Remote Sensing Digital Image Analysis: An Introduction
Remote Sensing Digital Image Analysis: An Introduction
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The Indonesian earthquake took place on 26 December 2004 at 00:58 GMT (moment magnitude 9.3) in the Indian Ocean, offshore the west coast of Sumatra, at a depth of about 30 km. This earthquake is one of the largest of the past 100 years, comparable only with those in Chile (1960) and Alaska (1964). The earthquake originated in the subduction zone of the Indian and Burma plates, moving at a relative velocity of 6 cm/year. The aftershocks were distributed along a plate boundary of about 1000-1300 km between Sumatra and the Andaman Islands. Some hours after the earthquake a destructive tsunami followed and hit the coastlines of the surrounding regions, causing widespread destruction in Indonesia, India, Thailand and Sri Lanka. The European Space Agency (ESA) made available a data package composed of European Remote Sensing Satellite Synthetic Aperture Radar (ERS-SAR) and Environment Satellite Advanced SAR (ENVISAT-ASAR) data covering the affected area, acquired before (four acquisition dates) and after (five acquisition dates) the earthquake. A total of 26 frames were analysed. We used this dataset to evaluate the effects of the earthquake and tsunami on the human settlements and the physiographic conditions along the coast. The proposed method is based on a visual comparison between pre-and post-seismic SAR intensity images, and on an analysis of their correlation coefficients. No complex data were made available by the ESA to exploit phase coherence. Analysis of pre-and post-earthquake SAR backscattering showed wide uplift areas between the Andaman Islands and Simeulue Island, and large modifications of the coastline of Sumatra. Subsiding areas were detected along the southeast coast of Andaman up to the west coast of Nicobar Island. Tidal effects were filtered out of the SAR images to identify the consequences of the earthquake. Global Positioning System (GPS) stations in the Andaman provided results confirming the surface displacement pattern detected by SAR. The analysis enabled us to draw a boundary line separating the uplift and subsidence.