Today we keep track of earthquakes with smart machines—digital black boxes known as seismographs. These "quake-writers" create a complete record of a seismic event, including the time it starts, its strength, and its every jolt and jiggle. Anything less is a mere seismoscope—a dumb quake detector. But we can learn from even the dumbest ones.
The ancient Chinese made such devices, like the dragon jar that you see in every geology textbook. A swinging weight inside a brass jar opened the dragon jaws around its rim when disturbed, dropping balls into the mouths of brass frogs to indicate the earthquake's direction. But the technology never advanced. However, the wonderful Newtonian physics that the Chinese could never invent allows us today to extract seismic data from some unexpected objects.
The Grocery Earthquake Scale
When the Imperial Valley earthquake of 15 October 1979 (M=6.4) struck southernmost California, lots of scientists showed up from the U.S. Geological Survey. The flat, open farmland displayed many kinds of ground disturbance to advantage. But Robert Nason, a veteran USGS researcher, went indoors to chain grocery stores, examining the mess that the quake left behind and treating it like a strong-motion seismogram. Weak earthquake shaking tosses a few cans into the aisles, while stronger shaking throws everything off the shelves, then knocks down the shelves themselves. Nason showed that your average 7–Eleven can be a pretty dependable gauge of seismic intensity. He turned his research into "Food store disturbance as a seismic intensity indicator" in the Bulletin of the Seismological Society of America.
Ten years later, during the World Series earthquake of 17 October 1989, grocery stores in the San Francisco Bay area made seismological news again. Many convenience stores had their surveillance cameras running that afternoon, and the videotapes caught the quake in action. And in Japan after the 15 January 1995 Kobe quake, researchers used store videos to figure out local intensities using the motion of shopping carts.
The Standing Stone Scale
The same thinking goes into the work of James Brune, who looks for the geologic version of soup cans—balanced rocks. The desert of the American West creates these by physical weathering, for example in Arches National Park in Utah or Joshua Tree National Park in California. Brune and his co-workers have been publishing papers for several decades about their work in establishing ages for balanced rocks.
In a 1998 article in Geology, for instance, Brune's group showed that many have been standing for ten thousand years, even in one place just 35 kilometers from the San Andreas fault. They used rock varnish and chlorine-36 dating to estimate the ages. If these stacks of big boulders haven't been knocked down for such a long time, he says, then maybe that part of the fault is less dangerous than the rest. In the same paper he showed that the Yucca Mountain site in Nevada, where we plan to bury our nuclear waste for the next few millennia, has been undisturbed for twice as long.
Since then, Brune has presented more work on what balanced rocks mean for the behavior of thrust faults. There are areas, such as between the Elsinore and San Jacinto faults of Southern California, where our planning models call for severe shaking, but where ancient balanced rocks are still on their toes.
Brune holds that the motions—rather, the lack of motions—recorded by these crude seismometers should make us revise our models. We may be able to save money with a more realistic standard, and we may have a better idea of how other faults, like the New Madrid zone of the American Midwest, behave over the long term.
PS: The Omori intensity scale used in Japan is partly based on similar common objects: stone lanterns, Buddhist temples, and tombstones.