Numerical analysis of the relationship between time-variant coda-Q and the variation in crustal stress

Abstract

SUMMARY Coda-Q is a stochastic parameter reflecting the heterogeneities of medium that seismic waves travel through. We confirmed that coda-Q would vary with the stress loaded to an elastic medium using numerical simulations of seismic wave propagation. When the stress is loaded, cracks in the crust could either close or newly open. The closure and opening of the cracks are not random but depending on the magnitude and the direction of the stress and the crack aspect ratio. The cracks in the medium after loading stress could be aligned in a specific orientation, and elastic wave velocity field would become anisotropic due to the alignment of specific crack orientations. Elastic wave velocity is in general faster along the direction corresponding with the crack orientation while slower along the perpendicular direction. In the numerical simulation, the effect of anisotropy in elastic wave velocity field due to the selective closure and opening of the cracks is calculated using a 2-D finite difference method assuming elastic wave velocity to be a function of the magnitude of loaded stress. The coda-Q calculated from seismic waves simulated for a model varies when the averaged normal stress changes. Our simulation indicated that the sensitivity of coda-Q −1 , that is the reciprocal of the coda-Q, would be 1.0 × 10 −2 (1.0MPa ‐1 ) against the magnitude of the confining pressure and 1.0 × 10 −3 (1.0 deg ‐1 ) against the direction of principal stress. We would like to conclude that coda-Q, a stochastic parameter reflecting heterogeneities of subsurface medium, could become a quantitative state indicator of the stress field of the medium where seismic waves propagate through. Spatiotemporal variation of coda-Q reflects change in the stress field in the crust.