OSB archive
OSB archive

Satellites see the earth move

Pete Wilton

If you want to see how volcanoes interact with their surroundings then get a view from space.

That’s what Juliet Biggs, of Oxford University’s Department of Earth Sciences, and colleagues have been doing by using satellites to investigate volcanic rifts in intimate detail.

I asked Juliet about how satellite radar images are giving insights into what’s happening beneath our feet, and could even help us to tap geothermal energy from inside the planet…

OxSciBlog: Why do we need satellites to study volcanic activity?
Juliet Biggs: The East African Rift extends for over 3000 km, from Mozambique to Djibouti; many of the volcanoes are in remote areas which can be hard to access on the ground and even fewer have ground-based monitoring networks.

Satellite measurements offer a unique opportunity to study the East African Rift on the plate-boundary scale, giving us insight into the development of magma activity as the rift matures and the behaviour of individual volcanic systems.

Even where available, ground-based measurements of surface deformation are taken at a limited number of stations (usually less than 10 per volcano) so satellite images give a much higher measurement density.

OSB: How is radar used to pick up very small changes on the ground?
JB: A radar image can be divided into two components, the amplitude and phase of the radar wave. By taking the difference between the phase of two radar images, we can measure the change in the distance between the satellite and ground surface to an accuracy of better than 1cm.

The resulting map is called an interferogram (see image above) and shows the ground displacement at high resolution (<100m) over large areas.

OSB: What is the significance of the tiny changes you've found?
JB: We have so far detected surface deformation at four of the volcanoes in the Kenyan Rift and two in the Main Ethiopian Rift. Although these volcanoes are not currently erupting, these observations show there are significant pressure changes going on in the plumbing system.

The volcanoes are neither dormant nor extinct, with active magma systems at depths of 2-5 km. Along with colleagues specialising in volcanology, petrology and seismology and structural geology, we are still trying to understand the mechanism behind the deformation: is it the result of new magma moving into the system, the build-up and release of gases, or an unstable hydrothermal system?

The high percentage of volcanoes which have been seen to be deforming is surprising and indicates a ubiquitous magma supply with implications for models of continental rifting, caldera volcanoes, geothermal resources, and volcanic and seismic hazard.

OSB: How could these findings help us understand volcanic activity near Nairobi/Addis Ababa?
JB: Several of these volcanoes lie close to the heavily populated capital cities of Nairobi and Addis Ababa. There is little information available about the eruptive history of most of these volcanoes, but widespread ash layers show they have the potential for major explosive eruptions.

The satellite observations can help us identify the presence of active magma chambers and understand the patterns of magma recharge. Several of these volcanoes are potential sites for geothermal power stations, so a clearer understanding is necessary to determine the level of risk for personnel, infrastructure and productivity.

OSB: What do you hope the rest of your studies will reveal?
JB: These observations formed part of the pilot study on the Kenyan Rift: I am currently working with ESA as part of the Changing Earth Science Network to produce a map of the spatial and temporal distribution of activity along the East African Rift.

When complete, this map will show the role of magmatic fluids in continental extension with implications both for the East African Rift and the development of other rift systems. The map will also provide a database of information for seismic hazard, volcanic hazard and geothermal exploration.

In December 2009, a sequence of four medium earthquakes hit Northern Lake Malawi causing significant damage and killing four people. The deformation patterns seen in the satellite images are consistent with the rupture of a shallow, west-dipping fault, which had not been previously mapped. Although previous studies have shown that magma has an important influence on continental rifting even in immature sections of the East African Rift System, the satellite images show no evidence for the involvement of magmatic fluids in these events.