Evidence for Holocene Earthquakes along the Húsavík-Flatey Fault in North Iceland: Implications for the Seismic Behavior of Oceanic Transform Faults

Understanding the long-term seismic behavior of oceanic transform faults is challenging because their location underwater generally prevents the use of classical paleoseismological techniques. The Húsavík-Flatey fault (HFF) in northern Iceland, however, is a partially emerged oceanic transform fault accommodating 6–9 mm/yr of deformation, offering a unique opportunity to apply classic inland paleoseismic methods to decipher the Holocene earthquake history of an oceanic transform fault. We excavated three fault-orthogonal paleoseismic trenches at two locations on the HFF and identified nine surface rupturing earthquakes in the last 6–8 ka. We observe little to no deformation associated with the most recent large earthquakes of 1872 (Mw ∼6.5) and the penultimate earthquake in 1755 (Mw ∼7), suggesting that these earthquakes may have occurred mainly offshore, ruptured a fault strand not sampled here, or that their magnitudes may have been overestimated. From our observations, we estimate a return time of 600 ± 200 yr for the largest earthquakes on the HFF (Mw 7.2–7.3), and we suggest that the known historical earthquakes are likely not representative of the largest possible earthquakes on the fault. Furthermore, our observations suggest a quasi-periodic behavior and support the quasi-repeating earthquake sequences observed from instrumental earthquake catalogs on several oceanic transform faults

Ancient sedimentary DNA shows rapid post-glacial colonisation of Iceland followed by relatively stable vegetation until the Norse settlement (Landnám) AD 870

Understanding patterns of colonisation is important for explaining both the distribution of single species and anticipating how ecosystems may respond to global warming. Insular flora may be especially vulnerable because oceans represent severe dispersal barriers. Here we analyse two lake sediment cores from Iceland for ancient sedimentary DNA to infer patterns of colonisation and Holocene vegetation development. Our cores from lakes Torfdalsvatn and Nykurvatn span the last c. 12,000 cal yr BP and c. 8600 cal yr BP, respectively. With near-centennial resolution, we identified a total of 191 plant taxa, with 152 taxa identified in the sedimentary record of Torfdalsvatn and 172 plant taxa in the sedimentary record of Nykurvatn. The terrestrial vegetation at Torfdalsvatn was initially dominated by bryophytes, arctic herbs such as Saxifraga spp. and grasses. Around 10,100 cal yr BP, a massive immigration of new taxa was observed, and shrubs and dwarf shrubs became common whereas aquatic macrophytes became dominant. At Nykurvatn, the dominant taxa were all present in the earliest samples; shrubs and dwarf shrubs were more abundant at this site than at Torfdalsvatn. There was an overall steep increase both in the local accumulated richness and regional species pool until 8000 cal yr BP, by which time ¾ of all taxa identified had arrived. The period 4500-1000 cal yr BP witnessed the appearance of a a small number of bryophytes, graminoids and forbs that were not recorded in earlier samples. The last millennium, after human settlement of the island (Landnám), is characterised by a sudden disappearance of Juniperus communis, but also reappearance of some high arctic forbs and dwarf shrubs. Notable immigration during the Holocene coincides with periods of increased incidence of sea ice, and we hypothesise that this may have acted as a dispersal vector. Thus, although ongoing climate change might provide a suitable habitat in Iceland for a large range of species only found in the neighbouring regions today, the reduction of sea ice may in fact limit the natural colonisation of new plant species