Scientists unearth evidence of centuries-old aftershocks

Author: By Steve Connor, Science Editor

Earthquakes usually occur at the boundary of two or more tectonic plates ? the
massive chunks of the earth?s crust that grind slowly against one another.
However, they can also occur many hundreds of miles from a fault line and it
is these earthquakes that scientists believe may be the result of long
aftershocks rather than background seismic activity.

A laboratory study that tested how tectonic faults work has found that the
further away an earthquake is from such fault line, the stronger the
likelihood that it could be the long aftershock of a previous earthquake
that has taken many years to make itself felt in terms of a second series of
violent ground movements.

The finding could explain many unexpected earthquakes in the centre of
continental shelves, such as the disastrous quake in Sichuan in the heart of
China in May 2008 which killed at least 68,000 people and injured up to
400,000 more. At 7.9 on the earthquake scale it was one of the most deadly
quakes in history.

Mian Liu, professor of geological sciences at the University of Missouri in
Columbia, said that scientists have tried to predict the occurrence of
larger earthquakes by looking at the frequency of smaller ones, which is why
the Sichuan earthquake took seismologists by surprise.

?Until now, we?ve mostly tried to tell where large earthquakes will happen by
looking at where small ones do,? Professor Liu said. But in Sichuan there
had not been many earthquakes in the past few hundred years, he added.

The study, reported in the journal Nature, found that aftershocks near to
tectonic boundaries continue for only a few years but further away they can
occur over a timescale of decades and centuries. Recent earthquakes in
Canada?s Saint Lawrence valley, for instance, may be the aftershocks of an
earthquake that occurred in 1663.

Similarly, a magnitude 7 earthquake that occurred near a town called New
Madrid in Mississippi in 1811 is still causing aftershocks that can be felt
in the American mid-west because these shocks are the result of movements
that are 100 times slower than the movements that occur near to a tectonic
fault line.

?A number of us had suspected this because many of the earthquakes we see
today in the Midwest have patterns that look like aftershocks. They happen
on the faults we think caused the big earthquakes in 1811 and 1812, and
they?ve been getting smaller with time,? Professor Liu said.

?Aftershocks happen after a big earthquake because the movement on the fault
changed the forces in the earth that act on the fault itself and nearby.
Aftershocks go on until the fault recovers, which takes much longer in the
middle of a continent,? he said.

The aim of the research is that it may help to make better predictions about
when and where an earthquake is likely to happen, said Professor Seth Stein
at Northwestern University.

?Instead of just focusing on where small earthquakes happen, we need to use
methods like GPS satellites and computer modelling to look for places where
the earth is storing up energy for a large future earthquake. We don?t see
that in the Midwest today, but we want to keep looking,? Professor Stein
said.

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