ALBERT

Description: A long, complex aftershock sequence has followed an Mw 6.1 mainshock in February 2008, in Storfjorden, Svalbard. Relatively located aftershock epicenters and focal mechanisms from moment tensor inversion suggest that the series took place along a NE–SW trending and steeply SSE dipping oblique-normal fault, with the total aftershock extent exceeding the mainshock rupture. The magnitude of the mainshock and the duration of the aftershock activity render the Storfjorden sequence important in terms of earthquake hazard assessment in the Svalbard region. We have evaluated all involved uncertainties and combined seismological information with background knowledge for the area, in order to achieve as sound a seismotectonic interpretation as possible. Our results clearly indicate that the activity in Storfjorden should be attributed to a previously unknown tectonic structure. In addition, there are indications of activation of secondary structures and possibly of stress triggering. These new findings affect our understanding of the region's earthquake potential.

Description: Accurate seismotectonic studies on and around Spitsbergen, European Arctic, are impeded by a number of difficulties. The largely glaciated environment and the geographic distribution of land and sea restrict geotectonic mapping, as well as the deployment of dense, local seismic networks. The nature of tectonic seismicity in the region, which is episodic and dominated by moderate to low event magnitudes, imposes additionally its own challenges. So far, the best studied activity has been the Storfjorden 2008-2016 earthquake series (Mw 6.1, 21 February 2008; Mw 5.1, 29 March 2016) and background seismicity in the offshore area east of southern Spitsbergen, and to a lesser extent, seismicity in Heerland, on central Spitsbergen (mb 5.5, 18 January 1976). Despite the aforementioned challenges, the improvement of the regional seismic network from 2010 onward, has been offering enhanced insight into Svalbard seismotectonics, in particular in areas best covered by seismic station distribution. One such case, is the Hornsund area, where the calculation of moment tensor solutions was possible for two moderate magnitude, onshore events (Mw 4.3, 4 July 2015; Mw 4.6, 31 May 2017). The nearby broadband station facilitates event focal depth determination, insights into the local tectonic structure enabled by geological mapping. We will present the focal characteristics of the activity associated with these earthquakes and discuss their placement within the local and regional geotectonic setting.