Using repeated sources to quantitatively determine temporal change of medium properties: Theory and an example

[1] We develop a theory of using difference wavefields of repeated sources to locate and quantify temporal medium change and apply the theory to locate temporal change of seismic properties beneath the Japan subduction zone using repeated earthquakes. Our theory states the difference wavefields of two repeated sources in a temporally changed medium can be equivalently treated as wavefields propagating from conceptual sources, with their location at the place of temporal change and their strengths equal to the product of magnitude of medium property change and magnitude of the initial wavefields from the repeated sources. When the medium change extends to a finite region, the conceptual sources become volumetric sources distributed over the region of the medium change and propagating in the direction of the initial wave. The conceptualization establishes a theoretical framework for possible applications of using difference wavefields to locate and quantify temporal medium changes in geological sciences, ultrasonic experiments, civil engineering and medical imaging. We search repeating earthquakes occurring in the Japan subduction zone, formulate an empirical procedure to extract the difference wavefields between repeating earthquakes and determine temporal change of seismic properties using a back-projection method. We locate the temporal change of seismic properties beneath the Japan subduction zone to be at (37.2 N, 142 E), and estimate the magnitude of the conceptual body force associated with the temporal change to be 1.15 10N, or as a reference, a 0.87% density change for an assumed volume of temporal change of 10 km.