Glaciers, not eruption: False alarm volcano mystery solved

Mýrdalsjökull glacier, atop the caldera of Iceland's Katla volcano.

When deep, long-period earthquakes started shaking the area around the Katla volcano on the southern tip of Iceland in 2001, officials feared it was a sign of an imminent eruption, as such quakes can be. So they were surprised when nothing happened. A new study identifies the source of the spurious signals: collapsing glaciers around the volcano, not the volcano itself. The finding may help researchers more accurately monitor other glacier-covered volcanoes.

When pressure changes in the magma-filled conduits in a volcano, the conduits can oscillate, producing small earthquakes, says Chris Bean, a seismologist at the University College Dublin in Ireland, who was not involved in the new study. The ground-shaking signals of these earthquakes last longer and have longer periods (lower frequencies) than quakes associated with shifting in Earth’s crust, Bean says, so scientific observers have traditionally interpreted long-period quakes as signs that a nearby volcano might be revving up for an eruption.

Although the Katla volcano has not erupted since 1918, it remains one of the largest and most active volcanoes in Iceland, says geophysicist Kristín Jónsdóttir, who led the new study as part of her doctoral research at Uppsala University in Sweden. Katla is also covered by a glacier, she says, which corks the volcano until it reaches explosive levels, then melts from the eruption’s heat and produces dangerous floods — all reasons for concern when the Iceland Meteorological Office noticed an upswing in long-period quakes beginning in 2001.

But when Jónsdóttir and her colleagues set up a series of seismometers around Katla in the spring of 2007, they found that all of the long-period quakes arose from west of the volcano, kilometers away from its caldera. In fact, the events centered largely on a 100-meter-high, 700-meter-wide cliff.

There, Jónsdóttir says, 100-meter-wide slices of the glacier fall over the cliff as the glacier advances, creating seismic signals that mimic long-period quakes as the chunks slam onto the ground below. This makes sense given the shallow origin and seasonality of the quakes, she says, which seem to peak in autumn with rains that filter to the base of the glacier and speed up the glacier’s slide over the cliffs.

Although other recent studies have found that long-period signals can stem from glaciers, the study by Jónsdóttir and colleagues, published in Geophysical Research Letters, is the first to apply to volcanoes, Bean says. “If you’re in an environment where you have a volcano covered by a glacier, your task of trying to estimate the likely hazard on the volcano has become a lot more difficult,” he says. Examples may include volcanoes in the Andes, Bean says, and the volcanoes in Alaska, Jónsdóttir adds.

Still, some long-period quakes could come from a volcano preparing to erupt, so it’s important that volcano observatories not count out that possibility, Bean says. More and denser seismic systems — not typical of volcano observatories, Bean says — could help scientists more accurately pinpoint whether long-period signals are coming from a volcano or the glaciers around them.

Jim Dixon, a seismologist with the Alaska Volcano Observatory in Fairbanks, says the results by Jónsdóttir and her team will aid the observatory in tracking volcanic activity on heavily glaciated volcanoes such as Redoubt, which erupted in March of this year. In the meantime, Jónsdóttir says the Iceland Meteorological Office, which assisted her team in the research, is not “so scared” of the long-period events it has been receiving — although it continues to monitor Katla closely. “They will pay attention to any changes in the activity,” she says.

Brian Fisher Johnson
Wednesday, October 7, 2009 - 12:31