Earthquakes occurring below oceans have been the subject of intense speculation of seismologists across the globe. Theories of the past have failed to pinpoint the exact cause of why or how earthquakes ensue. A new theory now tries to demystify the incidences of tectonic imbalances occurring in certain areas while bypassing the rest
THE oceanic crust of the earth is in perpetual motion. The numerous plates
forming the crust are either moving
towards each other or away from each
other. Where they diverge, the gap is
filled up by freshly formed igneous
rocks and where they converge, one
plate moves under the other till it
merges with the underlying mantle. At
places, this process, known as subduction, is accompanied by a major earthquake; in other areas, but for the knowledge of seismologists, the event under
the sea passes off quietly.
Why this happens has been
explained by Chris Scholz of Colombia
University's Lamont-Doherty Earth
Observatory and Jaime Campos of the
University of Chile in Santiago.
Working on a proposal forwarded in the
late'70s by seismologists Seiya Uyeda of
Tokai University in Japan, Hiroo
Kanamori of Caltech and others, Scholz
and Campos arrived at the conclusion
that the direction in which the resistant
force - known as sea anchor - acts,
determines whether subduction will
Potent power."the sea anchor force can turn
earthquakes on or off
be accompanied by an earthquake or
not (Science, Vol 270, No 5239).
When the subduction zone
cruises over the underlying mantle,
the descending slab sweeps into the
viscous mantle rock generating a
resistant force. If this force acts in
the direction opposite to the movement
of the descending slab, it bends it
inwards and subduction takes
place without an accompanying
earthquake. It also opens up a nascent
ocean in the back arc basin formed due
to the pulling apart of the overriding
plate. But when this force acts in the
direction of the plate movement, it
pushes the lower slab upwards against
the upper plate, setting the stage for a
massive earthquake.
The 7.8 Richter scale earthquake
which struck the subduction zone
near Guam in 1993, forced seismologists
to take a second look at the traditional
theory. This subduction zone is the
same as the one which spreads north
towards the Pacific Mariana Islands
onto Japan and is generally
peaceful. As the subducting plate
is of the same age and moves with
near even velocity, the occurrence of the earthquake only at
Guam had put the seismologists
in a quandary.
Uyeda and others had
suggested that the descending
plate is laterally fixed in the
mantle due to the viscosity of
the surrounding rocks. If the
upper plate moves towards the
lower plate, subduction occurs,
producing earthquakes. On the
Me other hand, if the upper plate is
moving away from the descending plate, then subduction occurs
peacefully.
Elaborating on the picture, Scholz and Campos
have framed a model of the
subduction zone forces by assuming that the descending slab is not
fixed, but sweeps below the upper slab
into the mantle. This model explains the
anomalous behaviour of the subduction
plate near Guam. Scholz and Campos
applied their model to 30 subduction
zones around the globe to check if their
predictions on the occurrence of earthquakes and spreading of back arc basins
hold good. They achieved 80 per cent
positive results which is the highest ever
in geophysical modelling and keeps
them optimistic with regard to the
acceptance of their theory.
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