Seismicity crisis at the Katla volcano, Iceland—signs of a cryptodome?

Katla, one of the most active and hazardous Icelandic volcanoes, has shown signs of unrest since 1999. In July that year a small glacial flood lasting a few hours was observed, accompanied by abnormal seismicity. It was probably caused by a short-lived, shallow magma intrusion or possibly a small subglacial eruption. In 2002 seismic activity increased dramatically, particularly in the Goðabunga area west of the caldera, and earthquakes have occurred continuously ever since. A network of four portable seismometers was run at Katla in the spring–summer 2003, with one station directly at the locus of the Goðabunga activity. These observations are combined with the data of the permanent Icelandic digital seismograph network, run by the Icelandic Meteorological Office. The earthquakes at Goðabunga are concentrated in a small area with a diameter of 3 to 4 km, and the events are shallow, mainly within the uppermost 2 km. We propose that the unusual pattern of seismicity indicates an intruding hot and acidic cryptodome. Apparently this dome has been slowly rising for decades but its propagation towards the surface accelerated some time around 1999, possibly triggered by a basaltic intrusion. This may lead to a silicic dome-building eruption in the Goðabunga area, which should be taken into consideration when estimating the eruption hazard at Katla.