Physics of Earthquakes
The Great Sumatra Earthquake
The massive earthquake off the west coast of Indonesia on December 26, 2004,
registered a magnitude of nine on the new "moment" scale (modified Richter scale) that
indicates the size of earthquakes. It was the fourth largest earthquake in one hundred years
and largest since the 1964 Prince William Sound, Alaska earthquake.
The devastating mega thrust earthquake occurred as a result of the India and Burma
plates coming together. It was caused by the release of stresses that developed as the India
plate slid beneath the overriding Burma plate. The fault dislocation, or earthquake, consisted
of a downward sliding of one plate relative to the overlying plate. The net effect was a
slightly more compact Earth. The India plate began its descent into the mantle at the Sunda
trench that lies west of the earthquake's epicentre.
The Indonesian earthquake calculated it affected Earth's rotation, decreased the length
of day, slightly changed the planet's shape, and shifted the North Pole by centimetres. The
earthquake that created the huge tsunami also changed the Earth's rotation.
All earthquakes have some effect on Earth's rotation. When one tectonic plate beneath
the Ocean was forced below the edge of another, "it had the effect of making the earth more
compact and spinning faster" - like an ice-skater spinning faster by tucking in the arms. It's
just they are usually barely noticeable. "Any worldly event that involves the movement of
mass affects the Earth's rotation, from seasonal weather down to driving a car". Anything that
redistributes Earth's mass will change Earth's rotation. "Earth's rotation changes all the time
as a result of not only earthquakes, but also the much larger effects of changes in atmospheric
winds and oceanic currents," he said. "Over the course of a year, the length of the day
increases and decreases by about a millisecond, or about 550 times larger than the change
caused by the Japanese earthquake. The position of Earth's figure axis also changes all the
time, by about 1 meter (3.3 feet) over the course of a year, or about six times more than the
change that should have been caused by the Japan quake."
Study reveals changes in polar motion that is shifting the North Pole. The "mean
North pole" was shifted by about 2.5 centimetres (1 inch) in the direction of 145º East
Longitude. The earthquake decreased the length of day by 6.8 microseconds and shifted
Earth's axis by 2.32 milliarcseconds (about 7 centimetres, or 2.76 inches). Physically this is
like a spinning skater drawing arms closer to the body resulting in a faster spin. The quake
also affected the Earth's shape. It is found, Earth's oblateness (flattening on the top and
bulging at the equator) decreased by a small amount. It decreased about one part in 10 billion,
continuing the trend of earthquakes making Earth less oblate.
To make a comparison about the mass that was shifted as a result of the earthquake,
and how it affected the Earth, compare it to the great Three-Gorge reservoir of China. If filled
the gorge would hold 40 cubic kilometers (10 trillion gallons) of water. That shift of mass
would increase the length of day by only 0.06 microseconds and make the Earth only very
slightly more round in the middle and flat on the top. It would shift the pole position by about
two centimetres (0.8 inches).
The researchers concluded the Sumatra earthquake caused a length of day (LOD)
change too small to detect, but it can be calculated. It also caused an oblateness change barely
detectable, and a pole shift large enough to be possibly identified. They hope to detect the
Dr. N. Venkatanathan
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Physics of Earthquakes
LOD signal and pole shift when Earth rotation data from ground based and space-borne
position sensors are reviewed.
Chilean Quake May Have Shortened Earth Days
The Feb. 27 magnitude 8.8, earthquake in Chile may have shortened the length of
each Earth day. Using a complex model, he and fellow scientists came up with a preliminary
calculation that the quake should have shortened the length of an Earth day by about 1.26
microseconds.
Perhaps more impressive is how much the quake shifted Earth's axis. The quake
should have moved Earth's figure axis (the axis about which Earth's mass is balanced) by 2.7
milliarcseconds (about 8 centimetres, or 3 inches). Earth’s figure axis is not the same as its
north-south axis; they are offset by about 10 meters (about 33 feet).
Even though the Chilean earthquake is much smaller than the Sumatran quake, it is
predicted to have changed the position of the figure axis by a bit more for two reasons.
1. Unlike the 2004 Sumatran earthquake, which was located near the equator, the
2010 Chilean earthquake was located in Earth's mid-latitudes, which makes it
more effective in shifting Earth's figure axis.
2. The fault responsible for the 2010 Chilean earthquake dips into Earth at a slightly
steeper angle than does the fault responsible for the 2004 Sumatran earthquake.
This makes the Chile fault more effective in moving Earth's mass vertically and
hence more effective in shifting Earth's figure axis.
Japan Earthquake 2011 also, shortened the days
The preliminary theoretical calculation of how the Japan earthquake—the fifth largest
since 1900—affected Earth's rotation. Calculations indicate that by changing the distribution
of Earth's mass, the Japanese earthquake should have caused Earth to rotate a bit faster,
shortening the length of the day by about 1.8 microseconds.
The calculations also show the Japan quake should have shifted the position of Earth's
figure axis (the axis about which Earth's mass is balanced) by about 17 centimetres (6.5
inches), towards 133 degrees east longitude. Earth's figure axis should not be confused with
its north-south axis; they are offset by about 10 meters (about 33 feet). This shift in Earth's
figure axis will cause Earth to wobble a bit differently as it rotates, but it will not cause a shift
of Earth's axis in space—only external forces such as the gravitational attraction of the sun,
moon and planets can do that. Both calculations will likely change as data on the quake are
further refined.
In comparison, following last year's magnitude 8.8 earthquake in Chile, Gross
estimated the Chile quake should have shortened the length of day by about 1.26
microseconds and shifted Earth's figure axis by about 8 centimetres (3 inches). A similar
calculation performed after the 2004 magnitude 9.1 Sumatran earthquake revealed it should
have shortened the length of day by 6.8 microseconds and shifted Earth's figure axis by about
7 centimetres, or 2.76 inches. How an individual earthquake affects Earth's rotation depends
on its size (magnitude), location and the details of how the fault slipped.
Dr. N. Venkatanathan
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