oa Estimation of Stress Change in Ductile Part of the Crust Inferred from Seismic Scattering

In the past, stress field in the subsurface has been measured by various techniques. For example, borehole-breakout, stress release method, hydraulic fracturing, strain gauge buried in the ground are used to measure the stress field after costly drilling the subsurface. Beside the techniques requiring direct access to the subsurface, the stress field is also measured indirectly. Electro Distance Meter, Global Positioning System, etc. are used to measure a surface deformation in time. From the surface deformation, stress change in the subsurface is estimated. However the estimated stress change in the subsurface is largely affected by inhomogeneities located near the surface, landslide, slope failure, etc. Thus, stress field in the subsurface can be obtained directly and indirectly although some problems described above are still remaining. Here we focus on seismic scattering wave, particularly coda-Q value, to measure a stress field change in the subsurface. Coda-Q, which is derived from the attenuation of coda envelope, is perceived to be an indicator of the inhomogeneity in the subsurface. In this study, we hypothesize that coda-Q indicates stress change in a deep subsurface since coda-Q is obtained from seismic waves traveling over a wide range of the crust including ductile part of the crust. Numerically it has been revealed that coda-Q indicates the magnitude of the stress change in the deep subsurface, while the surface strain distribution is largely affected by inhomogeneities located near the surface. In this study we confirm that coda-Q indicates stress change in the deep subsurface using real field data that was acquired at the time of the Iwate-Miyagi Nairiku earthquake in 2008. The stress change estimated from coda-Q corresponds with that in the ductile part calculated by a fault model, which is acquired by a seismic wave analysis, whereas the estimated stress change dose not correspond with that from GPS measurement. It means that coda-Q could indicate stress change in the deep subsurface more accurate than GPS measurement.