Edinburgh University scientists have developed a seismic interferometry technique to monitor movements beneath the Earth’s surface.
The new method, which uses data collected from earthquakes, could potentially allow the Earth’s seismic activity to be mapped more comprehensively.
When an earthquake occurs within the Earth, waves are sent out in all directions. These eventually come to the surface where they are recorded by seismometers. The recordings provide an indication of how the earthquake ‘sounded’ once the sound propagates to the surface of the Earth.
Now, Prof Andrew Curtis at Edinburgh University has shown that by using the knowledge gained about how energy from a first earthquake travels from its epicentre to the surface, it is possible to ‘back-project’ the recordings of a second earthquake from the surface down to the location of the first.
The result is a calculation of how the second earthquake would have ‘sounded’ at the location of the first, turning the first earthquake into a virtual seismometer.
‘Up until now, we have been limited to exploring the most interesting and dynamic regions of the Earth’s crust and mantle remotely and have only been able to use seismometers located on the Earth’s surface, perhaps hundreds of miles away,’ said Curtis. ‘Now, using the new method, we can place virtual seismometers deep below the Earth’s surface, directly within the Earth’s most dynamic regions of interest.’
It is not only earthquakes that can be used to listen to the Earth’s deepest secrets: any well-recorded source of seismic energy can also be turned into a virtual seismometer. Alternatives range from explosive shots used in the oil-and-gas exploration industry to underground nuclear weapons tests.
There is one other interesting aspect about the virtual seismometers. Whereas traditional seismometers measure how the ground surface shakes at the location of the seismometer because of an earthquake, the new virtual seismometers record how the Earth stretches or strains beneath the surface.
This is important because changes in strain are indicators of changes in stress. Stress changes on fault zones inside the Earth, whether caused by tectonic forces or by passing seismic waves, can trigger other earthquakes. So, for the first time, measurements of strain and inferred stress can be made non-invasively using virtual seismometers located inside a fault zone itself.
‘Perhaps this will take us a step closer to understanding the dynamics of earthquake triggering, contributing to solving the earthquake forecasting puzzle,’ added Curtis.
The research, published in Nature Geoscience, was carried out in collaboration with the British Geological Survey and Utrecht University.