CRUSTAL STRESS IN THE NORTHERN NORTH-SEA AS INFERRED FROM BOREHOLE BREAKOUTS AND EARTHQUAKE FOCAL MECHANISMS

The regional stress field in the northern North Sea (offshore western Norway) has been studied through the acquisition and analysis of direc tions of maximum horizontal compression (sigma H) as extracted from bo rehole breakouts and from earthquake focal mechanism solutions. The re sults indicate that the regional stress field is dominated by NW-SE co mpression, with good consistency between shallow borehole breakouts (2 -5 km depth) and deeper earthquakes (10-25 km depth). The broad spatia l consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the res ults are in good agreement with the western Europe trend found in earl ier investigations. The Tampen Spur region in the northern North Sea h as been subjected to particularly complex deformation, with two domina ting fault directions trending NW-SE and NE-SW. From Tampen Spur in th e west to the Sogn graben in the east an anomalous stress field is ind icated, with NE-SW oriented maximum horizontal compressions. This anom aly is clearly seen both in the borehole breakout data and in the eart hquake data. Possible sources for this anomaly are discussed, and incl ude postglacial uplift and/or lateral variations in the physical prope rties of the crust.