Low-frequency earthquakes at the Torfajِkull volcano, south Iceland

Torfajökull is a large rhyolitic volcanic edifice with a 12-km-diameter caldera and abundant high-temperature geothermal activity. It is located in the neovolcanic zone in south Iceland, at the junction of the eastern rift zone and a transform zone with the intraplate volcanic flank zone of south Iceland. The latest eruption at Torfajökull occurred about 500 years ago. Torfajökull is a source of persistent small-scale seismicity, where two types of earthquakes occur. High-frequency events are concentrated in the western part of the caldera and low-frequency events cluster in the south. Small low-frequency earthquakes have been observed at Torfajökull since the installation of a local analogue seismograph station in 1985. They typically occur in swarms; up to 300 earthquakes per day have been observed. The low-frequency events have a frequency content of about 1–3 Hz, and are difficult to locate, because of the emergent nature of their phases. The 160 events located during the years 1994–2000 using the permanent Icelandic seismic network cluster in the southern part of the Torfajökull caldera. A closer study of low-frequency events was carried out between May and October 2002, with a dense network of twenty Güralp 6TD broadband seismometers in the Torfajökull area. No distinct swarm activity was observed during this period, but small low-frequency events occurred almost on a daily basis. About 330 low-frequency events were detected during the study period. They are located in the southern part of the caldera, between two small glaciers. Areas of intensive geothermal activity surround the cluster of low-frequency events. It is argued that these earthquakes are associated with active magma in the south part of the Torfajökull caldera, possibly a rising cryptodome.