Houses are shown in flame while the Natori river floods over the surrounding area by tsunami tidal waves in Natori city, Miyagi Prefecture, northern Japan, March 11, 2011, after strong earthquakes hit the area. (AP Photo/Yasushi Kanno, The Yomiuri Shimbun) |
LOS
ANGELES (AP) — Scientists say the massive earthquake that struck off
the coast of Japan ranks as the fifth largest jolt in the world since
1900.
The
magnitude-8.9 “megathrust” quake is similar to what happened during the
2004 Sumatra quake and the one last year in Chile. In all these cases,
one tectonic plate is shoved beneath another.
Such earthquakes are responsible for the most powerful shifts in the Earth’s crust.
More
than 80 aftershocks greater than magnitude-5 have been felt since the
Japanese quake—a number that scientists say is normal for a quake this
size.
U.S.
Geological Survey seismologist Lucy Jones said a friend who was in
Tokyo for a tsunami planning meeting noted the shaking after the initial
shock lasted for about five minutes.
Hundreds killed in tsunami after 8.9 Japan quake
Tsunami waves hit Hawaii, sweep across islands
Big aftershocks are not unusual. In February, a 6.6-magnitude aftershock
ruptured near Maule, Chile—almost a year after what is now the sixth
largest earthquake in recorded history, a magnitude 8.8, hit in the same
region.
Information page at U.S. Geological Survey
Excerpt from “Science Behind Japan’s Deadly Earthquake” report by Brett Israel at LiveScience.com:
Aftershock alert
The rule of thumb for seismologists is that an earthquake’s largest
aftershock will be one magnitude smaller than the main shock, Caruso
said. That means a 7.9-magnitude earthquake could hit the region even a
year from now. Yet aftershocks are already hitting northern Japan now—35 larger than magnitude 5.0 and 14 larger than magnitude 6—according
to the UGSS.
Leighton Ah Cook, Hawaii State Civil Defense training branch chief, left, and state civil defense logistics planner Glenn Badua discuss tsunami response efforts at civil defense headquarters in Honolulu on Friday, March 11, 2011. A tsunami watch has been issued for Hawaii in the wake of a massive earthquake in that has struck off the northeastern coast of Japan. (AP Photo/Audrey McAvoy) |
The Japanese earthquake ruptured near the boundary between the Pacific and North American
tectonic plates—huge, moving slabs of the Earth’s crust. The quake
was a megathrust earthquake, where one of the plates is pushed over a
higher one, in this case the Pacific plate thrusts underneath Japan at
the Japan Trench. The seafloor was pushed away from Japan sending waves
roaring toward Hawaii and the West Coast of the U.S.
“The tsunami wave speed in deep water, open ocean, is about the same as
a commercial jet’s ground speed,” said Ken Hudnut, a USGS geologist in
Pasadena, Calif.
The epicenter of today’s quake was about 15.2 miles (24.4 kilometers)
deep, according to the USGS, which is near enough to the surface to set
off a tsunami.
“Generally we don’t get a tsunami unless we have a shallow quake, and that’s exactly what happened,” Caruso said.
SOURCES: The Associated Press and LiveScience.com
