Dating of paleolandslides in western Finnish Lapland

Abstract

The dating of landslide‐buried organic materials potentially indicates non‐stationary seismicity in northern Finland attributable to the release of lithospheric stresses during and after retreat of the Scandinavian Ice Sheet. The landslide age data reveals three episodes of increased slope instability and formation of landslides, from 9000 to 11 000 cal BP, from 5000 to 6000 cal BP, and from 1000 to 3000 cal BP. While a seismogenic origin cannot be unequivocally established, we interpret that at least the early Holocene episode reflects increased seismic activity in northern Finland in association with late‐glacial and postglacial faulting. The foci of slope instabilities changes through time, implying that different segments of the postglacial fault systems were active at different times during the Holocene. We also show that the correlation of landslide ages with the surface roughness and backwall slope is complicated. The morphology of landslide scarps is significantly affected by thickness of glacial sediments, liquefaction during landslide formation, and accumulation of peat upon landslide scars and deposits, and thus, the time‐dependent erosional smoothing of the surfaces should be considered as descriptive and non‐qualitative. Copyright © 2018 John Wiley & Sons, Ltd.

Postglacial reactivation of the Suasselkä PGF complex in SW Finnish Lapland

“…The Vänköselkä landslide, located about 10 km SE of the Retu site ( Fig. 2), is the only landslide discovered within 30 km of the present study areas (Ojala et al 2018(Ojala et al , 2019. Landslides are common farther SW of the study sites, in the vicinity of the Sirkka PGF system and Isovaara-Riikonkumpu PGF complex, where topographical relief is higher.…”

Geomorphological evidence of paleoseismicity: Surficial and underground structures of Pasmajärvi postglacial fault

“…The Ruokojärvi case shows subglacial breakdown structures of the esker intersected by the fault occurring probably around 11300-11500cal BP, and deformation structures within a nearby esker formed approximately around 10 000-10500cal BP. In addition to the new evidence, nearby landslides have buried organic matter around 10200cal BP (Ojala et al, 2018) and basal dates of peat bogs dammed by the Pasmajärvi fault present minimum ages of 9200-9500cal BP (Ojala et al, 2018). Therefore, the Pasmajärvi PGF multi-ruptured like many other PGFs in northern Sweden and Finland, and it probably ruptured twice between about 9200 and 11500cal BP.…”

“…Although the age of the seismic activityand especially slip events of the same and/or adjacent fault segmentsis thus far rather poorly constrained in Fennoscandia, most available information indicates that slip on the fault occurred mostly during or shortly after deglaciation, at around 10000-11300cal BP (e.g. Lagerbäck and Witshard, 1983;Lagerbäck and Sundh, 2008;Sutinen et al, 2009;Olesen et al, 2013;Smith et al, 2014Smith et al, , 2017Mikko et al, 2015;Lund et al, 2017;Ojala et al, 2018), which agrees well with the modelled bedrock stress field under a glacial loading and unloading (Wu et al, 1999;Hampel et al, 2009). The present evidence from and around the Ruokojärvi study site is consistent with this age trend, and highlights the complexity and fault reactivation during and soon after deglaciation.…”

“…In the vicinity of PGFs in western Finnish Lapland are inferred earthquake-induced landforms, such as paleolandslides, which give indirect evidence of the moment magnitude and age of past seismic activity (Ojala et al, 2018(Ojala et al, , 2019. Landslides close to the postglacial faults have been interpreted as subaerial co-seismic structures (Lagerbäck and Sundh, 2008;Sutinen et al, 2014bSutinen et al, , 2018Ojala et al, 2017Ojala et al, , 2019, whereas soft-sediment deformation structures interpreted to be seismites developed in a submerged environment below the highest shoreline of the Baltic Sea basin in Finland or an (ice) lake (Lagerbäck and Sundh, 2008;Alsop and Marco, 2011;Brandes and Winsemann, 2013). Co-seismic features might also be indicators of possible strike-slip faulting, when the ALS DEM is unable to reveal PGF scarps in the terrain, as mentioned by Ojala et al (2019) when considering landslides.…”

Potential secondary events caused by early Holocene paleoearthquakes in Fennoscandia – a climate-related review

“…1). If related to fault scarps many of the abundant landslides and landforms might be caused by old faults reactivated during the early Holocene (Lagerbäck & Sundh, 2008;Mörner, 2009;Lindqvist, 2010;Berglund & Dahlström, 2015;Mikko et al, 2015;Palmu et al, 2015;Mangerud et al, 2018;Ojala et al, 2018a).…”