The Nordland region, Northern Norway, situated in an intraplate continental setting, has the highest seismicity rate in mainland Norway. Better understanding of factors that influence the seismicity in Nordland can help increase knowledge of intraplate seismicity in general. However, the exact cause of seismicity in this region is still debated. Here, we address this problem with using a high-resolution 3-D VPand VP/VS ratio images of the crust in Nordland using seismic travel-time tomography. These images show the existence of a localized, 10 - 15 km Moho step that runs parallel to the coast. The north-south extent of this step coincides with the region that exhibits the highest rates of seismicity. Focal mechanisms of selected earthquakes computed in this study are dominated by normal and oblique-normal, indicating a coast-perpendicular extension. The coast-perpendicular extensional stress regime deviates from the regional compression imposed by the ridge push from the North Atlantic. This deviation is thought to stem from the additional interference with local flexural stress caused by sediment redistribution and glacial isostatic adjustment, and possibly exacerbated by gravitational potential energy stress associated with the Moho jump. The deformation due to the extensional regime is localized on pre-existing faults and fractures along the coastline. The tomography result shows that two distinct seismic swarms occurred in the coastal area with low VP and variable VP/VS ratio anomalies, pointing towards fractured crust and possibly the presence of fluids. The existence of fluids here can change the differential stress and promote seismic rupture.

Over the last decades, the receiver function technique has been widely used to image sharp discontinuities in elastic properties of the solid Earth at regional scales. To date, very few studies have attempted to use receiver functions for global imaging. One such endeavour has been pursued through the project “Global Lithospheric Imaging using Earthquake Recordings” (GLImER). Building on the advances of GLImER, we have developed PyGLImER - a Python-based software suite capable of creating global images from both P-to-S and S-to-P converted waves via a comprehensive receiver function workflow. This workflow creates a database of receiver functions by downloading seismograms from selected earthquakes and analysing the data via a series of steps that include pre-processing, quality control, deconvolution, and stacking. The stacking can be performed for common conversion points or single stations. All steps leading to the creation of receiver functions are automated. To visualise the generated stacks, the user can choose the desired survey area in a graphical user interface, and then explore the selected region either through 2D cross-sections or a 3D volume. By incorporating results from two independent seismic phases, we can combine the advantages of both phases for imaging different discontinuities. This results in an increased robustness and resolution of the final image. For example, we can use constraints from S receiver function images, which are multiple-free but relatively low resolution, to differentiate between real lithospheric/asthenospheric structures and multiple-induced artefacts in higher-resolution P receiver function images. Our preliminary results agree with those from recent regional and global studies, confirming the workflowís robustness. They also indicate that the new workflow combining P and S receiver functions has the potential to resolve global lithospheric discontinuities such as the lithosphere-asthenosphere boundary (LAB) or the midlithospheric discontinuity (MLD) more reliably than approaches using only one type of incident phase. PyGLImER will be distributed as open-source software, providing an easily accessible tool to rapidly generate high-resolution images of structures in the lithosphere and asthenosphere over large scales.