Wednesday, October 28, 2015

Nepal earthquake was less catastrophic than expected: Study

Scientists studying the earthquake combined data from the sparse seismic stations within Nepal with global observations and swift fieldwork. 

KATHMANDU: The devastating earthquake that struck Nepal on April 25 this year produced less damage and weaker shaking than might be expected from a magnitude 7.8 quake in the area, according to new research. 

In a region of major faulting and massive tectonic plate collisions, with an especially dense population centred on the country's capital of Kathmandu, seismologists had expected the worst from a major earthquake. 

The quake and its major aftershocks caused more than 8,000 fatalities, 22,000 injuries and hundreds of thousands of collapsed or damaged buildings. 

But the damage was not as catastrophic as expected, said US Geological Service geophysicist Susan Hough, guest editor of the Gorkha focus section papers - a group of ten new articles published in journal Seismological Research Letters. 

"The world community of earthquake professionals expected an earthquake like this directly beneath Nepal would take a far greater toll on property and lives. So it poses a major challenge to understand and explain the shaking from the earthquake," said Hough. 

Hough and others conclude that the shaking was less damaging than expected because of how the Kathmandu valley's "bowl" of ancient lake bed sediments responded. 

"We know that valleys like this greatly amplify shaking from small and moderate earthquakes, but in very large earthquakes, something that we call a nonlinear effect kicks in," Hough said. 

"We can think about the valley as a bowl of jello that shakes, but in a very big earthquake a valley is more like a bowl of sand," she said. 

"When shaken strongly, the grains aren't able to transmit energy in the same way that a solid rock does," said Hough. 

Other papers published in the journal section map out the main earthquake's rupture within the Himalayan area's complex fault system, concluding that the fault broke adjacent to the rupture from a 1934 magnitude 8.0-8.4 earthquake. 

The historical and geologic record suggests that much larger earthquakes have occurred both east and west of the fault. 

Scientists studying the earthquake combined data from the sparse seismic stations within Nepal with global observations and swift fieldwork. 

In some cases, the data were collected by cutting-edge "earthquake science from space," such as the before-and-after satellite imagery of the region, reported in a study by NASA's Jet Propulsion Laboratory (JPL) researcher Sang-Ho Yun and colleagues. 

Newspaper and other accounts provided a wealth of information that Earth Observatory of Singapore researcher Stacey Martin and his colleagues considered in detail to map out the severity of shaking throughout Nepal and neighbouring countries. 

Robb Moss of Cal Poly Pomona and his colleagues examined soil liquefaction within Kathmandu valley and several key landslide sites, and University of Nevada researcher Steven Angster and his team confirmed initial indications that the fault rupture did not reach the surface.

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