Each year, the world experiences about 1,700 earthquakes large enough to cause damage (magnitude 5.0 or greater). And thanks to global seismic networks and instant-news services, it's possible to hear about them all. This means that most people are at least passingly familiar with the earthquake-magnitude scale. But, while we know it's logarithmic, we don't easily realize how much that compresses the scale: only a few points spell the difference between the gurgling of lava beneath a volcano and a major catastrophe.
Nowhere is this more obvious than with the approximately one-point range that differentiates really big earthquakes from truly colossal ones. Alaska's 2002 "Denali" quake, for example, was a 7.9 whose effects were felt as far away as California. But the world sees temblors of that magnitude about once every two years.
Go just one more point up the scale, though, and quakes get rarer (thank goodness). So rare, in fact, that in the interval from 1960 to 2004, there had been only two. One (estimated at magnitude 9.5) was offshore from Chile in 1960. The other (officially estimated at 9.2, but now thought to have been bigger) was Alaska's 1964 "Good Friday" quake. Other than those two, there had been only one temblor bigger than 8.5.[2]
[2. Even the great San Francisco quake of 1906 probably wasn't much bigger than about 8.0. See the U.S. Geological Survey's website, quake.wr.usgs.gov/info/1906/index.html.]
Then in 2004, on the day after Christmas (a holiday known to the British as Boxing Day), the Indonesian island of Sumatra was hit by a quake nearly as large as Alaska's Good Friday mega-quake.
Everyone knows the story. The offshore quake set up a tsunami that swept the Indian Ocean, killing 300,000 people and wreaking unimaginable damage. Lesser known is the fact that three months later, on March 28, 2005, the second-largest earthquake in 40 years (magnitude 8.6) hit another portion of the same fault zone. It failed to make major headlines only because this one did not create a substantial tsunami.
The human story of the Boxing Day earthquake is well known. But the scientific story is just beginning to be revealed because the great quake of December 26 and its March 28 successor were the first mega-quakes to be studied with the tools of modern geophysics.
)
