Geotimes - December 2002

Geotimes

Geophenomena

Mount Etna roars

What began as a swarm of minor earthquakes and a series of erupting fissures in the predawn hours of Oct. 27, became an unusual display of activity from Italy’s Mount Etna that continued into mid-November. Two days after the eruption began, a magnitude-4.4 earthquake left thousands homeless in the town of Santa Venerina on the volcano’s southeast flank. That same week, lava flows from the Northeast Rift slowly cut across the road connecting the towns of Linguaglossa and Piano Provenzana, devastating a hotel complex and igniting several forest fires along the way.

The plume of ash streaming from Mount Etna varies with the wind. On Nov. 12, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this true-color image of the plume blowing east. Snow appears on the northern flanks of the volcano. Image courtesy of Jeff Schmaltz, MODIS Rapid Response Team, NASA GSFC.

Geologist Boris Behncke of the University of Catania says he found the eruption “haunting for many reasons, for the devastation of the tourist facilities on the northeastern flanks, the pine forest, but most of all for the explosiveness, which was far beyond Etna standards.” According to a report in the Catania-based journal La Sicilia, the eruption produced 10 to 11 million cubic meters of lava and more than 20 million cubic meters of tephra as of Nov. 11, Behncke says.

Although that is only half the volume of lava emitted during the two month-long (July to August) eruption of last year, the volume of tephra is at least twice that emitted in 2001, and the greatest emitted during any flank eruption of Etna since 1879, he explains.

By Nov. 5, the eruption had calmed on the northeast flank, but the south flank continued to send fire fountains and black streams of ash into the sky. Then at 1:40 p.m. on Nov. 12, the plume of ash that had trailed with the wind across Sicily, periodically shutting down the Catania airport for days at a time, suddenly stopped. At the same time, volcanic tremors on the south flank doubled in amplitude. The next day, a fissure in the base of a new cinder cone burst, sending a lava flow down the south flank. By Nov. 14, the ash again had returned and the new flow had reached 1,200 meters in length and was traveling at a rate of 2 to 3 meters per minute.

Christina Reed

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Alaska rumbles

In fewer than two weeks, two major earthquakes on the Denali Fault rattled central Alaska. Seismologists across the nation have mobilized to determine the effects of the magnitude-6.7 and magnitude-7.9 quakes that hit on Oct. 23 and Nov. 3 respectively, about 200 miles to the northeast of Anchorage. They were the biggest earthquakes felt in the area since 1932, and probably not the last.

Geologists have flocked to Denali Fault, scouring it from the air and placing instruments anywhere they can reach. Others are crunching data points in what promises to be a seismologically rich event. Seismic waves from the magnitude-7.9 earthquake triggered swarms in geothermal fields as far away as Yellowstone in Wyoming and Mammoth and the Geysers in California.

This aerial photo of the Trans-Alaska Pipeline System line near the Denali fault (looking west) shows where the line is supported by rails on which it can move freely in the event of fault offset. Here the line has moved toward the west end of the rails. Alyeska Pipeline Service Company reported no breaks to the line and therefore no loss of oil. Photo by Rod Combellick, DGGS

These events raise many questions — but one in particular is how the change in the stress regime will affect faults closer to home. The Castle Mountain Fault, just north of Anchorage and parallel to the Denali Fault, lies in the stress field that would be modified by a large earthquake.

Geologist Peter Haeussler of the U.S. Geological Survey (USGS) calls the fault Denali’s “little brother.” Castle Mountain is the only fault in the region that has a fault scarp that dates as recently as the Holocene. But it has not ruptured in about 650 years, and Haeussler and his colleagues recently concluded that it is due for a major event.

In the October GSA Bulletin, they presented a paleoseismic history of the Castle Mountain Fault taken from trenches dug across it. That cross-section showed the major events marked by where the fault branches and breaks. Samples taken from breaks along the fault gave radiocarbon dates.

Despite blurring of the sediment record, researchers were able to determine with relative assurance that over the past 2,700 years, major earthquakes occurred on the fault approximately every 700 years. After 650 years of quiet, “we’re getting into the window of the recurrence intervals of the earthquakes,” Haeussler says. That means that the Castle Mountain Fault may be in for a big event soon, though without a reliable statistical record, the study cannot give any indication of when.

What is certain is that, unlike the magnitude-7.9 event on the Denali Fault, a Castle Mountain earthquake will be much closer to Anchorage and other population centers — and will have possibly more devastating effects, even if the earthquake is smaller. The quake on Nov. 3 caused an estimated $25 million in structural damage to roads and bridges — far less than what would be expected in other parts of the world, but enough for Alaska Gov. Tony Knowles to declare a state of disaster.

In the meantime, Greg Anderson, a USGS seismologist in Pasadena, is crunching numbers to determine how the stress field may have changed after the earthquake. He plans to have some solid answers for this month’s meeting of the American Geophysical Union in San Francisco, where he and other researchers will present their results from the October and November earthquakes at a special session.

Michael Blanpied of the USGS in Menlo Park, says that uncertainties abound as to why the Alaska events affected geothermal fields so far away, and the nature and extent of the 7.9 earthquake remain to be seen. “Luckily, the 6.2 — what we now know was a foreshock — got people to put out temporary instruments on the fault,” he says, some of which were part of the Advanced National Seismic System (Geotimes, October 2002). “This will be a very good data set.”

Naomi Lubick
Geotimes contributing writer

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