Post by Dave on Jul 22, 2012 17:18:22 GMT -5
Sumatran zig-zag earthquake answers seismology puzzle
New Scientist, 19:00 19 July 2012 by Samuel Payler
"On 11 April, seismic sensors picked up evidence of a massive 8.6-magnitude earthquake off the coast of Indonesia. Tsunami alerts sounded and evacuations began across the region – but the destructive wave never came. A new analysis helps to explain why – and in doing so suggests some oceanic regions may be more vulnerable to potentially destructive quakes than we realised.
Initial data from the earthquake was surprising. Large quakes – like the 9.0-magnitude event that struck Japan in 2011 and the 9.1-magnitude Sumatra quake of 2004 – usually occur in a subduction zone where one tectonic plate slides beneath another. The April quake struck within an oceanic plate, hundreds of kilometres from the nearest subduction zone. It was the largest intraplate earthquake ever recorded.
So what happened? Using seismic data collected by a network of sensors, Lingsen Meng and his colleagues at the California Institute of Technology in Pasadena pieced together the events of the earthquake.
Confirming seismological hunches in the immediate aftermath of the quake, they found that the quake occurred along a vertical crack through the crust – a so-called strike-slip fault. Rock on either side of the fault moved horizontally. In contrast, subduction zone quakes usually involve rocks on one side of the fault lifting vertically relative to the other side, which displaces seawater and may trigger a tsunami.
But the actual pattern of rupture was unusual. An event of this scale suggests there should have been a rupture several hundred kilometres long – but Meng and colleagues found no evidence of such a long gash in the crust. Instead, they found that the rupture had been shared between four distinct strike-slip faults, three of which were orientated perpendicular to one another, creating an almost zig-zag shaped rupture pattern (see graphic).
"I was really amazed when I first saw it," says Meng. "This was a really complicated earthquake."
The nature of the compressional forces surrounding a fault in the middle of rupturing is such that the quake should be incapable of triggering movement along a second, perpendicular fault – let alone do so several times, and all within a few minutes. How exactly these forces were overcome is not yet clear, although it appears that the 2004 Sumatran quake may have been a factor: it occurred little more than 300 kilometres away and may have primed the area for April's zig-zag quake.
Shamita Das, a seismologist at the University of Oxford who was not involved in the study, has long suspected that these zig-zag ruptures occur. But demonstrating that all of the individual faults rupture at the same time – rather than in separate events separated by weeks or months – has proved tricky.
"This is a very important earthquake," she says. "We have a very clear case – there can be no doubt" that zig-zag ruptures occur.
The consequences of the new findings are far-reaching, says Meng. They demonstrate that earthquakes can grow to the large and potentially dangerous magnitudes associated with subduction zones, and can occur far from such zones by jumping from fault to fault within a tectonic plate. Oceanic crust is criss-crossed with fractures, so more quakes as large as the April event could potentially occur.
Fortunately, the lack of vertical displacement along any of April's zig-zag faults minimised the risk of a tsunami. Such faults can sometimes rupture vertically as well as horizontally, though, and could trigger a destructive wave. That suggests we need to reassess the earthquake risk in some intraplate areas, says Meng.
Journal reference: Science, DOI: 10.1126/science.1224030"
How else would you get cracks in the crust that move in opposing direction? EXPANDING EARTH - or more appropriately said - further evidence of Whole Earth Decompression
New Scientist, 19:00 19 July 2012 by Samuel Payler
"On 11 April, seismic sensors picked up evidence of a massive 8.6-magnitude earthquake off the coast of Indonesia. Tsunami alerts sounded and evacuations began across the region – but the destructive wave never came. A new analysis helps to explain why – and in doing so suggests some oceanic regions may be more vulnerable to potentially destructive quakes than we realised.
Initial data from the earthquake was surprising. Large quakes – like the 9.0-magnitude event that struck Japan in 2011 and the 9.1-magnitude Sumatra quake of 2004 – usually occur in a subduction zone where one tectonic plate slides beneath another. The April quake struck within an oceanic plate, hundreds of kilometres from the nearest subduction zone. It was the largest intraplate earthquake ever recorded.
So what happened? Using seismic data collected by a network of sensors, Lingsen Meng and his colleagues at the California Institute of Technology in Pasadena pieced together the events of the earthquake.
Confirming seismological hunches in the immediate aftermath of the quake, they found that the quake occurred along a vertical crack through the crust – a so-called strike-slip fault. Rock on either side of the fault moved horizontally. In contrast, subduction zone quakes usually involve rocks on one side of the fault lifting vertically relative to the other side, which displaces seawater and may trigger a tsunami.
But the actual pattern of rupture was unusual. An event of this scale suggests there should have been a rupture several hundred kilometres long – but Meng and colleagues found no evidence of such a long gash in the crust. Instead, they found that the rupture had been shared between four distinct strike-slip faults, three of which were orientated perpendicular to one another, creating an almost zig-zag shaped rupture pattern (see graphic).
"I was really amazed when I first saw it," says Meng. "This was a really complicated earthquake."
The nature of the compressional forces surrounding a fault in the middle of rupturing is such that the quake should be incapable of triggering movement along a second, perpendicular fault – let alone do so several times, and all within a few minutes. How exactly these forces were overcome is not yet clear, although it appears that the 2004 Sumatran quake may have been a factor: it occurred little more than 300 kilometres away and may have primed the area for April's zig-zag quake.
Shamita Das, a seismologist at the University of Oxford who was not involved in the study, has long suspected that these zig-zag ruptures occur. But demonstrating that all of the individual faults rupture at the same time – rather than in separate events separated by weeks or months – has proved tricky.
"This is a very important earthquake," she says. "We have a very clear case – there can be no doubt" that zig-zag ruptures occur.
The consequences of the new findings are far-reaching, says Meng. They demonstrate that earthquakes can grow to the large and potentially dangerous magnitudes associated with subduction zones, and can occur far from such zones by jumping from fault to fault within a tectonic plate. Oceanic crust is criss-crossed with fractures, so more quakes as large as the April event could potentially occur.
Fortunately, the lack of vertical displacement along any of April's zig-zag faults minimised the risk of a tsunami. Such faults can sometimes rupture vertically as well as horizontally, though, and could trigger a destructive wave. That suggests we need to reassess the earthquake risk in some intraplate areas, says Meng.
Journal reference: Science, DOI: 10.1126/science.1224030"
How else would you get cracks in the crust that move in opposing direction? EXPANDING EARTH - or more appropriately said - further evidence of Whole Earth Decompression
Attachments:
??