Geoscience Reference
In-Depth Information
it chose the Atlantic the other side of the Appalachian Mountains. Perhaps it is having an-
other try. Whatever its cause, another New Madrid earthquake today could cause untold
destruction.
The mystery of deep earthquakes
By plotting the depths of earthquakes, it is possible to trace the descent of ocean lithosphere
in a subduction zone such as the one where the Pacific plate descends beneath the Andes of
South America. For the first 200 kilometres or so, the rock is cold and brittle so fractures
and generates earthquakes as it would nearer the surface. But some earthquakes seem to
have their foci far deeper, up to 600 kilometres down, where heat and pressure should make
the rock soft and ductile so that it deforms rather than fractures. A possible explanation is
that these deep earthquakes might be due to a whole layer of crystals undergoing a phase
change from the olivine structure found in ocean lithosphere to the denser spinel structure
of the mantle. One argument against this theory is that this process can only happen once,
yet several earthquakes have been recorded from about the same place. But maybe it is just
successive layers of olivine transforming.
Awaiting the inevitable
In January 2001, northwestern India was rocked by a devastating quake centred on the town
of Bhuj in Gujerat. This was part of the continuing legacy of the intercontinental collision
of India with Asia. The relative motion between India and Tibet still adds up to about 2
metres per century. Though there were a number of severe Himalayan quakes during the
20th century, there are many areas that must have accumulated far more strain. A slip of 2
metres has the potential to produce a magnitude 7.8 earthquake. But there are some parts
of the thrust where India is pushing under the Himalayas that have accumulated a strain
equivalent to a slip of double that. In fact, some areas have not experienced a severe quake
for more than 500 years. Such a 'great quake' could be devastating indeed. Although build-
ing standards have improved in the past century, the evidence from Bhuj suggests that a
similar magnitude earthquake now is likely to kill a similar proportion of any population as
it could have done 100 years ago. But in the meantime, populations at risk have increased
by factors of 10 or more. If the 1905 Kangra quake were repeated today, 200,000 fatalities
are quite likely. Should one of the major cities in the Ganges plain be hit, the figure might
be worse by yet another order of magnitude. Another highly populous earthquake zone,
Tokyo, has not had a major quake since 1923. If a great quake were to strike there today,
even with Japan's improved building standards, it could cause an estimated $US 7 trillion
of damage, which might lead to the collapse of the global economy.
