Satellite records post-quake healing of Earth’s crust

Images recorded by European Space Agency’s Envisat satellite reveal that the surface of the earth has the capacity to ‘heal’ itself after an earthquake. The images of deformation and recovery of the Earth’s crust after the 2003 earthquake in Bam, Iran, are published in Nature this week. While scientists have known that fault zones are able to recover after an earthquake, the satellite images have enabled direct observation of this phenomenon for the first time.

Earthquakes radiate from slip on discrete faults, and can produce stress changes within a broader fault zone, especially near the Earth’s surface. These stress changes are caused by a process known as dilatancy in which the granular material expands in both porosity and volume as it shears within the fault zone.

From 12 days after the earthquake that devastated the city of Bam in eastern Iran, images were captured by the Envisat satellite and tracked through 3.5 years of deformation at the surface of the quake site.

The earthquake primarily ruptured a buried fault that is located directly under Bam and extends beneath a broad alluvial plain to the south. The earthquake had a large slip at depths of three to seven kilometers but very little slippage at the surface.

Eric Fielding from the Californian Institute of Technology and his colleagues used a remote sensing technique called InSAR to build up a compilation of radar images taken by the Envisat satellite.

The images reveal subsidence above the fault that the authors interpret as caused by a reversal of dilatancy generated during the earthquake. This is the first time that scientist have been able to measure the healing of the earth’s crust through the resulting volume decrease in granular material following an earthquake.