Discrimination of the Active Scattering Interfaces

Temporal variations of the scattering sources can be discriminated by the changes in reflectivity and efficiency of energy distributions of transformations of vibration modes (e.g. P-S and/or S-P) at the interfaces among heterogeneous media. The active scattering interface is caused by such temporal variations of material properties of the scattering sources due to various processes associated with crustal deformations and chemical reactions due to the tectonic activities. Occurrence of earthquakes and volcanic eruptions are in some way related to the migrations of super-critical fluid and re-distribution of localized stresses acting on the scattering interfaces. Recently discovered slow slip events and deep non-volcanic tremors are apparently related to the movement of supercritical water supplied by the dehydration of the subducted material, although the mechanisms to generate these phenomena remain uncertain. Intermittent occurrence of these events could suggest that the stress conditions in the crust and upper mantle could be a critical state and triggered by the movements of water vapor. The heterogeneity in the lithosphere can be investigated by monitoring such active scattering interfaces that could be caused by temporal changes of both stress state and heterogeneous distribution of interstitially migrating fluid through surrounding rocks. In order to demonstrate effects of changes in the material properties and/or physical conditions of an embedded low velocity layer, synthetic wave forms are estimated by calculating impulse responses of horizontally layered media with various conditions of scattering interfaces, such as thicknesses, material properties, stress distributions, and so on. Comparisons of changes in waveforms of reflected, transformed, and transmitted elastic waves are made as a function of incident angles, thickness of embedded low velocity layer, and amount of discontinuities of material properties.