengthens for larger earthquakes. It makes physical sense. Larger earthquakes last longer because it takes longer to rupture. In 1979, Tom Hanks and Hiroo Kanamori proposed the magnitude scale of the moment. The magnitude of the moment is based on the size of the entire fault fracture and uses most of the seismic signal.
The magnitude of the moment is calibrated to roughly match the original Richter scale – moderate earthquakes have values in the range M4-5 and major earthquakes are M7 and above. But the recalculation makes a difference for large earthquakes. On the current magnitude scale, the 1964 Alaska weighs 9.2 and the 1960 Chile a whopping 9.5; 1906 San Francisco is demoted to 7.9. Moment magnitude is now the norm for almost all moderate to large earthquakes.
The duration of the earthquake is part of the magnitude number as defined today. An M5 earthquake will only last about 10 seconds and an M8 earthquake about one minute. The last USGS earthquake page includes information on the duration of some larger earthquakes. Go to quake.usgs.gov and click on the latest earthquake map. For major earthquakes like the recent Alaska 8.2 or Haiti 7.0, you will find in the left menu a link to Finite Fault with a map of the fault rupture zone and, at the bottom, a graph of the duration of the rupture. 8.2 lasted almost two minutes; the duration for Haiti was about 25 seconds.
Duration does not tell the whole story. The nature of the rupture and the speed with which it occurs also affects the potential for damage. Many fault breaks propagate at the speed of sound, creating high frequency vibrations that you will feel and strongly shake structures. But some ruptures take place more slowly and produce little high-frequency energy. The magnitude of the moment may still be large, but the impact on the built environment may be small, and people may not experience a lot of tremors.
There is still a lot of confusion over greatness. As in Richter’s original definition, today’s magnitude tells you nothing about the damage or strength of the shaking. Rather, it is a measure of the potential for damage based on its location. The July 28 M8.2 in Alaska and the August 12 M8.1 in the South Atlantic each produced about 30 times the energy and lasted two to three times longer than the August 14 M7.2 in Haiti. However, the 8 caused no damage and the death toll in Haiti now stands at nearly 2,200.
Thanks Janet. More questions welcome – see how to contact me below.