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 directions of maximum horizontal compression (sigma H) as extracted from borehole breakouts and from earthquake focal mechanism solutions. The results indicate that the regional stress field is dominated by NW-SE compression, with good consistency between shallow borehole breakouts (2-5 km depth) and deeper earthquakes (10-25 km depth). The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the results are in good agreement with the western Europe trend found in earlier investigations. The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW-SE and NE-SW. From Tampen Spur in the west to the Sogn graben in the east an anomalous stress field is indicated, with NE-SW oriented maximum horizontal compressions. This anomaly is clearly seen both in the borehole breakout data and in the earthquake data. Possible sources for this anomaly are discussed, and include postglacial uplift and/or lateral variations in the physical properties of the crust.