11 May 2018–How big is an earthquake? Seismologists have been measuring this feature—the magnitude of an earthquake—for more than a century, but the answers are not always as straightforward as one would think, says Allison Bent, a research seismologist with Natural Resources Canada.
Numbers associated with the Richter scale and the moment magnitude scale (Mw) might be most familiar to the layperson, but seismologists have used different magnitude scales over the years that take into account the distance from an earthquake, the local geology and several other factors. A major part of Bent’s work is to reconcile these scales to make them more useful in seismic hazard assessments.
To understand the problems she grapples with, it may be helpful to start with the basics of what magnitude means to a seismologist. A magnitude scale is an equation, calculated using the amplitude of seismic waves on a seismogram, corrected for the amplification of the instrument and the instrument’s distance away from the earthquake, and sometimes the frequency of the measured wave and the depth of the earthquake. The scales are logarithmic, Bent explains. “That means that a difference of one unit in magnitude corresponds to a difference of a factor of 10 in terms of the amplitude of the seismic waves recorded at the same distance. So a magnitude 6.0 is 10 times greater than a magnitude 5.0 or a magnitude 2.4 is 10 times greater than a magnitude 1.4 earthquake, and so on.”
Magnitude scales don’t directly measure the energy released by an earthquake, she adds, “but a difference of one unit in magnitude corresponds roughly to a factor of 30 in terms of energy.”
Different magnitude scales have been adopted depending on earthquake distance, among other factors, but the moment magnitude scale is “now generally considered the best measure of earthquake size,” Bent says, since it can be related to the physical properties of an earthquake rupture and does not underestimate the size of the very largest (Mw 8.0 and greater) earthquakes.
“When assessing seismic hazard for an entire country—or even a smaller area—it is important that magnitudes are uniform in space and time,” Bent says. “That is to say that magnitude 6.0 should mean the same whether the earthquake occurred in 1948 or 2018 and whether it was in Montreal or Vancouver. To ensure this, it is necessary to develop reliable equations to convert from the various magnitude scales to Mw.”
Bent is now developing those equations, conducting research “that started out quite innocently” to answer questions posed by her colleagues working on seismic hazard. “It seems that each time I resolve one issue, another complexity arises. Magnitude is very simple in concept but more complex in reality.”
Since most of her work involves waveform analysis, Bent is usually behind a desk rather than in the field. But she has had some interesting seismology travel experiences, including one trip to El Salvador in 2001 to help a team of engineers and a landslide expert with their observations following a large earthquake. “We spent about a week driving and flying by helicopter all over the country. Not everything we saw was pleasant but I would do it again in an instant,” she recalls. “I learned so much and there is no substitute for seeing up close and personal the effects the waves I study have on peoples’ lives.”
A past member of the SSA Board of Directors and a current Associate Editor of the Bulletin of the Seismological Society of America, Bent has been working at Natural Resources Canada since receiving her Ph.D. from California Institute of Technology in 1990, following a lifelong interest in earthquakes and volcanoes. “I’m not really sure what sparked my interest originally but I decided at a very young age what I wanted to do when I grew up and never really changed my mind,” Bent says. “I love math and physics but I also like dealing with real-world problems and seismology offers me the opportunity to combine the two.”
Finding time to work on each of her many projects, rather than technical issues, is Bent’s biggest on-the-job challenge. She has a long list of “dream projects,” she says, “but one thing that I think would be really fun to do is pick a random topic in seismology—maybe put four or five in a hat and draw one—where I’ve had little or no experience and spend a year working on just that to see what I could accomplish. “
“Seismology covers such a wide range of topics that most of us are experts in a few areas but have only a general knowledge of the others. So my random topic idea might have to wait until I retire,” she says.