THE constant rumbling of earthquakes round the world is nudging the North
Pole towards Japan. Giorgio Spada of the University of Bologna, Italy, has now
shown that this is because the largest quakes, most of which occur along the
Pacific rim, tend to tilt the pole towards their epicentres.
Spada was following up a study last year by Benjamin Chao of NASA鈥檚 Goddard
Space Flight Center in Greenbelt, Maryland. Chao and his colleagues looked at
the records of earthquakes since 1977 to see how they have affected the Earth鈥檚
rotation. Quakes shift the Earth鈥檚 crust and the mantle underneath,
redistributing mass. This distortion of the Earth鈥檚 shape can change its axis of
rotation and speed, in the same way that spinning ice-skaters turn faster or
slower by moving their arms in or out.
Chao鈥檚 team showed that earthquakes since 1977 have slightly shifted the
North Pole towards Japan, at a rate of about 6 centimetres each century. The
same trend emerges for all quakes over the past 100 years. Chao could not
explain why the direction is always the same, but he suggested that there is
perhaps some mysterious link between the Earth鈥檚 rotation and the triggers for
quakes.
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鈥淚 was tremendously curious about this, probably because Chao invoked some
`behind-the-scene鈥� process,鈥� says Spada. But by looking at the contributions of
individual earthquakes, Spada was able to explain the effect simply in terms of
their brute force. The biggest quakes tend to be of the 鈥渄ip-slip鈥� type, in
which one plate of the Earth鈥檚 crust is sliding under another. During this kind
of earthquake, the crust and mantle bob up and down.
The redistribution of mass in these quakes depends on many factors, including
the angle of the dipping crust. But Spada calculated that, statistically, the
mass rearrangement is most likely to tug the North Pole along the line of
longitude containing the epicentre. And because most big earthquakes occur round
the Pacific rim, he says, this fully accounts for the slight polar motion
towards Japan (Geophysical Research Letters, vol 24, p 539).
No one has measured the polar motion due to quakes directly because it is
swamped by bigger effects. For instance, the mass distribution on the Earth
changes as glaciers melt over thousands of years. The resulting polar motion,
measured by tiny changes in the latitudes of stars, is about 10 centimetres per
year away from Japan. However, Spada says, over timescales of 10 to 100 million
years, the motion of the crust and mantle probably has the greatest
influence.
鈥淚鈥檓 convinced that Spada is correct,鈥� says Chao. He hopes to confirm this by
watching the effect of a single, huge earthquake. A quake of magnitude 8.5 on
the Richter scale should make the pole lurch by a few millimetres鈥攁 shift
that state-of-the-art astronomical detectors could pick up. 鈥淲e鈥檙e due for a big
earthquake like that,鈥� he says, 鈥渉opefully in the middle of an ocean.鈥�