Paleoearthquakes on the Húsavík-Flatey Fault in northern Iceland: Where are the large earthquakes?

Paleoseismology is key to study earthquake recurrence and fault slip rates during the Late Pleistocene-Holocene. The Húsavík-Flatey Fault (HFF) in northern Iceland is a 100 km-long right-lateral transform fault connecting the onshore Northern Volcanic Zone to the offshore Kolbeinsey Ridge and accommodating, together with the Grímsey Oblique Rift (GOR), ~18 mm/yr of relative motion between the Eurasian and North American plates. Significant earthquakes occurred on the HFF in 1755, 1838 and 1872 with estimated magnitudes of 6.5-7. However, historical information on past earthquakes prior to 1755 is very limited in both timing and size.

We excavated five trenches in a small basin (Vestari Krubbsskál) located 5.5 km southeast of the town of Húsavík and at 300 m.a.s.l. and one trench in an alluvial fan (Traðargerði) located 0.5 km north of Húsavík and at 50 m.a.s.l. In a cold and wet environment, such as in coastal parts of Iceland, one has to take into account periglacial processes affecting the topsoil to discriminate tectonic from non-tectonic deformation. We used tephra layers in the Vestari Krubbsskál and Traðargerði trenches as well as birch wood samples in Traðargerði to constrain the timing of past earthquakes. Tephra layers Hekla-3 (2971 BP) and Hekla-4 (4331±20 BP) are visible in the top half of all the trenches. In addition, a few younger tephra layers are visible in the top part of the trenches. In Traðargerði several dark layers rich in organic matter are found, including birch wood-rich layers from the Earlier Birch Period (9000-7000 BP) and the Later Birch Period (5000-2500 BP). In Vestari Krubbsskál the lower halves of the trenches display mostly lacustrine deposits whereas in Traðargerði the lower half of the trench shows alluvial deposits overlaying coarser deposits (gravels/pebbles) most likely of late-glacial or early post-glacial origins. In addition, early Holocene tephra layers are observed in some of the trenches at both sites and may correspond to Askja-S (10800 BP), Saksunarvatn (10300 BP) and Vedde (12100 BP). These observations provide good age constraints and suggest that both the Vestari Krubbsskál and Traðargerði trenches cover the entire Holocene.

Trenches at both sites show significant normal deformation in addition to strike-slip, well correlated with their larger scale topographies (pull-apart basin in Vestari Krubbsskál and 45 m-high fault scarp in Traðargerði). We mapped layers, cracks and faults on all trench walls to build a catalogue of Holocene earthquakes. We identified events based on the upward terminations of the cracks and retrodeformation. Our results yield fewer major earthquakes than expected, suggesting that large earthquakes (around magnitude 7) are probably rare and the more typical HFF earthquakes of magnitude 6-6.5 likely produce limited topsoil deformation.[yk1]  Our interpretation also suggests that the Holocene slip rate [yk2] for the fault section we are studying may be slower than the estimated geodetic slip rate (6 to 9 mm/yr)[yk3]  for the entire onshore HFF, although secondary onshore sub-parallel fault strands could accommodate part of the deformation.