Monday, October 27, 2008

Landslides Trace Kashmir Earthquake Fault

Via an article on the NASA Earth Observatory website on the use of satellite imagery for mapping the 2005 Kashmir earthquake fault trace.

This earthquake was unusual for the Himalayan region as it occurred along a shallow fault and caused a surface rupture. i.e the cracks and deformation was visible at the surface. Deformation due to earthquakes is usually mapped by spotting offsets in artificial and natural linear features like walls and fences or gulley's and streams. That is incredibly hard to do in remote steep terrains like the Kashmir Himalayas. So the researchers used before and after earthquake images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) aboard the NASA Terra satellite to try and spot this rupture. A team of geologists had mapped this fault using ground surveys but this is a time consuming process and so a complimentary image analysis was done to evaluate whether imagery can provide a reliable way to quickly identify zones of deformation to aid rescue efforts after an earthquake.

The images below are ASTER false color composites which clearly show the fault trace highlighted by a linear zone of white color. These are landslides that occurred along the hanging wall of the fault. Only a small section of the trace is seen in the images.

Source: NASA Earth Observatory

False Color Composites are prepared by assigning the three primary colors red, green and blue to the three wavelength bands which analysts feel contain the maximum information about surface features. This arbitrary assignment of colors to bands results in features appearing in colors unnatural to the human eye. Hence the term False Color Composite. Depending upon sensor design a range of wavelengths from the visible part of the wavelength spectrum (reflected energy) to the thermal part of the spectrum (emitted energy) can be collected for study. ASTER the sensor used in this study as the name implies senses energy in both the thermal and reflected part of the spectrum. Only the reflected portion of the spectrum was used to process the images in this study.

Satellite sensors break up the reflected or emitted energy coming from the earth's surface into discrete regions or bands to allow better discrimination of surface features. This works because different surface features reflect or emit efficiently in different wavelengths. A judicious selection of bands can then be used to create maximum contrast between different surface features.

When using reflected energy, usually this wavelength combination as is in this image is near infrared, red and green. The wavelength range is from about 0.5 microns (green) to about 1 microns for near infrared. In this case red color has been assigned to the near infrared band since healthy vegetation reflects a lot of near infrared. Water appears blue and built up areas and landslides which are zones where vegetation has been stripped off and fresh rock and soil exposed appear gray and white respectively, indicating a very high reflectance in the smaller wavelengths.

The surface rupture of the Kashmir earthquake extends over 75 km. Another paper which studied the tectonics of this earthquake has concluded that the rupture has occurred along some subsidiary faults and not along the major boundary fault where strain is apparently still accumulating.

This does not bode well for the Kashmir region.

Update: Robert Simmon a NASA researcher in an email to me points out an error in my post. I gave the impression that the fault trace was mapped using landslides as a guide. This is not so. The fault trace was delineated by mapping the ground deformation i.e the offsets caused by the slip along the fault were mapped by correlating the before and after images. Landslides in fact obscure the fault trace such that the before and after images can't be correlated along such patches.

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