Abstract
Abstract. A new seismic reflection survey for imaging deeper levels of the end-glacial Pärvie fault system in northern Sweden was acquired in June 2014. The Pärvie fault system hosts the largest fault scarp so far documented in northern Scandinavia, both in terms of its length and calculated magnitude of the earthquake that generated it. Present-day microearthquakes occur along the length of the fault scarp on the eastern side of the scarp, in general agreement with an east-dipping main fault. In the central section of the fault system, where there is a number of subsidiary faults east of the main Pärvie scarp, it has been unclear how the earthquakes relate to the structures mapped at the surface. A seismic profile across the Pärvie fault system acquired in 2007, with a mechanical hammer as a source, showed a good correlation between the surface mapped faults and moderate to steeply dipping reflections. The most pronounced reflectors could be mapped to about 3 km depth. In the new seismic survey, for deeper penetration an explosive source with a maximum charge size of 8.34 kg in 20 m deep shot holes was used. Reflectors can now be traced to deeper levels with the main 65° east-dipping fault interpreted as a weakly reflective structure. As in the previous profile, there is a strongly reflective 60° west-dipping structure present to the east of the main fault that can now be mapped to about 8 km depth. Extrapolations of the main and subsidiary faults converge at a depth of about 11.5 km, where current earthquake activity is concentrated, suggesting their intersection has created favorable conditions for seismic stress release. Based on the present and previous seismic reflection data, we propose potential locations for future boreholes for scientific drilling into the fault system. These boreholes will provide a better understanding of the reflective nature of the fault structures and stress fields along the faults at depth.
Highlights
During the last glaciation the ice that covered much of northern Europe, with thickness in excess of 3 km, gave rise to dynamic changes in the stress field of the Fennoscandian Shield (Pascal et al, 2010)
Important questions concerning the end-glacial faults are as follows: (i) what is the geometry of the faults at depth and their extent? (ii) What was the stress field that caused these ruptures during the last deglaciation? (iii) What is the stress field at depth that is generating the current earthquake activity along the faults
In order to resolve the geometry of the Pärvie fault system deeper than the previous studies (Juhlin et al, 2010), the new seismic survey in the area was acquired in June 2014 using an explosive source
Summary
During the last glaciation the ice that covered much of northern Europe, with thickness in excess of 3 km, gave rise to dynamic changes in the stress field of the Fennoscandian Shield (Pascal et al, 2010). It was expected that the new survey would verify the geometry of the main Pärvie fault and allow mapping of the subsidiary faults to deeper levels in the crust, but with a lower-resolution image of the uppermost few kilometers. This was the case, and together with re-migrated sections of the previous data with the velocity model obtained from Lindblom et al (2015) we show how the fault system can be mapped to greater depth and how the obtained image can be correlated with the earthquakes occurring in the area
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