Over 50,000 earthquakes occur every year on Earth. About a thousand of these are over magnitude 5 on the Richter scale and often cause damage to human settlements. The most powerful earthquakes, though rare, reach magnitude 9 on the Richter scale and cause devastation over kilometres.
Satellite data provide a unique opportunity to measure fine changes in the Earth's surface which are often precursors of an earthquake. Such data was produced by ESA with the Synthetic Aperture Radar instruments SAR and ASAR on ERS-2 and Envisat respectively. Monitoring earthquake prone regions with such instruments can assist in the forecasting of earthquakes, as well as in the management and evaluation of earthquake-associated risks.
With the launch of ERS-2 in 1995, a technique called SAR Interferometry (InSAR) became available. This technique involves combining two SAR images of the same area for use in displaying elevation or land change over time. This is obviously of particular use in the monitoring and analysis of earthquakes, as it can be used to pinpoint the extent of an earthquake and the variations in terrain.
Envisat was launched in March 2002, and carried the ASAR instrument, which sparked the interest for multi-polarisation. This uses PolInSAR (SAR Polarimetry and Polarimetric Interferometry) which can be used to combine multiple images of the same area, providing a more advanced application.
Japan was struck by the worst earthquake in its recorded history on 11 March 2011. The category 9 earthquake left 15,883 people dead and affected millions across Japan. It also caused a tsunami which lead to further damage and flooding. Hundreds of thousands of homes were either destroyed or suffered damage and a full recovery from the damage would take years.
A 5.8/5.9 magnitude earthquake struck L'Aquila, Italy on 06 April 2009. 297 people were killed and over 60,000 left homeless as houses were damaged or destroyed.
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