Constraining the dynamics of continental collisions using InSAR
PI: Dr T Wright
Sponsor: The Royal Society
Value: £166637
Dates: 1st May 2006 to 30th September 2009
Summary
We all live on the earth's continental crust, and yet scientists do not yet understand the physical laws that govern the way it deforms. The deformation of continental crust causes earthquakes, which have resulted in more than a million fatalities in the past century. I propose to use new satellite technology to map the present-day deformation of large areas of the earth's continental crust. I will use these new data sets to place constraints on the physical laws that govern the deformation of the continents.
It is a common misconception that plate tectonic theory can fully describe the relative motions of all points on our planet's surface. The theory has proved remarkably successful for oceanic crust, where a small number of rigid surface plates move relative to each other, and deformation (hence earthquakes) is concentrated in narrow zones along their boundaries. However, the earth's continents do not follow the simple rules of plate tectonics: they deform internally with earthquakes occurring in zones that are often many hundreds of kilometres wide.
A major obstacle to the development of a tectonic theory for the continents has been the lack of knowledge of the present-day surface deformation field. The largest deforming area on the planet is Tibet, where the ongoing collision of India with Asia has created the Himalayas and Tibetan plateau - an area more than 2000 by 1000 km in size with an average height of 5000 m. It is the natural testing ground for any proposed physical laws, but nowhere is the present-day surface deformation field less well known. Two competing models have used Tibet as a battle ground for more than 20 years. One proposes that the continents do follow the rules of plate tectonics; it is just that the plates are smaller. This theory predicts a small number of “plate-like" blocks in Tibet, with a few large, fast-moving faults between them. At the other extreme, it has been proposed that the continents behave like a viscous fluid - the Tibetan plateau, in this model, is analogous to the elevated region created in front of a spoon moving through treacle. The fluid continents theory predicts smaller slip rates on a larger number of faults. Detailed knowledge of the present-day surface deformation field within the Tibetan plateau will allow these models to be tested.
Over the past six years, I have worked on a new technique for mapping the deformation of the continents caused by the earthquake cycle: satellite radar interferometry (InSAR). Using data from satellites in orbit 800 km above the earth, InSAR enables deformation of the earth's surface to be mapped with a spatial resolution of a few tens of metres and a precision of a few millimetres. InSAR does not require equipment on the ground or expensive field campaigns, so it can gather information on the earth's surface deformation from remote areas such as the Tibetan plateau, where it is difficult to obtain any other deformation data with sufficient density. In essence, InSAR works by using the phase component of radar waves, with a wavelength of about 6 cm, transmitted and received from an orbiting satellite. If the ground has moved in the time between two different satellite images, acquired from the same position in space, then this causes a measurable phase shift. I use this information to build up very precise maps of the way the earth is deforming over large areas.
Over the past twelve years, the European ERS satellites have been acquiring radar data over the entire planet, including the Tibetan plateau. This data archive offers a rich resource that has yet to be fully exploited. During the next five years, I intend to work with data from the ERS archive, and newly acquired data, to map the present-day deformation field of much of Tibet. I will use these data to determine which of the contrasting models for continental tectonics is correct.
Furthermore, my previous studies have produced unexpected results, such as silent slip triggered on faults by distant earthquakes. I expect more surprises in the next five years.