The Katla volcanic system imaged using local earthquakes recorded with a temporary seismic network

Katla is one of the most active subglacial volcanoes in Iceland. A temporary seismic network was operated on and around Katla for 2.5 years. A subset of similar to 800 analyzed local earthquakes clustered geographically in four regions: (1) the caldera, (2) the western region, (3) the southern rim, and (4) the eastern rim of the glacier. Based on the frequency content of recorded seismograms, each event was labeled as volcano tectonic (VT), long period (LP), or 'Mixed'. The southern cluster consists of LP events only, and the eastern cluster consists of VT events, while the western cluster is 'Mixed' although primarily LP. The caldera seismicity is confined to a subregion centered in the northeastern part of the caldera above 1 km below sea level (bsl) and gradually deepens away from its center to about 4 km depth. Deeper events are almost all VT, whereas LP events in the center of caldera locate at shallow depths. This is also where the velocities are lowest in the top 3 km of the crust of our 3-D tomographic model. A high-velocity core (similar to 6.5 km/s) is found at 4 km bsl beneath this low-velocity zone. We propose that a subcaldera may be developing within the present caldera and suggest a conceptual model for Katla volcano with a thin volume (similar to 1 km thick) that may host hot rhyolitic material in the shallow crust below the relocated seismic activity and above the high-velocity core. We interpret this core to consist of mafic cumulates resulting from fractionation of mafic intrusions and partial melting of subsiding hydrothermally altered rocks.