By Heidi Soosalu
The Katla volcano under the Mırdalsjökull glacier is known to have seasonal seismic activity earthquakes concentrate to the latter part of the year. The seismicity is interpreted to be related to the deloading of the thin crust above the magma chamber due to the summer melting of the ice cap, and to high groundwater pressure in the caldera roof at the same time.
There are two distinct seismic areas under Mırdalsjökull. The most active area is located at the Goğabunga rise in the western part of the glacier, and the other one in the Katla caldera in the middle part. The earthquakes in both of the places are volcanic in nature. They start with rather high frequencies, and the onset can be anything from clearly impulsive to emergent. The continuation of the signal consists of lower frequencies only. No clear S-wave are seen in most of the seismic records, especially not in the signals from Goğabunga.
Due to the nature of the Katla events it is somewhat difficult to estimate their magnitudes. However, they are seldom larger than local magnitude (ML) 4. It is worth noticing that the local population rarely feels Katla earthquakes, though inhabited areas are situated quite near. The depths of the Katla events are difficult to estimate, as well, but they seem to be shallow, within 0-5 km.
Since 1999 Katla has been showing signs of unrest. In July 1999 a small glacial flood lasting less than 24 hours was observed. It is possible that it was caused by an intrusion of magma or even a small subglacial eruption. So far this has not been followed by further eruptive activity. The seismicity in the year 1999, on the other hand, had the typical seasonal pattern and was not exceptional. During the year 2002 the seismic activity in the Katla area has risen dramatically. In the spring the earthquakes of the fall 2001 did not cease to occur in a typical manner, but continued throughout the year on a daily basis. Their magnitudes have been mostly below 2.5, but a couple of events larger than ML 3 have been observed.