Stress aligned cracks in the upper crust of the Val d'Agri region as revealed by shear wave splitting

Details Stress aligned cracks in the upper crust of the Val d'Agri region as revealed by shear wave splitting Stress aligned cracks in the upper crust of the Val d'Agri region as revealed by shear wave splitting

Oct 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.179..601p&link_type=abstract

Geophysical Journal International, Volume 179, Issue 2, pp. 601-614.

Astronomy and Astrophysics

Astronomy

3

Seismicity And Tectonics, Seismic Anisotropy, Crustal Structure

Scientific paper

Shear wave splitting is measured at 19 seismic stations of a temporary network deployed in the Val d'Agri area to record low-magnitude seismic activity. The splitting results suggest the presence of an anisotropic layer between the surface and 15 km depth (i.e. above the hypocentres). The dominant fast polarization direction strikes NW-SE parallel to the Apennines orogen and is approximately parallel to the maximum horizontal stress in the region, as well as to major normal faults bordering the Val d'Agri basin. The size of the normalized delay times in the study region is about 0.01 s km-1, suggesting 4.5 percent shear wave velocity anisotropy (SWVA). On the south-western flank of the basin, where most of the seismicity occurs, we found larger values of normalized delay times, between 0.017 and 0.02 s km-1. These high values suggest a 10 percent of SWVA.
These parameters agree with an interpretation of seismic anisotropy in terms of the Extensive-Dilatancy Anisotropy (EDA) model that considers the rock volume pervaded by fluid-saturated microcracks aligned by the active stress field. Anisotropic parameters are consistent with borehole image logs from deep exploration wells in the Val d'Agri oil field that detect pervasive fluid saturated microcracks striking NW-SE parallel to the maximum horizontal stress in the carbonatic reservoir. However, we cannot rule out the contribution of aligned macroscopic fractures because the main Quaternary normal faults are parallel to the maximum horizontal stress.
The strong anisotropy and the seismicity concentration testify for active deformation along the SW flank of the basin.

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