Sedimentary Records of Past Earthquakes during the Last 2400 Years in Botnsvatn Lake on the Húsavík-Flatey Fault, North Iceland

Abstract:

The Húsavík-Flatey Fault (HFF), which is a part of the Tjörnes Fracture Zone (TFZ) in North Iceland, is a ~100 km-long dextral strike-slip fault. The fault has produced many magnitude 6-7 earthquakes in the past; however, the historical earthquake record in the region is mostly limited to the past 300 years. Almost 80% of the HFF is offshore. At the on-land part of the fault, glacial morphology and lack of well-developed fluvial systems limit the number of sites having faulting-related sedimentation, which is required for conventional paleoseismic trenching. Hence, Botnsvatn Lake, located in a small pull-apart basin close to the south-eastern end of the HFF, is a valuable site to study past earthquakes in the region. Several piston cores, maximum 4.1 m-long, were collected from the lake in 2013. The physical and geochemical properties of the sediments were investigated by means of magnetic susceptibility, gamma-ray density, total carbon/nitrogen/sulphur content, grain-size distribution analysis, radiographic images on u-channels and micro-XRF scanning. The chronology of the sediments was constructed by four radiocarbon dates and 62 tephra layers. The well-documented tephrochronology in Iceland provides a robust age control along the cores. Eight sedimentary events have been detected in the sedimentary sequence of Botnsvatn Lake. Some of the events can be attributed to increased sediment influx, probably due to seismically-triggered landslides in the catchment, while the other events appear as soft sediment deformations. The youngest event in the sequence temporally coincides with the historical 1872 AD earthquake (M=6.5) in the region. In the light of preliminary interpretations, “at least” eight earthquakes took place during the last 2400 years. In the coming months, the Botnsvatn sedimentary record will be compared in detail with the paleoclimatological records in the region in order to disentangle the climatic and seismic signals.