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Satellites helps scientists see effects of remote earthquakes

The unique capabilities of a NASA earth-observing satellite have allowed researchers to view the effects of a major earthquake that occurred in 2001 in Northern India near the border of Pakistan.

Lead author Bernard Pinty of the Institute for Environment and Sustainability in the Joint Research Centre of the European Commission, Ispra, Italy, and colleagues from the U.S., France and Germany, used the Multi-angle Imaging Spectro Radiometer (MISR) instrument on NASA’s Terra satellite to observe the effects of a massive earthquake in the Gujarat province of India.

Considered one of the two most damaging seismic events in Indian recorded history, the Gujarat earthquake struck with a magnitude of 7.7 (Richter scale) on January 26, 2001. About 20,000 people died and another 16 million people were affected. Local residents reported fountains of water and sediments spouting from the Earth following the earthquake. As a result of the earthquake’s intense ground shaking, loosely-packed, water-saturated sediments in the area liquefied, behaving more like a liquid than a solid. Ground water flowed up to the surface carrying sediments, flooding large areas including ancient riverbeds.

The earthquake’s epicenter was located about 80 kilometers (50 miles) east of the city of Bhuj, but the MISR instrument found dewatering, or release of water and sediment due to compression and liquefaction, as far as 200 km (124 miles) from the epicenter. Additionally, there was significant dewatering all along an 80-100 kms (50-62 miles) wide (south to north) ancient salt lake bed to the north of Bhuj, known as the Rann of Kutch. In the days to weeks following the earthquake, along with ground cracks and other types of deformation, water flowed to the surface and progressively evaporated in various places.
A year later, scientists could still observe the consequences of the earthquake across the Rann because the water that came up to the surface was very salty. After evaporation, the salt was left on the ground and MISR was able to detect it also. The MISR instrument views the sunlit face of the Earth simultaneously at nine widely spaced angles, and provides ongoing global coverage with high spatial detail. Its imagery is carefully calibrated to provide accurate measurements of the brightness, contrast, and color of reflected sunlight. One way MISR registers surface features is by picking up different wavelengths of light as they are reflected off the Earth’s surface. As the satellite passes overhead, MISR collects information over a 400 km (248 mile) swath at a spatial resolution of 275 meters (300 yards), instantaneously assessing surface features over large regions.

Since the bright soils of the Rann of Kutch reflect most of the Sun’s incoming near-infrared radiation, and water bodies absorb near-infrared radiation, MISR can detect the contrast and thereby tell where dewatering from the earthquake occurred. Changes in reflection at different view angles also proved advantageous to identify the presence of surface water in other regions.

Source: NASA, MISR Team