September 27, 2012
April’s Sumatra Quake May Have Been Largest Of Its Type On Record
Alan McStravick for redOrbit.com - Your Universe Online
In a report due for release tomorrow in the journal Nature, researchers from the University of Utah, the University of California, Santa Cruz, and seismologists from the United States Geological Service (USGS), contend that the massive April 11 earthquake, centered in the Indo-Australian plate, actually ripped at least 4 fault lines.
With its epicenters just northwest of Sumatra, Indonesia, the quake was in the same geological neighborhood as the 2004 Sumatran quake that triggered a massive tsunami and claimed more than 200,000 lives. April´s quake, however, was responsible only for 2 direct deaths and 8 heart attacks and represented an entirely different style of quake than its 2004 event predecessor.
The four confirmed ripped faults, termed "strike-slip" faults, get their name from the movement of earth in relation to the other side of the fault line. A strike-slip fault will move in a lateral, horizontal direction. Imagine standing on one side of a fault line with a tree directly in front of you on the other side of the fault line. After a quake event, that tree would remain on the same level as you but would have shifted either to the left or right.
The 2004 earthquake, by comparison, was an event at a fault line that is known as a “subduction zone”. Here, the older, denser side of the fault forces the younger side under it. When this occurs on land, large mounds and hills are created. When it happens underwater, waves are created that increase in size and force, creating, in the worst scenario, a tsunami on par with what was witnessed in the Indian Ocean on December 26, 2004.
During April´s quake, four distinct faults within the Indo-Australian plate experienced rupturing. The first, third and fourth faults, although separated by some distance, occurred parallel to one another, extending from a northwest to southeast direction. The second fault line tore perpendicular to the others, even crossing over fault 1, creating an ℠x´ pattern on the sea floor.
While scientists have long known that the Indo-Australian plate on which the entire continent of Australia and surrounding waters sits has been breaking apart — a process that started approximately 50 million years ago and expected to continue for millions more — they were amazed by the overall strength of the April 11 earthquake.
"We've never seen an earthquake like this," says study co-author Keith Koper, an associate professor geophysics and director of the University of Utah Seismograph Stations. "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."
According to the new study, the great quake of last April 11 is perhaps the most powerful strike-slip earthquake ever seismically recorded, although a similar-sized quake in Tibet in 1950 was of an unknown type. The research was led by two University of California, Santa Cruz, seismologists, graduate student Han Yue and Thorne Lay, a professor of Earth and planetary sciences.
The researchers cannot be certain whether the April great quake was the largest intraplate quake or the largest strike-slip quake because they are attempting to compare it with quakes that occurred long before the invention of modern seismometers, explained Koper.
According to Lay, the Indo-Australian plate is continuing to experience an upheaval that will eventually cause the plate to fragment into two separate plates. "Nobody was anticipating an earthquake of this size and type, and the complexity of the faulting surprised everybody I've spoken to about this," said Lay.
By looking at the tectonic plates in the region, it is easy to see how the break-up is progressing. As the plate attempts to move in a northeasterly direction, it is hampered on the northwest side by the continent of Asia. The eastern portion of the plate, unencumbered by any large land mass, moves far more easily as it 'subducts', or dives, under the island of Sumatra. This process of subduction is what makes these quakes so menacing for populations in the region as the production of large tsunamis is a very likely outcome.
Aftershocks can occur within a few hours of the initial event and can often bring far greater danger and damage than the first earthquake. The April 11 earthquake helped to broaden the understanding of aftershocks, causing researchers to largely discard their previous conceptions of the time-lag and locality of aftershocks.
For more than 6 days following the April 11 earthquake, seismic activity was recorded at higher than normal (and expected) levels across the globe.
The overall strength of the initial earthquake was quite large, ranking it as the tenth most powerful on record in the last 100 years. While it did trigger small aftershocks in the first 3 hours, this study shows that fault lines the world over, while not failing immediately, were primed for activity that occurred over the entire week that followed.
For those living in seismically active areas around the globe, this new information should serve as a warning that all is not necessarily clear if you escape the first few hours without any seismic incidents.
"Until now, we seismologists have always said, 'Don't worry about distant earthquakes triggering local quakes,'" said Roland Burgmann, professor of earth and planetary science at UC Berkeley and coauthor of the study. "This study now says that, while it is very rare — it may only happen every few decades — it is a real possibility if the right kind of earthquake happens."
"We found a lot of big events around the world, including a 7.0 quake in Baja California and quakes in Indonesia and Japan, that created significant local shaking," Burgmann added. "If those quakes had been in an urban area, it could potentially have been disastrous."
"This was one of the weirdest earthquakes we have ever seen," Burgmann said. "It was like the 1906 San Francisco earthquake, a strike-slip event, but it was huge — 15 times more energetic. This earthquake and an 8.3 that followed were in a very diffuse zone in an oceanic plate close to the Sumatra subduction zone, but it wasn't a single fault that produced the quake, it was a crisscrossing of three or four faults that all ruptured in sequence to make such a big earthquake, and they ruptured deep."