Geophysicists from the University of Colorado at Boulder have developed a new imaging technique to visualize the movement of rocks below the earth’s surface. And they applied this technique to the Himalaya to discover how the Indian plate moves as it passes beneath the Himalayan plateau. They used 29 broadband seismometers installed in Nepal and Tibet to record about 1,700 earthquakes between 2001 and 2003. Now, they think their technique can be used to assess earthquake hazards, even it can't predict them accurately. Read more...
"We imaged the boundary between the Indian and Asian tectonic plates by developing a new technique that highlights strongly deformed rocks beneath Earth's surface, and applied it to data we collected with a network of temporary seismic sensors deployed in Nepal and Tibet," said Vera Schulte-Pelkum, a researcher at the Cooperative Institute for Research in Environmental Sciences (CIRES).
The network included 29 broadband seismometers operated by the CU-Boulder and SUNY Binghamton teams. About 1,700 earthquakes from as far away as Europe, Alaska and Japan were recorded during an 18-month period starting in 2001. The study was funded primarily by the National Science Foundation.
Below is a map showing where the broadband seismometers in Nepal and Tibet were located (Credit: CIRES).
[This location map has two parts:] (a) Overview map with topography. The extent of the study area map in (b) is outlined in red. The location of INDEPTH profiles is indicated in blue. (b) Topography map of the study area. Stations deployed for this study are shown in black (three stations with little to no data owing to equipment problems or vandalism are shown in white). Hypocentres relocated with our network are colour coded by depth (scale in km).
The scientists found a shear zone above the base of the Indian crust beneath the Himalaya.
Shear zones are similar to faults, Schulte-Pelkum said. Faults are brittle structures at or near the surface of the earth, while shear zones are found at depths of 10 miles or more where heat causes more ductile, or flowing, rock movement.
In subduction zones such as where India and Asia collide, however, earthquakes along brittle faults can occur at depth because rock temperatures are cooler, the researchers said.
Of course, this will not help to forecast earthquakes -- only to understand them better.
With the team's new method, geophysicists can study the deep crust and determine the direction rocks are being sheared. The shearing is similar to a deck of cards being spread out on a table, said Sheehan, an associate professor of geological sciences at CU-Boulder and a CIRES researcher. "We can see how the deep crust has moved. Seeing where these structures are and how they have moved in the subsurface helps us better understand where local hazards are.
"If we can more accurately calculate the subsurface geometries, we can improve our estimations of how the ground will shake during an earthquake. We can't predict earthquakes, but we can get a better idea of how an earthquake's energy will radiate," explained Sheehan.
The research work has been published by Nature on June 30, 2005 under the name "Imaging the Indian subcontinent beneath the Himalaya." Here is a link to the first paragraph and this is an excerpt.
Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths.
If you want to read the full paper (PDF format, 4 pages, 574 KB), you need to be a subscriber to Nature or purchase the article for $30. The above illustration comes from this paper.
Sources: University of Colorado at Boulder news release, June 29, 2005; and various web sites
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