EGU21-13998, updated on 10 Jan 2024
https://doi.org/10.5194/egusphere-egu21-13998
EGU General Assembly 2021
© Author(s) 2024. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Remi Matrau1, Yann Klinger2, Jonathan Harrington1, Ulas Avsar3, Esther R. Gudmundsdottir4, Thorvaldur Thordarson4, Armann Hoskuldsson4, and Sigurjon Jonsson1
Remi Matrau et al.
  • 1King Abdullah University of Science and Technology, Crustal Deformation and InSAR, Thuwal, Saudi Arabia (remi.matrau@kaust.edu.sa)
  • 2Université de Paris, Institut de Physique du Globe de Paris, CNRS, Paris, France
  • 3Middle East Technical University (METU), Ankara, Turkey
  • 4Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland

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.

How to cite: Matrau, R., Klinger, Y., Harrington, J., Avsar, U., Gudmundsdottir, E. R., Thordarson, T., Hoskuldsson, A., and Jonsson, S.: Paleoearthquakes on the Húsavík-Flatey Fault in northern Iceland: Where are the large earthquakes?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13998, https://doi.org/10.5194/egusphere-egu21-13998, 2021.