This is possibly the first time we have observed the deep faultingcreated as a plate breaks free from part of itself in real time. Ofcourse, this behavior is evident in the rocks themselves and is longestablished and well understood.
At least, geology is now practiced everywhere and the globe isbecoming heavily instrumented. The result is that occurrences arenow been well observed and with computer technology, readilysimulated and tracked. There is still plenty to do but the data baseis nicely tightening up as is well demonstrated here. Of course, theprocess will take centuries.
Big quake was partof crustal plate breakup
by Staff Writers
Salt Lake City UT(SPX) Sep 27, 2012
Thismap of the Indian Ocean region shows boundaries of Earth's tectonicplates in the area, and the epicenters (red stars) of two greatearthquakes that happened April 11, 2012. A new study from theUniversity of Utah and University of California, Santa Cruz, says themain shock measured 8.7 in magnitude, about 40 times larger than theprevious estimate of 8.6. An 8.2-magnitude quake followed two hourslater.The scientists explain how at least four faults ruptured duringthe 8.7 main shock, and how both great quakes are likely part of thebreakup of the Indo-Australian Plate into separate subplates. Thenortheastward-moving plate is breaking up over scores of millions ofyears because the western part of the plate is bumping into Asia andslowing down, while the eastern part is sliding more easily beneathSumatra and the Sunda plate. Credit: Keith Koper, University of UtahSeismograph Stations.
http://www.terradaily.com/reports/Big_quake_was_part_of_crustal_plate_breakup_999.html
Seismologists haveknown for years that the Indo-Australian plate of Earth's crust isslowly breaking apart, but they saw it in action last April when atleast four faults broke in a magnitude-8.7 earthquake that may be thelargest of its type ever recorded.
The great Indian Oceanquake of April 11, 2012 previously was reported as 8.6 magnitude, andthe new estimate means the quake was 40 percent larger than had beenbelieved, scientists from the University of Utah and University ofCalifornia, Santa Cruz, report in the Sept. 27 issue of the journalNature.
The quake was causedby at least four undersea fault ruptures southwest of Sumatra,Indonesia, within a 2-minute, 40-second period. It killed at leasttwo people, and eight others died from heart attacks. The quake wasfelt from India to Australia, including throughout South Asia andSoutheast Asia.
If the four ruptureswere considered separate quakes, their magnitudes would have been8.5, 7.9, 8.3 and 7.8 on the "moment magnitude" scale usedto measure the largest quakes, the scientists report.
The 8.7 main shockbroke three faults that were parallel but offset from each other -known as en echelon faults - and a fourth fault that wasperpendicular to and crossed the first fault.
The new studyconcludes that the magnitude-8.7 quake and an 8.2 quake two hourslater were part of the breakup of the Indian and Australian subplatesalong a yet-unclear boundary beneath the Indian Ocean west of Sumatraand southeast of India - a process that started roughly 50 millionyears ago and that will continue for millions more.
"We've never seenan earthquake like this," says study co-author Keith Koper, anassociate professor geophysics and director of the University of UtahSeismograph Stations.
"This is part ofthe messy business of breaking up a plate. ... This is a geologicprocess. It will take millions of years to form a new plateboundary and, most likely, it will take thousands of similar largequakes for that to happen."
All four faults thatbroke in the 8.7 quake and the fifth fault that ruptured in the 8.2quake were strike-slip faults, meaning ground on one side of thefault moves horizontally past ground on the other side.
The great quake oflast April 11 "is possibly the largest strike-slip earthquakeever seismically recorded," although a similar size quake inTibet in 1950 was of an unknown type, according to the new study,which was led by two University of California, Santa Cruz,seismologists: graduate student Han Yue and Thorne Lay, a professorof Earth and planetary sciences. The National Science Foundationfunded the study.
The 8.7 jolt also "isprobably the largest intraplate [within a single tectonic plate ofEarth's crust] ever seismically recorded," Lay, Yue and Koperadd. Most of Earth's earthquakes occur at existing plate boundaries.
The researchers cannotbe certain the April great quake was the largest intraplate quake orthe largest strike-slip quake because "we are comparing itagainst historic earthquakes long before we had modern seismometers,"says Koper.
Why the Great QuakeDidn't Unleash Major Tsunamis
Koper says the2012 quakes likely were triggered, at least in part, by changes incrustal stresses caused by the magnitude-9.1 Sumatra-Andamanearthquake of Dec. 26, 2004 - a jolt that generated massive tsunamisthat killed most of the 228,000 victims in the Indian Ocean region.
The fact the 8.7 and8.2 quakes were generated by horizontal movements along seafloorstrike-slip faults - not by vertical motion along thrust faults -explains why they didn't generate major tsunamis.
The 8.7 quake causedsmall tsunamis, the largest of which measured about 12 inches inheight at Meulaboh, Indonesia, according to the U.S. GeologicalSurvey.
Without majortsunamis, the great earthquake caused "very little damage anddeath, especially for this size of an earthquake, because it happenedin the ocean and away from coastlines," and on strike-slipfaults, says Koper.
The researchersstudied the quake using a variety of methods to analyze the seismicwaves it generated.
Because the same datacan be interpreted in various ways, Koper says it is conceivable thatmore than four fault segments broke during the 8.7 quake -conceivably five or even six - although four fault ruptures is mostlikely.
Breaking Up is Hard toDo
TheIndo-Australian plate is breaking into two or perhaps three pieces(some believe a Capricorn subplate is separating from the west sideof the Indian subplate). The magnitude-8.7 and 8.2 great quakes onApril 11 occurred over a broad area where the India and Australiansubplates are being sheared apart.
"What we'reseeing here is the Indo-Australian plate fragmenting into twoseparate plates," says Lay.
The breakup of thenortheast-moving Indo-Australian plate is happening because it iscolliding with Asia in the northwest, which slows down the westernpart of the plate, while the eastern part of the plate continuesmoving more easily by diving or "subducting" under theisland of Sumatra to the northeast. The subduction zone off Sumatracaused the catastrophic 2004 magnitude-9.1 quake and tsunami.
Seismic analysis showsthe April 11 quakes "involve rupture of a very complex networkof faults, for which we have no documented precedent in recordedseismic history," the researchers write.
The analysis revealedthis sequence for the faults ruptures that generated the 8.7 quake,and the estimated fault rupture lengths and slippage amounts:
+ The quake began withthe 50-second rupture of a fault extending west-northwest toeast-southeast, with an epicenter a few hundred miles southwest ofSumatra. The fault ruptured along a roughly 90-mile length, breaking"bilaterally" both west-northwestward andeast-southeastward, and also at least 30 miles deep, "almostripping through the whole plate," Koper says. The seafloor onone side of the fault slipped about 100 feet past the seafloor on thefault's other side.
+ The second fault,which slipped about 25 feet, began to rupture 40 seconds afterthe quake began. This rupture extended an estimated 60 miles to 120miles north-northeast to south-southwest - perpendicular to thefirst fault and crossing it.
+ The third fault wasparallel to the first fault and about 90 to the miles southwest ofit. It started breaking 70 seconds after the quake began andruptured along a length of about 90 miles. This fault slipped about70 feet.
+ The fourth faultparalleled the first and third faults, but was to the northwest ofboth of them. It began to rupture 145 seconds after the quake beganand continued to do so for 15 seconds until the quake ended after atotal time of 2 minutes and 40 seconds. The fault rupture was roughly30 miles to 60 miles long. The ground on one side of this faultslipped about 20 feet past ground on the other side.
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