A U.S. seismologist tasked with estimating the magnitude of Monday’s earthquake in southwestern China says that number is not nearly as important as population figures. In this case, the quake’s location in the heavily populated Sichuan province, not far from the capital Chengdu, is likely to have a great influence on the death toll.
“Whether it’s a magnitude of 7.5, 7.6, 7.7 is not nearly as important as the fact that it’s near a population,” David Wald, a seismologist with the U.S. Geological Survey’s National Earthquake Information Center in Golden, Colo., told me. “We tend to not worry as much about small details in magnitude difference as in trying to convey whether there’s a population in the area that will shake the hardest. I always get a little bit miffed about the focus on magnitude.”
The magnitude does, however, convey vital information about a quake. It indicates the size of the fault, or the break in the Earth where the sudden movement occurred to produce the quake. And it indicates the amount of movement, called slip. A 7.8 quake involves a fault of roughly 150 to 200 kilometers, Dr. Wald said, with a slip of several meters. “That’s a huge area of rock that’s moving.” (These magnitude estimates are more properly called moment magnitude or energy magnitude; they’re comparable to estimates of older quakes that were based on the Richter scale, which has fallen out of favor among seismologists.)
To make its estimates, USGS relies on its stations and those from other countries that participate in the Global Seismographic Network. USGS maintains stations around the world, including several near Monday’s quake. But for big quakes, the closest stations tend to go “off-scale,” according to Dr. Wald, because the amount of shaking there is greatest. More-distant stations give more reliable numbers as waves from the quake spread across the Earth, within a matter of hours. (USGS does equip stations near U.S. faults, such as in California, so that they can reliably measure nearby quakes.)
As data come in from these stations, USGS estimates the magnitude and then estimates it again as new data arrive. “It’s not as simple as just going to some instrument and measuring an amplitude,” Dr. Wald said. Monday morning, USGS revised its figure to 7.8, in agreement with the figure reported by China’s State Seismological Bureau. As I spoke to Dr. Wald, he and his colleagues were examining the data again. “It will probably go to magnitude 7.9,” he said. (It did shortly after our phone call.)
Small changes in magnitude numbers have a big impact, because they’re on a logarithmic scale. An increase by one on the scale means the fault size increased by 10 times. So an increase from 7.5 to 7.8 suggests that the fault was twice as large as initially thought, and an increase from 7.8 to 7.9 increases the estimate by another 26%. But Dr. Wald argues that geography and population figures are more important because a quake of magnitude 6.8 with epicenter at a population center could create just as much damage. That said, the slip also increases as magnitude increases, though not at the same rate as fault size. An increase of magnitude by one is equivalent to multiplying the slip by about three, according to Dr. Wald; that means a 7.8 quake has a slip about 39% greater than a 7.5 quake. And the amount of shaking experienced near the epicenter correlates roughly with the slip.
“You could have a 6.8 magnitude earthquake in the wrong place and suffer extreme damage,” Dr. Wald said. “When you have a larger earthquake, though, you increase the odds [of significant damage].”