Analysis of patterns and temporal behavior of seismic activity at Katla volcano, Iceland

Katla is an ice-covered central volcano in the southern part of Iceland’s Eastern Volcanic Zone. It is one of the country's most hazardous volcanoes due to its frequent explosive hydro-magmatic, basaltic eruptions and proximity to inhabited areas. The central volcano has a large ~100 km², ice-filled caldera and an associated fissure swarm extending 70 km towards NE. Most frequently, eruptions are explosive and inside the caldera, but the largest effusive eruptions occur on the fissure swarm. In the last millennium, over 20 explosive eruptions have occurred inside the caldera, and these eruptions have been accompanied by large subglacial floods (jökulhlaups) causing severe flood risks in glacial rivers draining from the ice-cap. Several geothermal areas are located inside the caldera, where geothermal heat melts the overlying ice and meltwater accumulates at the glacier bed. These melt-water pockets regularly drain and cause smaller jökulhlaups in the surrounding glacial rivers. Manifestations of these areas are depressions, or ice cauldrons, on the overlying glacier surface and high seismicity, concentrated at shallow levels in the underlying crust. Research on the effects of glacial isostatic rebound of Katla on magma production in the underlying mantle has been ongoing in the ISVOLC project. In the presentation we focus on the analysis of seismicity recorded at Katla during the last 35 years and interpret the results together with other multidisciplinary observations during this period. Katla is a very seismically active volcano with nearly 38 thousand earthquakes recorded since the beginning of the national digital seismic network SIL (VI). Around 90% of this activity is equally divided between the caldera and the NW flank (Godabunga). The earthquakes are predominantly shallow, or 90% in the top 5 km and this is also the depth range of the largest earthquakes, whose magnitude can be up to Mw~4.5.  Only 4% of the activity is below 10 km. To improve earthquake location-accuracy and enable mapping of the volcano’s subsurface plumbing, as well as evolution of the seismicity, relative relocations (DD), using cross-correlation of waveforms from 15 thousand earthquakes in the magnitude range 0.4<M<3, was carried out. The relocated shallow seismicity reveals several distinct clusters within the caldera and the time and spatial evolution of these clusters are interpreted together with observations of cauldron developments and drainage, as well as observations of deformation through GPS over the last three decades. The relocations also reveal a distinct vertically elongated earthquake cluster at 17-25 km depth under the eastern caldera rim. This activity, which often comes in bursts, is most prominent from 2011-2021, peaking in 2014-2015. We infer that this cluster represents magmatic intrusions into the crust from the mantle below.