Ultraslow spreading mid‐ocean ridges have a low magma budget and melt is distributed unevenly along the ridge axis. There is little or no basaltic crust between isolated magmatic centers. The processes that focus melts to segments of robust magmatism are not yet understood. During a seismic survey of the ultraslow spreading Knipovich Ridge in the Norwegian‐Greenland Sea with ocean bottom seismometers, we discovered a seismic gap in the upper mantle beneath Logachev Seamount, where micro‐earthquakes clearly delineate a shallowing of the maximum depth of faulting. A topography of the lithosphere that allows melts to travel laterally along its base and rise in areas of thin lithosphere has been proposed as a possible mechanism to explain the focusing of melts at volcanic centers, but has never been confirmed observationally. Our results are the first geophysical evidence for an along‐axis variation of the lithospheric thickness at an ultraslow spreading ridge.

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Ambient seismic noise is caused by a number of sources in specific frequency bands. The quantification of ambient noise makes it possible to evaluate station and network performance. We evaluate noise levels in Norway from the 2013 data set of the Norwegian National Seismic Network as well as two temporary deployments. Apart from the station performance, we studied the geographical and temporal variations, and developed a local noise model for Norway. The microseism peaks related to the ocean are significant in Norway. We, therefore, investigated the relationship between oceanic weather conditions and noise levels. We find a correlation of low-frequency noise (0.125–0.25 Hz) with wave heights up to 900 km offshore. High (2–10 Hz) and intermediate (0.5–5 Hz) frequency noise correlates only up to 450 km offshore with wave heights. From a geographic perspective, stations in southern Norway show lower noise levels for low frequencies due to a larger distance to the dominant noise sources in the North Atlantic. Finally, we studied the influence of high-frequency noise levels on earthquake detectability and found that a noise level increase of 10 dB decreases the detectability by 0.5 magnitude units. This method provides a practical way to consider noise variations in detection maps.Electronic supplementary materialThe online version of this article (doi:10.1007/s10950-016-9566-8) contains supplementary material, which is available to authorized users.

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Norway is part of an intraplate environment and therefore experiences low to intermediate seismicity. This seismicity is a response to the stress field in the lithosphere, which is mainly influenced by ridge push from the Mid-Atlantic ridge. In order to analyse the seismicity in Norway, this thesis first quantifies the ambient seismic noise levels and the detection capability of the Norwegian National Seismic Network (NNSN). Following, it assesses Lg wave attenuation and estimates earthquake source parameters.The recordings of the NNSN in 2013 are analysed in terms of ambient seismic noise levels. A local noise model for Norway is derived, and geographic and temporal variations are assessed. The sources of ambient seismic noise have specific frequency bands, such as the microseismic peak (4-8 s), which relates to oceanic waves. This thesis obtains a correlation between the microseismic peak and wave heights up to 900 km offshore.Furthermore, a correlation between human activity, especially in the bigger cities, and daily noise level variations is observed. In particular, those noise level variations are used to quantify the station and network performance in Norway. The network capability to detect local and regional events decreases by 0.5 units of magnitude if high frequency noise generated by human activity increases by 10 dB. This observation is incorporated into the presented detection threshold map of Norway.In order to assess Lg wave propagation, this thesis analyses attenuation of Lg waves using 1369 observations from 279 earthquakes recorded between 1990 and 2017. Initially, Lg wave propagation is quantified through Lg/Pn amplitude ratios. High Lg/Pn ratios, as obtained for onshore regions, imply efficient Lg wave propagation. Offshore regions show mainly inefficient Lg propagation. In order to study this in more detail, Lg wave attenuation was calculated. The calculation obtains an average attenuation of Q Lg (f ) = 529f 0.42 for mainland Norway. Using a tomographic inversion approach, three tomographic maps are presented. These show Lg wave attenuation at 2 Hz, 4 Hz and 6 Hz. The maps reveal significant variations between on-and offshore regions, with higher attenuation offshore. Changes in crustal structure and unconsolidated sediii iments are thought to be the cause of the relatively high Lg wave attenuation.Finally, this thesis presents estimated earthquake source parameters from events located in Norway and the Svalbard archipelago. The earthquake source parameters are derived using the empirical Green's function method. Between January 1990 and May 2018, the database of the NNSN contains 263 earthquake pairs to which the method was applied. The corresponding 107 master events have a local magnitude range of 1.3-3.4. Assuming a Brune source model, stress drops between 0.4 bar and 355 bar are obtained. We observe increasing stress drop with increasing seismic moment, which contradicts earthquake self-similarity. iv List of publications and authorship statementThe main part of the thesis, which consi...