Postglacial reactivation of the Suasselkä PGF complex in SW Finnish Lapland

Abstract

The Suasselkä fault complex is recognized from high-resolution digital elevation models (LiDAR DEMs) as a complex of 37 surface ruptures striking SW–NE and cutting through Weichselian glacial sediments, confirming the postglacial nature of the rupturing event. The analysis of LiDAR DEMs, geophysical profiles and trenching of glacial sediments at two sites along the Suasselkä postglacial fault (PGF) system has provided evidence of earthquakes that post- and predate the Late Weichselian deglaciation. At the Suaspalo trenching site, at least two different rupture events are observed: the first one predates the deposition of the lowermost till unit and potentially dates to the Early or even pre-Weichselian. The second rupture event, on the other hand, deforms overlying tills and is thus a post-Weichselian event. Our results indicate that multiple rupturing events can take place within ‘postglacial fault complexes’, which is the second time such a characteristic has been proven for PGFs in northern Fennoscandia. Furthermore, the present results provide the first evidence of either Early or even pre-Weichselian faulting in Finland, suggesting that the same fault complexes were also active after glacial phases that predated the last glacial maximum of the Late Weichselian. These results warrant re-evaluation of the long-term seismic risk associated with the faults as the assumption of single rupture events is erroneous. Considering that the Suasselkä PGF complex is composed of four PGF systems and 37 isolated 150–7500-m-long segments, we calculated the potential moment magnitudes based on the rupture length and mean/maximum vertical displacement for each individual segment and system, taken that they potentially ruptured independently, and finally for the entire Suasselkä PGF complex. For the entire complex rupturing in a single event, the observed displacement and length of the complex yields a moment magnitude estimate of Mw ≈ 6.7–8.1. However, based on geomorphological and sedimentological evidence, we believe that this is a conservative estimate of the moment magnitude of the paleoearthquake. If isolated PGF systems are considered to have ruptured individually, the moment magnitudes estimated based on surface rupture lengths range from Mw ≈ 5.5–6.7, and based on mean and maximum cumulative displacement values, the estimated values range from Mw ≈ 6.2–7.3 and Mw ≈ 6.7–8.0, respectively. As we cannot with the available data distinguish the exact number of events, the age of events or which segments potentially ruptured at the same time, we conclude that the events that took place in the complex ranged in moment magnitude from c. Mw ≈ 5.5 to 8.1, the Mw ≈ 8.1 estimate representing conservative upper bound for the maximum moment magnitude.