Breaking up is hard to do. Even for
Earth’s tectonic plates, separation is studded with sudden releases of pent up
stress, such as the twin tremors that rocked the Indian Ocean on April 11,
2012.
The magnitude 8.7 and 8.2 earthquakes that struck off the coast of Sumatra that day herald the breakup of the Indo-Australian plate along an unclear boundary beneath the Indian Ocean southeast of India, according to two studies published online today in Nature.
In one of the reports, seismologists from the University of Utah and University of California, Santa Cruz, say the main shock—the combined outcries of four separate faults rupturing in a span of 160 seconds—measured 8.7 in magnitude. That’s about 40 times larger than the previous estimate of 8.6. The 8.2-magnitude quake followed
along a fifth fault two hours later.
Two of the largest strike-slip earthquakes ever recorded, these earthquakes struck where the Indo-Australian plate is being torn asunder as it marches to the northeast. The trouble happens because the west of breakup region is not keeping pace with the segment to the east. The western portion of the plate is slowed by its ongoing
collision with Asia, whle eastern part of the plate moves relatively unimpeded
as it dives, or subducts, under the island of Sumatra.
Sudden vertical motion along this
subduction zone off Sumatra caused the catastrophic magnitude-9.1
Sumatra-Andaman earthquake of Dec. 26, 2004—a jolt that generated massive
tsunamis that killed most of the 228,000 victims in the Indian Ocean region.
By changing stress patterns in the
earth’s crust, that 2004 catastrophe probably helped to trigger the 2012
quakes, which were much less upsetting from humanity’s point of view: only ten
people are known to have died as a result.
The difference? Among other
reasons, the horizontal movements along the strike-slip faults out in the
middle of the Indo-Australian plate simply cannot move as much water as the vertical
motion along its subducting edges can. (The first April 11, 2012, quake did
cause small tsunamis, but none more than 12 inches high, according to the U.S.
Geological Survey.)
That doesn’t mean last year’s twin
strike-slip jolts were impotent. A third paper published online today in Nature reports a
fivefold increase in the rate of
remote earthquakes (those greater than 1,500 kilometers from the epicenter)
with magnitudes of 5.5 or greater during the six days following the initial events.
This map drives home the point. The four earthquakes of magnitude 6.0 or greater noted off
western North America all occurred within the first 24 hours of the Indian
Ocean events. Such a brazen cluster of
tremors that size is highly unusual; the global average is one every three
days.
“We’ve never seen an earthquake
like this,” the University of Utah’s Keith Koper said in a press release. “This
is part of the messy business of breaking up a plate….This is a
geologic process. It will take millions of years to form a new plate boundary
and, most likely, it will take thousands of similar large quakes for that to
happen.”
IMAGES:
Map of the Indian Ocean region showing boundaries of
Earth’s tectonic plates near the epicenters (red stars) of two great
earthquakes that happened April 11, 2012. (Keith Koper, University of
Utah Seismograph Stations)
Epicenters of four remote magnitude 6.0 or greater earthquakes
(red beach balls) that occurred within 24 hours of the April 11, 2012, east
Indian Ocean earthquake (black and white beach ball). (Fred Pollitz)
