Looking inside the Continents from Space: Insights into Earthquake Hazard and Crustal Deformation

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As two tectonic plates move together or apart, any continent trapped between them deforms, causing major geological features such as mountain belts or sedimentary basins to develop. As the brittle, near-surface crust tries to accommodate the deformation, earthquakes occur on faults inside the earth. The need to understand how the continents deform, and where earthquakes will occur, is compelling - between 1.4 and 1.7 million people have died in earthquakes in the continental interiors since 1900.We can measure the way the continents are actively deforming using satellites. GPS can provide very precise measurements of how individual points on the ground move, but such points are often sparsely distributed. Over the past two decades, satellites designed by the European Space Agency (ESA) have demonstrated the ability of satellite-borne radar to measure displacements of the earth's surface. The radar repeatedly sends out bursts of a microwave signal that scatters back from the surface and is measured when it returns to the spacecraft. We use differences in the radar returns acquired by the satellite at two different times to measure the displacement of that point over the intervening time interval. Displacements of a few millimeters or less can be measured in this way.As the continental crust deforms, the rocks continue to bend, building up strain that will be released in future earthquakes. When assessing earthquake hazard, in addition to knowing where the faults are on which the earthquakes will occur, it is essential to know the rate at which this strain is growing. These rates are small, however, and not easy to measure using radar in the presence of noise caused by changes on the ground from which the radar scatters and in the properties of the atmosphere through which the radar signal passes. In addition, errors in our knowledge of the position of the satellites affect our measurements. Methods can be devised to counter these difficulties, but the opportunities to apply them has been limited with the current satellites by the irregular and infrequent acquisition of radar images over many parts of the seismic belts.We are motivated to bring the efforts of a team of investigators to bear on these questions because of the planned launch by ESA in mid-to-late 2013 of Sentinel-1A, a new radar satellite. An identical partner, Sentinel-1B will be launched 18 months later. Each spacecraft will pass over a given point on the earth's surface every 6 days; once both are in orbit any point will be revisited every 3 days. This short time interval, plus the fact that observations will be made for every pass of the spacecraft and its position will be carefully controlled and well known, will mean a radical improvement in our ability to measure rates of motion and strain. By combining the measurements from all available satellite tracks, together with any GPS data available, we will be able to map in detail over large areas the rates at which strain is building up. We plan to look at what happens inside the continents as they deform by using such observations to test and constrain physical models. Thus the displacements occurring in an earthquake measured by radar can be used to infer the movements that have taken place on the fault at depth. The way the earth's surface in the vicinity of an earthquake continues to move immediately after it tells us about the mechanical properties of the surrounding region, knowledge essential to understanding how the forces around a fault vary with time. On a larger scale, the spatial distribution of strain in the continents tells us about changes in the strength of the crust. With these constraints we can test competing hypotheses about how the continents deform and what are the major factors controlling where the deformation occurs.