ALBERT

Notes: A set of 517 recordings of Lg waves from 151 earthquakes in and around Norway has been used for determination of seismic moment Mo, corner frequency fo and anelastic attenuation Q(f). The data used have been recorded at source-receiver distances of 20 to 1200 km, with ML magnitudes between 0.8 and 5.0, and the parameters were estimated by inverting Fourier spectra from all of the recordings simultaneously. The observed spectra were represented by a source term, a spreading term, and an attenuation term, and the inversion was made assuming the geometrical spreading to be known. Because of the non-linear behaviour of the spectral shape, the inversion was done iteratively by minimizing the differences between observed and computed spectra.A standard ω-2 source model was used in the inversion, supported by near-field observations of small-magnitude earthquakes at the regional NORESS array. A model for geometrical spreading was then established by investigating the decay of Lg waves using synthetic data modelled without anelastic attenuation, for a realistic crustal structure with a Moho depth of 40 km. The results support the standard model of spherical spreading at short distances and cylindrical spreading at longer distances, but with a transition distance closer to 200 km than to the more commonly used value of 100 km. The decay rate beyond this distance was found to be close to –1/2 in the frequency domain, equal to the theoretical value for cylindrical spreading, and –3/4 in the time domain, where the theoretical value for an Airy phase is –5/6.The data used in the inversion were amplitude-displacement spectra corrected for instrument response and site effects, reduced (by smoothing) to 64 spectral values and weighted to represent equidistantly spaced points in the log-frequency domain. Only spectral values with signal-to-noise ratios of at least four were accepted, with an upper limit set to 10Hz, and a lower frequency limit determined by the instrument response. The computed Mo values are quite stable, and exhibit a linear dependency on ML. The corner frequencies are less well constrained, however, especially for smaller events. In using a model of the type fo∝Mo-δ we found a δ around 3.4, indicating slightly increasing stress drop, with values of less than 10 MPa in all cases (using a Brune model). The resulting Q models of the type Q(f) = qfη yield q values around 440 and η values around 0.7. This is reasonably close to other anelastic attenuation models found in this area.

Notes: Data collected by temporary seismic networks and individual stations over a 7-yr period in the Svalbard Archipelago are integrated and used to study the seismicity and present-day tectonics of the archipelago and surrounding regions. Most of the continental seismic activity occurs in three concentrated zones, one in Heer Land near the eastern coast of the island of Spitsbergen and two, in close proximity to one another, on the island of Nordaustlandet. All three zones are in regions which have been devoid of major orogenic activity since the late Devonian. the Heer Land zone and at least one of the zones in Nordaustlandet define active faulting which has not been identified by surface mapping. the other zone in Nordaustlandet occurs in the region of a mapped system of faults, all branches of which are relatively minor. A number of smaller concentrations and individual earthquakes occur throughout much of the archipelago and surrounding continental shelves. Seismicity is currently low in the region of Tertiary orogenic activity in western Spitsbergen. A roughly linear pattern of minor activity in southern Spitsbergen occurs west of the Heer Land zone and extends at least 50 km southward. the major N-S faults in Spitsbergen which are thought to have accomodated large displacements in Devonian time are currently inactive, except perhaps at the southern termini of the Billefjorden and Lomfjorden fault zones, where they approach the Heer land seismic zone. Earthquakes located near the western continental margin of Spitsbergen may occur on segments of rifting and shearing which reflect the opening of the Greenland Sea during the Tertiary. the limited extent of the currently active zones of activity, their spatial stability over several years, and the lack of activity on the major faults of Svalbard argue against the existence of plate boundaries, along which motion is currently occurring, in the Svalbard region.All fault-plane solutions on Svalbard are consistent with a stress field characterized by E-W compression, whereas previously published fault-plane solutions for the ridge system west of Svalbard indicate E-W extension. the magnitude of the compressive stress increases from small values near the ridge system to values sufficiently large to dominate the regional seismicity of Svalbard over distances of 200–300 km.

Notes: 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 (s̀H) 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.