Iceland: Restless Katla Volcano Heightens Fears Of Possible Eruption

From Irish Weather ONline: Restless Katla Volcano Heightens Fears Of Possible Eruption

An increase overnight of seismic actvity in the vicinity of the Katla volcano in southern Iceland has heightened fears of a possible eruption.

Katla is located on the Mýrdalsjökull glacier, which is the southernmost glacier in Iceland and is almost 600 km2. It is situated to the north of Vík í Mýrdal and to the east of the smaller glacier Eyjafjallajökull, where an eruption in 2010 caused major disruptions to air traffic throughout western and northern Europe in April and May 2010. The caldera is 10 km (6 mi) diameter and is covered with 200–700 metres (660-2,300 ft) of ice.

Sixteen eruptions have been documented at Katla between 930 and 1918 at intervals of 40–80 years. It has not significantly erupted for 93 years, although there may have been small eruptions that did not break the ice cover in 1955 and 1999. The 1918 eruption resulted in extending the southern coast by 5 km due to laharic flood deposits.

In the early hours of today, 05 October, an intense swarm of earthquakes was registered in the Katla caldera; the largest of these earthquakes had a local magnitude of ~3.7. Most of the ongoing seismicity is sourced at shallow (< 5 km) depths.

Commenting before the latest (14-15 Oct., 2011) earthquake swarm, the IMO said there were no measurable signs that an eruption of Katla was imminent.

“However, given the heightened levels of seismicity”, the IMO added, “the situation might change abruptly. Monitoring teams at IMO are following the ongoing activity closely, and sensor-based networks around the volcano ensure that all seismological, geodetic, and hydrological changes are detected.”

“It is definitely showing signs of restlessness,” commented Pall Einarsson, a professor of geophysics at the University of Iceland.

According to Jón Frímann, the author of the popular Icelandic volcano and earthquake website: “Earthquake activity continues in Katla volcano as before. Most of the earthquakes are as before just small ones. Currently the earthquakes do not appear to be from dike intrusion as happened last week (5 October, 2011… After this large earthquake swarm, activity dropped considerably but it has been picking up again slowly during the week. But earthquake observation has been difficult due to frequent storms during the past two weeks that have been passing over Iceland.”

A glacial flood or jokulhlaup from the Katla volcano badly damaged a bridge leading to the closure of a busy road on Saturday, 09 July last. Iceland’s Civil Protection Agency (CPA) said flooding took place near the volcano, most likely caused by the melting of its ice cap. An aerial observation of the Mýrdalsjökull at the time reported cracks in two calderas in the southernmost part of the glacier.

In recent weeks residents of Vik (population 300 approx.), located at the foot of Katla, have participated in emergency evacuation drills in the event of a volcanic eruption and subsequent glacial floods affecting the small coastal town.

Meanwhile, an earthquake swarm has also been detected at the Askja volcano, which is situated in a remote part of the central highlands of Iceland.

Experts say an eruption is not imminent but that pressure is continuing to build beneath the volcano. Askja was virtually unknown until the tremendous eruption which started on March 29, 1875. It last erupted in 1961.

The most significant, recent earthquake (3.9 magnitude) to hit Iceland occurred at 01:16 AM on Sunday 16 October. The epicentre was located 37 km S Grindavík; 67 km SW Hafnarfjörður; and 75 km SW Reykjavík

Earthquake Swarms
Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey (USGS) points out that an event may last for days, weeks, or months.

Harmonic Tremors
Harmonic tremor describes a long-duration release of seismic energy, with distinct spectral (harmonic) lines, that often precedes or accompanies a volcanic eruption. More generally, a volcanic tremor is a sustained signal that may or may not possess these harmonic spectral features.