Abstract
On June 8, 2008, at 12:25 GMT, a M W 6.4 earthquake, the Movri Mountain earthquake, occurred in the area of northwest Peloponnese, western Greece. The epicenter was located in the municipality of Movri, 35 km southwest of Patras. For this study, a crustal anisotropy analysis was performed in the epicentral area of the Movri Mountain earthquake. Specifically, the shear-wave splitting phenomenon and its temporal evolution in relation to the Movri Mountain earthquake was studied, using the cross-correlation method. The data analysis revealed the presence of shear-wave splitting in the study area. Both before and after the Movri Mountain earthquake, the polarization directions of the fast component of the shear waves followed a general NNW-SSE direction. The observed mean fast polarization direction was not consistent with the regional stress field, which showed a general E-W direction of the maximum horizontal compressive stress. The differences between the estimated fast polarization directions and the properties of the regional stress field suggest the presence of a local stress field in the area around the fault. An increase in time delays was observed soon after the Movri Mountain earthquake. The average value of the delay times before the earthquake was ca. 18 ±2.6 ms, while after the earthquake this was ca. 40 ±4.6 ms. This increase in the time delay indicates changes in the crustal properties, which were possibly caused by variations in the preexisting micro-crack system characteristics related to the Movri Mountain earthquake, and the possible involvement of over-pressured fluids.
Highlights
Shear-wave splitting is a phenomenon in which shearwaves are separated into two components with different polarization directions and propagation velocities
The two splitting parameters that can be measured through shear-wave data processing are the polarization direction { of the fast component of the shear waves, and the time delay dt between the two components
The most widely accepted physical model, which is known as the extensive dilatancy anisotropy model [Crampin 1978, 1993, Crampin et al 1984b], explains the principal cause of the shear-wave splitting phenomenon as S-wave propagation through stress-aligned, fluid-saturated micro-cracks with an orientation parallel/ sub-parallel to the direction of maximum horizontal compression
Summary
Shear-wave splitting is a phenomenon in which shearwaves are separated into two components with different polarization directions and propagation velocities. The most widely accepted physical model, which is known as the extensive dilatancy anisotropy model [Crampin 1978, 1993, Crampin et al 1984b], explains the principal cause of the shear-wave splitting phenomenon as S-wave propagation through stress-aligned, fluid-saturated micro-cracks with an orientation parallel/ sub-parallel to the direction of maximum horizontal compression. This model has since been modified, and led to the anisotropic poro-elasticity model [Crampin and Zatsepin 1997, Zatsepin and Crampin 1997], which explains the way in which such microcracks evolve in response to changing conditions in permeable rock. Several studies on local earthquakes worldwide have revealed shear-wave splitting in various geological settings that indicate anisotropic media [Kaneshima 1990, Crampin and Lovell 1991, Gao et al 1998, Liu et al 2008]
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