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

Abstract

ABSTRACT 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.