Two eruptions from Katla (in 1721 and 1918) and one eruption from Hekla (the so named Hekla Z eruption which took place somewhere between 550 and 950 BC) were chosen as the test case studies for this project. Thickness measurements and samples were collected across the footprint of each deposit and combined with previously published and unpublished datasets. The grain size distribution was analysed at 18 locations for Katla 1721, 37 locations for Katla 1918, and 7 locations for Hekla Z. The Katla 1721 and 1918 deposits are very fine grained and well sorted at most outcrops, consisting dominantly of ash (<2 mm in diameter). The Hekla Z deposit is considerable coarser and with <15% ash at the most proximal locations.
Deposit thickness was converted to tephra load (i.e. mass per area) using an average measured bulk deposit density of 920 kg m-3 for Katla 1721, 1010 kg m-3 for Katla 1918, and 520 kg m-3 for Hekla Z and used as the input for inversion modelling. Eruption Source Parameters for each eruption were estimated using the TEPHRA2 model in inversion mode, run on the High Performance Computing (HPC) computer cluster at University of Geneva. For Katla 1721, the erupted mass was estimated to be 2.1×1011–2.2×1011 kg, equivalent to a volume of 0.2–0.3 km3, with a plume height of about 19 km. For Katla 1918, the erupted mass was estimated to be 2.1×1011–3.9×1012 kg, equivalent to a volume of 0.2–3.9 km3, with a plume height in the range of 15–23 km. For Hekla Z, the erupted mass was estimated to be 6.8×1010–2.7×1011 kg, equivalent to a volume of 0.1–0.5 km3, with a plume height of about 16 km.
On the volcanic explosivity index, Katla 1721 was a VEI 4, Katla 1918 a 4–5, and Hekla Z a 4. On the magnitude scale, Katla 1721 was a M 4.3–4.5 Katla 1918 a 4.3–5.6 and Hekla Z a 3.8–4.4. On the intensity scale, Katla 1721 was a 10.6 Katla 1918 a 10.2–11.0 and Hekla Z a 10.3.
