In the News
“ differences in social structures, not necessarily cognitive advances, allowed our species to cross the barrier to cumulative
cultural evolution. ” — Joseph henrich, page 13
size of March 11
earthquake took
Japan by surprise
geologists expected a 7.5 in
devastated region, not a 9.0
story one
By Devin powell
Japan has been expecting and pre- paring for the “big one” for more than 30 years. But the magnitude 9.0 temblor that struck March
11 — the world’s fourth biggest quake
since 1900 — wasn’t the catastrophe the
island nation had in mind. The quake’s
epicenter was about 129 kilometers east
of the city of Sendai, in a strip of ocean
crust previously thought unlikely to be
capable of unleashing such energy.
Predicting where big
earthquakes will strike is
extraordinarily difficult,
and nowhere more so
than Japan. The north-
ern part of the island
nation sits at the inter-
section of four moving
pieces of the Earth’s
crust. Where one tec-
tonic plate slides beneath
another, forming a
subduction zone, sudden slippages
can unleash tremendous amounts
of energy.
The Sendai quake occurred at the
Japan Trench, the junction of the
westward-moving Pacific plate and the
plate beneath northern Japan. Histori-
cal records, one of seismologists’ best
tools for identifying areas at risk, suggest
that this segmented fault
produced several earth-
quakes bigger than 7.0 in
the 20th century — but
none bigger than 8.0.
That’s why the Japa-
nese government has long
focused on the nation’s
southern coast, where
the northward-moving
Philippine plate has a
proven ability to spawn
big quakes. Quakes larger
than 8.0 tend to strike the
a map of the northern
half of Japan’s main island
shows the highest shak-
ing intensity in orange.
sendai
“This area has a long history of earthquakes, but [the Sendai quake] doesn’t
fit the pattern,” says Harold Tobin, a
marine geophysicist at the University
of Wisconsin–Madison. “The expectation was high for a 7.5, but that’s a
hundred times smaller
than a 9.0.”
The magnitude 9.0 earthquake off northeastern Japan on March 11 generated a
tsunami more than 7 meters high that reached the coast within 15 minutes.
Tokai region in central Japan every 150
years or so, with the last big one in 1854.
In 1976 researcher Katsuhiko Ishibashi of
Kobe University warned that the Suruga
trough, a subduction zone just off the
coast of Tokai, was due for another big
one. Since then, the Japanese government and research community have
braced for such an event — deploying GPS
systems to monitor the movements of
islands on the Philippine plate and even
generating computer simulations of how
crowds in train stations might react.
Current thinking about the mechanisms governing megaquakes also favors
the Philippine plate as the site of greatest risk. About 80 percent of all quakes
above magnitude 8.5 occur at the edges of
such geologically young tectonic plates.
Kilometer-thick sediment layers carried by these plates are thought to grind
smooth patches that allow long stretches
of fault to rupture at once. The Pacific
www.sciencenews.org
april 9, 2011 | science news | 5
