Aftershock whopper not a surprise

The whopping 7.2-magnitude
aftershock that rattled Chile Thursday is nothing unusual following such a
large original earthquake, scientists say.

The aftershock may sound
surprisingly strong, given that it is bigger than the original earthquake
that decimated Haiti
in January, but it wasn’t unexpected to
scientists, said Don Blakeman, a geophysicist with the United States Geological
Survey.

The 8.8-magnitude earthquake that
struck off the coast of the Maule region of Chile on 27 February was one of the
strongest
ever recorded
. After such a strong quake, aftershocks that are
themselves substantial are par for the course.

Aftershocks are typically smaller
in magnitude than the original quake, with many small aftershocks and a handful
of larger ones. With an original quake that was more than an 8 in magnitude
“we would expect at least a couple of 7’s,” Blakeman said.

Aftershocks are earthquakes
themselves that occur because the fault that ruptured in the original temblor
is still readjusting itself. They typically occur within the same zone along a
fault that ruptured in the original quake. And “the bigger the quake, the
larger the aftershock zone,” Blakeman said, which explains why this
aftershock was not in the same spot as the epicentre of the original earthquake.

The rupture zone for the original
quake was 250 miles long, “so it’s a very big zone,” Blakeman said.
The aftershock struck about 93 miles southwest of the capital, Santiago.

Aftershocks can last for weeks to
months. They may not occur continuously, with periods of quiet in between
sequences of aftershocks.

“It kind of runs in
spells,” Blakeman said.

Not long after the 7.2-magnitude
aftershock today, a powerful 6.9 aftershock shook the area again.

So far, the original Chile
earthquake has spawned just the one aftershock over
a 7 in magnitude and about 10 in between a 6 and a 7, Blakeman said. But
“I know we’ve had at least 150 in the 5 or greater range,” he added.

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