Databank

Databank

Posiva publishes Working Reports and Posiva Reports. From the year 2006 nearly all the reports have been published on our webpage and they can be found in the databank. In the databank you can also find our Annual Reviews and some other publications as well. You can also find print-quality pictures and useful links in the databank.

Recent publications


POSIVA Report 2019-1

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Name:

Postglacial Faults in Finland -a Review of PGSdyn Project Results

Writer:

Ojala, A. E.K., Mattila, J., Ruskeeniemi, T., Markovaara-Koivisto, M., Palmu, J-P., Nordbäck, N,. Lindberg, A,. Sutinen, R., Aaltonen, I., Savunen, J.

Language:

English

Page count:

118

ISBN:

978-951-652-271-8 / ISSN 2343-4740

Summary:

Intraplate fault scarps that cross-cut glacial sediments, often referred to as ‘postglacial faults’ (PGFs), are distinctive features in northern Fennoscandia. Their existence indicates that the Fennoscandian Shield has experienced high-magnitude (Mw ≈ 6.5–8.2) postglacial earthquakes attributable to the release of lithospheric stresses during or after the retreat of continental glaciers, which were much larger than those recorded in historical and instrumental times. The earthquakes also triggered hundreds of landslides in glacial tills and seismically-induced soft sediment deformation structures (SSDS) that are common in the vicinity of the faults. The PGSdyn project combined remote sensing, geological and geophysical mapping, and drilling and trenching of bedrock and glacial sediment across the fault scarps and through the landslides in order to understand details of the reactivation mechanisms, timing and internal geometrical characteristics of the faults, as well as estimate the potential magnitude of palaeoearthquakes related to seismically-induced features in Finland. These estimates are important from a seismic hazard point of view, for example in the safety assessment of deep nuclear waste repositories, which need to consider seismic risks over a time period of up to one million years.

This report reviews the main results and conclusions of the PGSdyn project (2014–2018). The analyses are based on national-scale remote sensing using high-resolution digital elevation models (LiDAR DEM), which enhance the discovery and allow the detailed geomorphological analysis of PGF scarps and landslides, and site-specific studies with trenching, drilling, drillhole instrumentation and shallow geophysics, such as GPR and EM-anisotropy surveys. Accordingly, the discovered PGFs and palaeolandslides are concentrated in the SW part of Finnish Lapland, roughly in the area of Sodankylä, Kolari and Kittilä. In addition, the PGSdyn project revealed two previously unknown PGF systems at Lauhavuori in Isojoki, SW Finland, and at Korteaavankummut in Savukoski, eastern Finnish Lapland, which probably represent late- or postglacial seismicity. Of the discovered and mapped features, we conducted site-specific fieldwork on seven PGFs and drilled through 11 landslides in the vicinity of the PGFs to discover and date landslide-buried organic matter. Regional fieldwork was also conducted in the Olkiluoto and Kuusamo areas to discover possible sediment disturbance structures (SSDS) and liquefaction spreads and craters that could be connected to late- or postglacial earthquakes.

The main conclusions of the PGSdyn project are as follows: (I) The PGF zones are generally characterized by a number of discrete 0.2–9.1-km-long fault scarp segments that often run parallel to each other and form longer PGF systems. A set of PGF systems further forms a PGF complex that is typically tens of kilometres long. Concurrently, we have identified 18 PGF systems that make up 9 PGF complexes in Finland. (II) The PGF systems and complexes in northern Fennoscandia generally strike in SW–NE directions and represent the reactivation of pre-existing bedrock fault zones. (III) Several of the excavated sites indicate that multiple rupturing events can take place within PGF complexes where different and/or the same PGF segments were activated or reactivated at different times. The results provide evidence of non-stationary late- and postglacial seismicity in Finnish Lapland, which indicates, from the perspective of seismic hazard assessment, that the assumption of postglacial earthquakes taking place in one event is overly conservative. (IV) Stratigraphic sections of the trenched PGF segments, age data on landslide-buried organic matter and the observed SSDS in lacustrine and marine sediments indicate that palaeoseismicity in the Fennoscandian Shield area is not limited to a short period during and after the Late Weichselian deglaciation. Instead, the non-stationary seismicity has probably appeared (episodically) throughout the Holocene and the same PGF complexes were also active after glacial phases that predated the Late Weichselian glacial maximum. The spatial arrangement of the landslide age data from northern Finland also indicates that certain ages do not cluster around specific postglacial faults but rather occur sporadically and, furthermore, that landslides associated with a specific PGF system and complex may have different ages. (V) Considering that the PGF complexes are often composed several PGF systems and isolated segments, we calculated the potential moment magnitudes based on the rupture length and mean/maximum vertical displacement for each individual PGF segment and system, taken that they potentially ruptured independently, and finally for the entire PGF complexes using scaling laws for faults from stable continental regions. Assuming a single rupture event for each segment, surface length values indicate moment magnitudes of Mw ≈ 2.9–6.1 and displacement data Mw ≈ 2.9–8.0 or even up to unrealistic Mw ≈ 9.5. At the PGF complex scale, the rational length-derived moment magnitude estimates range between Mw ≈ 4.9–7.4 and the displacement-derived moment magnitudes between Mw ≈ 4.6–8.7. Accounting for PGF system scale assessment of moment magnitudes, the surface rupture length data yield moment magnitudes in the range of Mw ≈ 4.9–7.0 and displacement data values in the range of Mw ≈ 4.6–8.7. Excluding the most extreme moment magnitude estimates, we consider that a realistic moment magnitude range for the earthquakes that took place in Finnish Lapland is in the range of Mw ≈ 4.9–7.5. Based on current understanding, we consider that the system-based moment magnitude estimates yield the most realistic values, as these consist of closely-associated segments that are most likely linked to the same rupturing event. The defined complexes, on the other hand, may present larger linked fault zones, but due to the large non-ruptured gaps between the complexes, we consider that these represent isolated rupture events. The estimates are in accordance with maximum moment magnitude calculations of Mw ≈ 6.9–7.5 based on landslide volume-area data.

Keywords:

Paleoseismicity, Earthquake, Moment magnitude, Postglacial fault, Landslide, Finland, Fennoscandian Shield

File(s):

Posiva 2019-01_web (pdf) (21.8 MB)


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