The devastating earthquake that struck Japan earlier this year was
powerful enough to slightly alter the pull of gravity under the
affected area, scientists have found.
Anything that has mass has a gravity field that attracts objects
toward it. The strength of this field depends on a body's mass.
Since the Earth's mass is not spread out evenly, its gravity field
isstronger
in some places and weaker in others.
The magnitude
9.0 Tohoku-Oki temblor in
March was the most powerful earthquake to hit Japan and
the fifth-most
powerful quake ever recorded. To see how the temblor
might have deformed the Earth there, scientists used the Gravity
Recovery and Climate Experiment (GRACE) satellites to analyze the
area's gravity field before and after the quake.
The researchers found the Tohoku-Oki quake reduced the gravity
field there by an average of two- millionths of a gal by slightly
thinning the Earth's crust. In comparison, the strength of the
gravitational pull at the Earth's surface is, on average, 980 gals.
(The gal, short for Galileo, is a unit of acceleration; one gal is
defined as one centimeter per second squared.)
"The most important implication of our findings is that the massive
Tohoku-Oki earthquake brings significant changes to not only the
ground but also the underground structure of Japan," researcher
Koji Matsuo, a geophysicist at Hokkaido University in Japan, told
OurAmazingPlanet.
The GRACE satellites had previously detected gravity changes caused
by the magnitude 9.1 to 9.3 2004 Sumatra-Andaman quake, the
third-most powerful earthquake ever recorded, and the magnitude 8.8
earthquake that hit Chile in 2010, the eighth-most powerful on
record. These reduced the gravity fields in the areas struck in
much the same way as the Tohoku-Oki quake, since they were all
similar types of earthquakes.
The researchers are now interested in seeing if they can detect
post-quake gravity field changes as the crust settles back into
place.
Matsuo and colleague Kosuke Heki detailed their findings online
Sept. 22 in the journal Geophysical Research Letters.