Fault zone monitoring with passive image interferometry

The technique of passive image interferometry applies interferometric methods to correlation functions of seismic noise to monitor small temporal variations of the Earth interior. We computed the autocorrelation function of seismic noise recorded at a single seismometer located in the vicinity of the source region of the M-w = 6.6 Mid-Niigata earthquake. Analysing the temporal evolution of the autocorrelation function, which is interpreted as the source receiver collocated elastic wave Green's function, we detect a sudden decrease of relative seismic velocity in the Earth crust of -0.6 per cent that coincides with the occurrence of the earthquake. This drop of seismic velocity is likely to be caused by the change of stress in the source region of the earthquake.