Ambient noise levels, QLg wave tomography and earthquake source parameters in Norway

Abstract

Norway is part of an intraplate environment and therefore experiences low to intermediate seismicity. This seismicity is a response to the stress eld in the lithosphere, which is mainly in uenced by ridge push from the Mid-Atlantic ridge. In order to analyse the seismicity in Norway, this thesis rst quanti es 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 speci c 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 o shore. 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 quanti ed through Lg/Pn amplitude ratios. High Lg/Pn ratios, as obtained for onshore regions, imply e cient Lg wave propagation. O shore regions show mainly ine cient Lg propagation. In order to study this in more detail, Lg wave attenuation was calculated. The calculation obtains an average attenuation of QLg(f) = 529f0:42 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 signi cant variations between on- and o shore regions, with higher attenuation o shore. Changes in crustal structure and unconsolidated sediments 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.