Shear‐wave anisotropy across the geothermal field of Milos, Aegean volcanic arc

Abstract

The drilled geothermal reservoir of Milos island, Aegean sea (Greece) is densely sampled by seismic waves from local earthquakes occurring at depths of 4‐6 km, recorded by numerous three‐component seismographs. During the eight months of observation seismic activity on the island occurred in the form of a few swarms located in three different areas. The focal mechanisms of all earthquakes share an area of dilatational quadrants indicating a nearly vertical maximum compressive stress. This implies an extensional field consistent with the regional tectonics of the Aegean sea. Shear‐wave splitting is widely observed. Most of the observed leading shear‐wave polarizations are aligned around the N‐S direction and they are parallel to the structural trends of faults observed at the surface. The other seismograms show a leading shear wave polarized along a N40d̀E direction. These polarizations are observed particularly at stations located between the geothermal drillholes, indicating probably the orientation of microfractures aligned in the geothermal reservoir by the present‐day stress field. Variations of the anisotropy magnitude are deduced from the study of shear‐wave delays in the vicinity of geothermal drillholes, and their distribution appears to correlate with the limits of a fractured medium detected by P and S velocity anomalies. Copyright © 1991, Wiley Blackwell. All rights reserved