Big quake country
Posted: Wednesday, November 14, 2007 12:34 PM by Alan Boyle
USGS
This chart shows the 118 earthquakes reported worldwide over the past week, with Wednesday's Chile earthquake highlighted in red. Many of the quakes occurred along the Pacific Ring of Fire. Click on the image for a larger chart.
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In some parts of the world, a magnitude-7.7 earthquake like the one experienced today in Chile would be a shatteringly rare occurrence - but not for the home of the most powerful quake ever recorded. "This is a normal earthquake for this part of the world," observed John Bellini, a geophysicist at the U.S. Geological Survey's National Earthquake Information Center.
In fact, the No. 1 quake on USGS' list of most powerful shakers is a magnitude-9.5 monster that hit southern Chile in 1960. About 1,655 were killed in the region, and a resulting tsunami killed scores more when the waves swept over Hawaii and the Philippines.
Because quakes are rated on a logarithmic scale, that 1960 quake was almost 100 times as strong as today's event - and there's no sign so far that the loss of life will come close to that earlier toll. Nevertheless, today's quake demonstrates how location is everything when it comes to powerful quakes.
Chile has a front-row seat on the Pacific Ring of Fire, a seismic zone that encircles the Pacific Ocean and is responsible for the spate of strong quakes experienced in Indonesia. Bellini said Chile is in a particularly active seismic zone because that's where the Nazca crustal plate is diving beneath the South American contintental plate. This chart shows how the plates play out, and this backgrounder from USGS explains the process that resulted in today's quake.
The clash of plates is what made the region what it is today: The upthrust of the South American plate created the Andes, and the high plateau on the west side of the mountains rates as one of the driest places on Earth. In fact, Chile's Atacama Desert is so dry that it serves as a good scientific stand-in for Mars.
The depth of today's epicenter - nearly 37 miles, or 60 kilometers - means that the quake should be widely felt but not as catastrophic as a shallower quake. As another example, an even stronger and deeper quake in a nearby region of Bolivia in 1994 (magnitude 8.3; depth 397 miles, or 636 kilometers) was felt as far away as Minnesota but caused no major damage.
So how big a deal is a 7.7 quake? Bellini notes that there are about 18 seismic events that go higher than magnitude 7 every year, and the USGS' comprehensive list of historic worldwide earthquakes points up plenty of big quakes that barely made a dent in history.
If anything, today's event demonstrates that you can't judge a quake completely on the basis of one number.
For more information about the science of earthquakes, check out our interactive graphic. If you ever experience a rumble yourself, feel free to let the USGS know. And if it's a biggie, let us know as well.
