More Faults Found in Oil-Rich Alaska Quakes could rupture underwater pipelines

David Perlman, Chronicle Science Editor Thursday, September 28, 2000

A newly discovered network of seismic faults beneath Alaska's Cook Inlet poses a serious threat to underwater pipelines carrying oil and gas from offshore drilling platforms, geologists are warning.

The newfound faults are far shallower than the one that caused the Anchorage earthquake of 1964, the most destructive in U.S. history, but they could produce damaging quakes with magnitudes of 6.5 or higher, according to a report released yesterday.

Peter J. Haeussler, a U.S. Geological Survey geologist in Anchorage, said that the drilling platforms themselves would probably be strong enough to withstand the seismic shock waves. But he said any of the faults could produce quakes big enough to rupture the pipelines.

The monster earthquakes that have frequently struck the Alaskan coast originate deep underground where the Pacific plate, a great slab of the earth's crust, has been sliding underneath the North American plate for millions of years in a process called subduction.

It was one of these deep temblors that hit Anchorage and its vicinity in 1964 with a magnitude of 9.2 -- the largest ever recorded in the United States. It produced a tsunami -- a giant sea wave -- that devastated towns all along the Gulf of Alaska and hit as far south as Crescent City in Del Norte County, and as far west as Hawaii.

The new seismic faults, revealed in oil company data, run parallel to the inlet and lie much nearer the surface, perhaps eight miles deep or less, Haeussler said. They have been created in folds in the earth's strata that are constantly growing.

In an interview yesterday, Haeussler said the faults beneath the Cook Inlet could produce quakes similar to the magnitude 6.9 temblor that hit the Alaskan village of Beluga in 1933. The quake caused little damage because the area was so sparsely settled, but today the toll might be much higher, he said.

Cook Inlet itself is flanked by two major faults -- the Castle Mountain Fault to the northeast and the Border Ranges Fault to the southwest. They may both be active, Haeussler said, and within only the past 4,100 years or so the Castle Mountain Fault could have ruptured several times with quakes as large as magnitude 7.

In their search for new oil reserves, the big oil companies use reflected sound waves from underwater explosions to trace the outlines of the earth beneath the seabed, and their findings are normally guarded as super-secret.

Scientists at ARCO-Alaska, Inc., recently made their Cook Inlet data available to Haeussler and his colleagues. From those charts the scientists were able to develop a clear picture of the faults and the moving geologic folds within which the faults lie.

Haeussler has published a detailed report on his findings in this month's issue of the Bulletin of the Geological Society of America. His colleagues were Ronald L. Bruhn of the University of Utah and Thomas L. Pratt of the USGS and University of Washington in Seattle.

The Cook Inlet is a dramatic body of water with some of the highest tides in the world -- up to 32 feet high at times. Fierce winters often see ice pans stretching over 40 acres moving in and out with the tides and crashing against the 16 drilling platforms.

Oil was first discovered there some 40 years ago, and although production has dwindled to a meager 30,000 barrels a day now, the inlet has yielded a total of about 6 billion barrels of crude since production began, according to Tim Ryherd, a geologist with the Alaska Department of Natural Resources.

http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2000/09/28/MN15001.DTL

-- Martin Thompson (mthom1927@aol.com), September 28, 2000