Excerpted from “Japan earthquake is a seismic curveball
” By Joel Achenbach, “Achenblog” (blog), The Washington Post
, Posted at 10:20 AM ET, 03/11/2011, my added emphasis
he epicenter of the earthquake was about 15 miles below the sea floor and about 80 miles east of the coastal city of Sendai. Tremors are common throughout Japan, and this one was near the Japan Trench, where the Pacific plate, the speediest of the earth's major slabs of crust, dives beneath the islands of Japan in what's called a subduction zone. There was a major tremor, magnitude 7.9, just two days ago - what now looks like a foreshock.
But although this is a seismic zone, part of the so-called Ring of Fire that lines much of the Pacific, until recently it wasn't considered one of Japan's most vulnerable areas
. A 2009 paper by Japanese scientists discussed the possibility of a major earthquake in this part of Japan. Science does not instantly alter public policy, however.
The Japanese government has been prepared since the 1970s for the Tokai Earthquake, the idea of which emerged from the study of previous events along the Nankai Trough, another plate boundary that slides along the underside of Japan. This is among the most complex seismic zones in the world.
The plate boundary off the coast of Sendai had not had a "mega-quake" in the modern era. It may not have suffered a major rupture like this for more than 1,000 years. The closest analog may be a tremor recorded by monks in the year 869, according to Dave Applegate, a senior earthquake specialist at the U.S. Geological Survey.