Brussels, 04 Nov 2003
A powerful earthquake that shook Denali, Alaska, in November 2002 also sent shockwaves through a section the Earth's atmosphere, a European Space Agency (ESA) supported study has found.
The discovery could have a major impact on earthquake detection techniques in areas not covered by seismic networks, such as the deep ocean and near islands.
The research was carried out by the institute of global physics in Paris and the California institute of technology, and focussed on a region of the atmosphere called the ionosphere. The ionosphere surrounds the Earth from altitudes of 75 to 1000 kilometres, and is filled with charged particles that can disrupt radio signals, particularly global positioning satellite (GPS) signals.
This very quality allowed researchers to map fluctuations in the ionosphere in near real time, creating a detailed, three dimensional impression of this region of the atmosphere. This fact, combined with the ionosphere's ability to act as an amplifier of seismic waves on the Earth's surface, led the team to hope they would be able to detect an earthquake by monitoring distortions in the GPS signal.
When the Denali earthquake hit Alaska on 3 November 2002, it generated the largest type of seismic wave, known as a Rayleigh wave, and was powerful enough to crack motorways. Around 660 seconds after these waves shook the Earth, the team observed GPS variations two or three times larger than normal background noise, and in a pattern consistent with the seismic activity on the ground.
Although the signals were weak and only sampled every 30 seconds, the team hopes that the technique can be refined and improved, and eventually used to provide earthquake monitoring in areas not equipped with seismic detectors.
A particularly promising future development will be the launch of Europe's Galileo satellite navigation network, as team member Vesna Ducic explains: 'Galileo will double the number of satellites and therefore will allow much more precise maps of the ionosphere. We can also foresee that Europe will develop a dense network of Galileo/GPS stations that will take part in the monitoring of these phenomena.'
Indeed ESA, together with partners in France, is already funding a pre operational project called SPECTRE (service and products for ionosphere electronic content and tropospheric refractive index over Europe from GPS), devoted to mapping the ionosphere. This initiative, together with the Franco-US project, form part of ESA's space weather applications pilot project, which aims to develop a range of application oriented services based on space weather monitoring.