Fault could cause major earthquake

Quake of 6 or 7 magnitude could hit within 100 years

Posted: Monday, December 30, 2002

ANCHORAGE - The Castle Mountain fault, a little-known fault that runs through the Matanuska and Susitna valleys, could produce a strong earthquake, according to a scientific study published this fall.

The Castle Mountain fault is smaller than the Denali fault, which ruptured in early November, causing $20 million in damage to Alaska's Interior. But it is similar.

"It's the only active fault that comes to the surface in Southcentral Alaska," geologist Peter Haeussler, a leading earthquake researcher with the U.S. Geological Survey in Anchorage, told the Anchorage Daily News.

The Castle Mountain fault cuts under the Parks Highway near the Alaska Railroad overpass in Houston, about 30 miles north of Anchorage. Its western trace creases the forest and swamp between Big Lake and Willow for miles.

Haeussler and two other geologists found that magnitude 6 or 7 temblors have ruptured the Castle Mountain fault every 700 years over the past 2,800 years. It has been at least 650 years since the last one struck, so a quake of similar force could hit any time within the next 50 to 100 years.

"I'd personally try to avoid living within six miles of this thing," Haeussler said. "You wouldn't want to put any critical facilities on top of it."

In a November disaster workshop sponsored by the U.S. Army Corps of Engineers, a "worst-case" magnitude 7.5 quake in a fault near the Chugach Mountains devastated Anchorage. The scenario projected up to 270 deaths and 10,000 injuries. Some 9,000 buildings might be destroyed, and 52 bridges might fall. Power would fail, phone lines go down. Damage could run as high as $5.9 billion.

The Castle Mountain fault is part of a vast network of cracks and folds in the surface layers of the Earth, all of them generated by the collision of immense crustal plates. Its band of pulverized, shattered bedrock may have drawn the paths taken by the Matanuska Glacier and the Little Susitna River, Haeussler said.

"The early history of all these faults is a bit mushy," he said. "But there is strong evidence that the Castle Mountain fault has been active on the order of 45 million years."

Castle Mountain isn't the only Southcentral fault with such potential. A few years ago, Haeussler and other scientists published a different study that found hidden earthquake danger beneath Cook Inlet in the same geologic structures that trapped oil and natural gas reserves. These faults could also produce magnitude 6 to 7 quakes, the scientists said.

Most people living near the Castle Mountain fault probably don't know it exists. Ken Hudson, chief of code enforcement for the Matanuska-Susitna Borough, said the borough has no building codes or earthquake zoning, so the location of the fault isn't drawn on property maps.

Houston Mayor Dale Adams said Houston residents understand they live in earthquake country.

"It's just living in Alaska, and we know it's going to happen someday," he said. "I'm not sure what we can do about it, other than have an emergency plan."

Bea Adler, a hazards planner working with the Matanuska-Susitna Borough, said people need to be prepared to be completely on their own for at least a week.

"That means food, water, diapers, medicine, an alternative heating source. ... In a situation like this, Alaska becomes an island. It is perfectly believable to think that we could have areas completely isolated," she said.

The eastern section of the fault has rattled Sutton twice in recent years, producing a 5.7 magnitude quake in 1983 and a 4.6 quake in 1996.

Sutton resident Clancey O'Rourke was working in the coal digs at Wishbone Hill during the 1996 event. The ground seemed to roll, and rocks slid in the mountains, but not much else happened.

"There was always a little bit of ground shaking going on up there on Castle Mountain," said O'Rourke, who lived for seven years by the fault itself. "Maybe we get a little shake, rattle and roll, and it's over. It stirs your hot chocolate a little bit and that's it."



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