EGU2020-21749
https://doi.org/10.5194/egusphere-egu2020-21749
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Large magnitude earthquakes of late Holocene age in the Precambrian of Finnmark, Northern Norway

Odleiv Olesen1, Lars Olsen1, Steven Gibbons2, Tormod Kværna3, Bent Ole Ruud4, and Tor Arne Johansen4
Odleiv Olesen et al.
  • 1Geological Survey of Norway, Geophysics, Trondheim, Norway (odleiv.olesen@ngu.no)
  • 2NGI, Oslo, Norway,
  • 3NORSAR, Kjeller, Norway
  • 4University of Bergen, Bergen, Norway

The 80 km long Stuoragurra postglacial fault occurs within the c. 5 km wide Precambrian Mironjavri-Sværholt Fault Zone in the northern Fennoscandian Shield. Deep seismic profiling and drilling show that the fault dips at an angle of 30-40° to the southeast. The reverse fault can be traced down to a depth of c. 2.5 km on the reflection seismic profile. A total of c. 100 earthquakes has been registered along the fault between 1991 and 2019. Recordings at the ARCES seismic array in Karasjok c. 40 km to the SE of the fault and other seismic stations in northern Norway and Finland have been utilized. The maximum moment magnitude is 4.0. The Stuoragurra fault constitutes the Norwegian part of the larger Lapland province of postglacial faults extending southwards into northern Finland and northern Sweden. The formation of these faults has previously been associated with the deglaciation of the last inland ice. Trenching of different sections of the fault and radiocarbon dating of buried and deformed organic material reveal, however, a late Holocene age (between c. 700 and 4000 years before present at three separate fault segments). The reverse displacement of c. 9 m and segment lengths of 9-12 km of the two southernmost fault segments indicate a moment magnitude of c. 7. The results from this study indicate that the maximum magnitude of future earthquakes in Fennoscandia can be significantly larger than the existing estimate of c. 6.

How to cite: Olesen, O., Olsen, L., Gibbons, S., Kværna, T., Ruud, B. O., and Johansen, T. A.: Large magnitude earthquakes of late Holocene age in the Precambrian of Finnmark, Northern Norway, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21749, https://doi.org/10.5194/egusphere-egu2020-21749, 2020

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Presentation version 1 – uploaded on 06 May 2020
  • CC1: Comment on EGU2020-21749, Dan Clark, 07 May 2020

    This is a very interesting finding! I have some questions, if I may?
    1) does the Stuoragurra fault show evidence for an event immediately after ice retreat as well as your younger event? Looking at the trench log (slide 10) perhaps the boulders in the green sedimentary package relates to a colluvium from a 8.5-9.1 kyr BP event? If so, how is the slip distributed between the two events?
    2) is the fault mechanism consistent with the current tectonic crustal stress field, or the stress field from glacial rebound, or a mix?
    3) Can the Stuarogurra Fault be distinguished in the landscape as having hosted a younger event because it looks more youthful? On a related note, how many of the other Fennoscandian post glacial faults to you think might have hosted late Holocene events?  

    Thanks! 

    Dan Clark (Geosceince Australia)

    • AC1: Reply to CC1, Odleiv Olesen, 07 May 2020

      Hello Dan,

      Thanks a lot for your response and questions.

      1) We have currently C14 dates from three trenches. Faulting occurreed after c. 4000, c. 1200 and c. 800 years. Noen of the three trenches have signs of faulting 9000-10,000 years ago. We will make a new trench this summer. It will be located some distance away from the swamp that you can see in slide 10. We hope to reach deeper into the footwall block to look for signs of older faulting. The Maze fault system has a high fault scarp compared to the length. This could indicate two or more fault events even though we have not found any evidence for this yet.

      2) Most of Norway shows a compressional rock stress regime. GIA modelling generally assumes a prestress regime.

      3) Different periods of postglacial faulting are also evident from age dating of landslides in northern Finland (Ojala et al., 2018). Radiocarbon age data revealed three episodes of increased landslide formation associated with postglacial faulting, from 9000 to 11 000 cal BP, from 5000 to 6000 cal BP and from 1000 to 3000 cal BP.

      I can send you a preprint of chapter 6.1 in the Cambridge University Press book on Glacially-triggered Faulting. It was actually accepted today.

      With the best regards from

      Odleiv Olesen

      Geological Survey of Norway

      • CC2: Reply to AC1, Dan Clark, 08 May 2020

        Hi! I would very much be interested in reading your chapter. I had the pleasure of reviewing a different chapter in this book. Very interesting topic!

  • AC2: Comment on EGU2020-21749, Odleiv Olesen, 08 May 2020

    Thanks again for your interest.

    I hust sent a pdf file attached to an Email.

  • CC3: Comment on EGU2020-21749, Stéphane Baize, 13 May 2020

    Dear Odleiv, Nice results! Did you compare the timing of earthquakes with those documented in Finland?

    • AC3: Reply to CC3, Odleiv Olesen, 14 May 2020

      Radiocarbon age data revealed three episodes of increased landslide formation associated with postglacial faulting in northern Finland, from 9000 to 11 000 cal BP, from 5000 to 6000 cal BP and from 1000 to 3000 cal BP (Ojala et al. 2018). These ages deviates somewhat from the ages obtained for the the Stuoragurra fault complex in Norway (ca. 800 and younger that 1200 and 4000 cal BP).  We have not so far found any support for large-scale earthquakes occurring during the deglaciation period 9000-10000 years ago. We will, however, continue with more trenching and other field work during the coming autumn.