Earthquake or Aftershock? An Academic Question

Earthquake or aftershock? For thousands of Indonesians digging out from the latest devastating geologic jolt, the question is academic. Monday's earthquake was a catastrophic exclamation point on what has been a harrowing three months on Sumatra and surrounding islands.

But for scientists, the magnitude 8.7 quake also poses a vexing problem. Was the event a seismological shrug following the cataclysmic Dec. 26th earthquake -- the fourth-largest on record, spawning a tsunami that left nearly 300,000 people dead or missing throughout the Indian Ocean basin?

Or, should it be considered an independent historic event?

One thing is certain: Monday's disaster was no coincidence.

One prominent study published in the journal Nature on March 17 suggested with remarkable accuracy that stresses were accumulating on adjacent portions of the Sumatra Trench. Something like this was likely to occur -- perhaps within weeks, predicted John McCloskey and his partners at the University of Ulster-Coleraine in Northern Ireland.

If anything, they underestimated how wild things are getting off the coast of Sumatra, where several plates of the Earth's crust grind and dive beneath each other in a geologic wrestling match that researchers call a subduction zone.

"Subduction zone earthquakes are often coupled,'' McCloskey reported. "An earthquake of magnitude 7-7.5 would seem to represent the greatest immediate threat.''

Geophysicists for the U.S. Geological Survey and other laboratories initially described Monday's disaster as an aftershock, and for good reason.

Aftershocks are additional, smaller earthquakes that occur after the main shock and in the same geographic area. They can rattle a region for months or years. Generally, the larger the main shock, the more intense the aftershocks will be and the longer they will persist.

Since Dec. 26, Sumatra has been rattled by powerful aftershocks regularly. Residents probably should expect them to continue for months at least.

When data from instruments around the world started streaming into the agency's Earthquake Information Center in Golden, Colo., on Monday morning, it looked like a magnitude 8.0 had occurred offshore less than 100 miles from the epicenter of the big December quake.

Anything 8.0 or greater is considered to be a great earthquake in its own right. The entire planet might absorb one temblor like it over an entire year.

But December's event still was vastly more powerful. In explosive power, December's disaster was equal to 100 million pounds of TNT. It caused the seabed to spring up as much as 60 feet.

In contrast, Monday's wallop was a fraction of that.

"Technically, it's an aftershock,'' said Kate Hutton, a seismologist at the California Institute of Technology. Hutton added she was surprised there was no tsunami danger afterward.

"I would not have necessarily expected a huge tsunami like the one in December, but I would have expected something,'' she said.

Within hours, USGS was revising its estimate upward to magnitude 8.7. That prompted some researchers to reconsider.

"The bigger it gets, the less it looks like an aftershock,'' said Northwestern University geophysicist Seth Stein. "We'll be looking at it for the next few days.''

And while Monday's quake also occurred in the Sumatra Trench, it appears it occurred on a different segment, and the displacement was heading south for about 150-300 miles.

December's quake ruptured a segment of the fault, extending for more than 700 miles to the north.

Aftershocks typically occur on the same fault as the big one, but are at least a full magnitude smaller. The latest quake was followed by at least five true aftershocks on the southern segment that measured between 6.7 and 5.5.

"If this one broke to the south, then it's not an aftershock in any meaningful sense,'' Stein said. "At this point it looks more like triggered earthquake.''

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