Virtual seismometers in the subsurface of the Earth from seismic interferometry

The Earth’s interior can be imaged by analysing the records of propagating seismic waves. However, the global array of permanent seismometers that record seismic energy is confined almost exclusively to land-based sites. This limits the resolution of subsurface images, and results in relatively few local measurements from areas of great geological and tectonic interest (for example, the mid-ocean ridges and the Tibetan plateau)1. Here we use an unconventional form of seismic interferometry2–5 to turn earthquakes into virtual seismometers located beneath the Earth’s surface. Seismic waves generated by one earthquake lead to transient strain in the subsurface at other locations around the globe. This strain can be quantified from seismograms of independent earthquakes that have occurred in those locations. This technique can therefore provide information on the subsurface strain in regions of the globe that lack instrumental networks. Applying our method to earthquakes in Alaska and the southwestern United States, we show that the information that can be obtained from these earthquakes about other such events is consistent with that provided by instrumental seismometers. Our approach may allow realtime, non-invasive, subsurface seismic strain monitoring, particularly in areas remote frominstrumental networks.