Creating 'virtual seismometers' deep inside the Earth

An earthquake in a subduction zone receives sound waves like a microphone. By using earthquakes themselves as virtual microphones that record the sound of the Earth’s internal movements, we can listen to the Earth’s stretching and cracking from directly within its most interesting, dynamic places.
Comparison of real (a/c) and virtual (b/d) recordings of the Sichuan earthquake, China, recorded at stations in California and Alaska and synthesised using earthquakes adjacent to the stations. CLICK TO ENLARGE

Seismologists use networks of sensors, called seismometers, at hundreds of locations over our planet's surface to record signals from the earthquakes generated by the huge tectonic forces deep inside the Earth. Typically, these signals are then used to characterize earthquakes and also to build up a picture of the interior of the Earth.

However, the networks are generally restricted to land areas, with very few seismometers in the deep oceans. This means there are relatively few local measurements from areas of great interest such as mid-ocean ridges and subduction zones where some of the largest earthquakes occur.

Virtual seismometers

New research by Curtis et. al, in Nature Geoscience Virtual seismometers in the subsurface of the Earth from seismic interferometry, 30 August 2009, shows how earthquakes located deep within the Earth can be turned into virtual seismometers. This builds on recent research that shows how information about the Earth can be extracted from background noise recorded at pairs of seismometers, an approach commonly known as seismic interferometry.

However, Curtis et. al, (2009) show how the theory can be modified to turn any energy source into a receiver. This then allows us to use earthquakes as virtual sensors in places where there are no actual seismometers.

Whereas normal seismometers measure how the ground surface shakes due to an earthquake, the new virtual seismometers record how the Earth strains beneath the Earth’s surface. Changes in strain are good indicators of changes in stress, and stress changes on fault zones inside the Earth trigger other earthquakes. So, for the first time we can measure of strain and inferred stress deep inside fault zones, perhaps bringing us a step closer to solving the earthquake puzzle.

Further information

Visit the BGS Earthquakes site or contact Dr Brian Baptie for further information.