Intraplate seismicity in Nordland, Northern Norway: Insight from Seismic Tomography

The Nordland region has the highest seismicity rate in mainland Norway. Earthquakes occur mostly along the coast and offshore along the passive margin. The largest known earthquake was the M 5.9, 1819 Lurøy earthquake, which was widely felt throughout Scandinavia. In recent times, earthquake swarms were recorded in several locations along the coast of Nordland . The exact cause of seismicity is not known, but a combination of the following factors has been proposed: (1) post-glacial deformation, (2) sedimentary loading and topography effects, (3) ridge push effect, (4) continental margin effect, and (5) lateral variation within the earth lithosphere. Better understanding the causative mechanisms of seismicity in Nordland can help increase knowledge of intraplate seismicity in general. An investigation of the causative mechanism requires a high-resolution image of crustal structure in the region. This kind of model is not yet available, although the offshore region have been previously mapped in detail by active seismic surveys. The aim of this study was thus to address this shortcoming by producing high-resolution images of the crustal structure in Nordland (between 66 ^o N to 69 ^o N) using seismic travel-time tomography. We developed 3-D V _P , and V _P /V _S models using data from 530 selected earthquakes recorded by 80 seismic stations in Northern Norway and the surrounding regions. Our tomographic images recover structure in the crust, down to approximately Moho depths. These images show the existence of a localized, ~10 to ~15 km Moho step paralleling the coast. The north-south extent of this step coincides with the region that exhibits the high est rates of seismicity. Th is Moho step was not resolved by previous models, in which the transition between Lofoten and the mainland appeared to be relatively smooth. W e also computed the focal mechanisms of earthquakes in the coastal region of Nordland using ray parameters computed in the 3-D V _P model. The focal mechanisms are dominated by normal faulting mechanisms with strike directions parallel to the coastline, which indicates an extensional stress regime in this area. We use the 3-D velocity model and focal mechanisms to explain the effect that lateral variation s in crust al depth together with other processes may have on the generat ion of intraplate seismicity in the region.