On a computer generated diagram of seismic profiles from Nepal and Tibet, John Nabelek traces a thin blue line. “That’s the interface between the Indian and the Eurasian tectonic plates,” he says. The earthquake-prone, mountainous terrain above it is home to an estimated 40 million people.
“It is very steep. In earthquakes, landslides come tumbling down,” says Nabelek, an associate professor in Oregon State University’s College of Oceanic and Atmospheric Sciences. “Construction is not up to par, so there, you’re looking at a huge disaster.”
With support from the National Science Foundation (NSF), Nabelek leads an international team of scientists on a quest to understand the underlying geology of the Himalayas. In 2009, they created the most complete seismic image of the Earth’s crust and upper mantle in the region and discovered some unusual geologic features that may explain how it has evolved. The study is known as Hi-CLIMB, Himalayan-Tibetan Continental Lithosphere during Mountain Building.
“The research took us from the jungles of Nepal, with its elephants, crocodiles and rhinos, to the barren, wind-swept heights of Tibet in areas where nothing grew for hundreds of miles and there were absolutely no humans around,” Nabelek says. “That remoteness is one reason this region had never previously been completely profiled.”
Waterbed Geology
A lack of scientific consensus on how two continental plates collide has led to competing theories about the Himalayas. Some researchers have proposed that the heat generated by the collision has melted so much rock that the Tibetan plateau floats above it as though on a waterbed.
“There could be small pockets of fluid, but our study shows there are not large amounts of fluid here,” says Nabelek. “The building of Tibet is not a simple process. In part, the mountain building is similar to pushing dirt with a bulldozer, except in this case, the Indian sediments pile up into a wedge that is the lesser Himalayan mountains.”
The interface between the subducting Indian plate and the upper Himalayan and Tibetan crust is the Main Himalayan thrust fault, which reaches the surface in southern Nepal. The new images show that it extends from the surface to mid-crustal depths in central Tibet, but the shallow part of the fault sticks, leading to historically devastating mega-thrust earthquakes.
“The deep part is ductile and slips in a continuous fashion. Knowing the depth and geometry of this interface will advance research on a variety of fronts, including the interpretation of strain accumulation from GPS measurements prior to large earthquakes,” Nabelek adds. The study is continuing with funding from NSF and the Air Force Research Laboratory.
Nabelek also studies the Cascadia subduction zone, in which the relatively dense Juan de Fuca plate dives beneath North America. “The advantage of working in Tibet is that you can get on top of it. You can work on it. With the Cascadia, most of the mega-thrust is offshore about 100 miles.”
His emphasis in Cascadia is in the southern portion of the Juan de Fuca plate offshore from the Oregon-California border, a region known as the Gorda Deformation. Scientists don’t yet know why so much seismic activity occurs in this area. Most of the Juan de Fuca plate is relatively calm.
In another project funded by the NSF-EarthScope program, Nabelek will use the crustal imaging techniques employed in Nepal and Tibet to study the Earth’s crust under parts of Nevada. That project is scheduled to start this summer.
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