October 25, 2005

DARRELL L. BREESE\Frontiersman reporter

PALMER - It was 5:36 p.m. on March 27, 1964, when the major earthquake shook Southcentral Alaska. The magnitude 9.2 temblor caused an estimated $311 million in damage and is still one of the biggest seismic events in world history.

But that could all change if expert predictions turn out to be correct. Geologists with the U.S. Geological Survey have forecasted that Alaska's next major earthquake is likely to occur on the Castle Mountain fault at the base of the Talkeetna Mountains. The fault also runs directly through the site proposed for 450 residential lots as part of the Hatcher Pass Ski Area development partnership between JL Properties and the Matanuska-Susitna Borough.

A study conducted following a 7.9 earthquake along the Denali fault in November 2002 by the USGS shows that the Castle Mountain fault may be ready to produce a magnitude 7.0 earthquake. Peter Haeussler, the principal USGS investigator in the study, said his research demonstrated that an earthquake of at least that magnitude may occur along this fault in the near future.

&#8220What 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,” Haeussler said. &#8220The Denali fault quake in 2002 caused an estimated $25 million in structural damage to roads and bridges - far less than what would be expected in more populated parts of the state, but enough for the governor to declare a state of disaster back then.”

Haeussler's research demonstrates that major earthquakes occurred on this fault on average every 700 years or so in the last 2,700 years, and that the last significant earthquake along the fault occurred about 650 years ago.

The 1964 earthquake was related to a slip of the Pacific plate beneath southern Alaska. In contrast, Haeussler said, earthquakes that occur on shallow geologic structures, such as the Castle Mountain fault and the related field of structures in Cook Inlet, may not produce great earthquakes, but they are closer to where people live and work, and as a result may have more impact when they occur.

The fault spans the Matanuska and Susitna valleys. The 12-mile deep crack extends nearly 100 miles from the mountains northeast of Sutton across the front of the Talkeetna Mountains and out toward Mount Susitna, crossing under the Parks Highway near the Alaska Railroad overpass in Houston. Its western trace creases the forest and swamp between Big Lake and Willow for miles.

According to the study, the fault is the only active fault that comes to the earth's surface in the region. The eastern part of the fault, near Palmer, produced light to moderate magnitude 5.7 and 4.6 earthquakes in 1984 and 1996, respectively. The fault is part of a larger field of folds and faults in the Cook Inlet region that are capable of producing magnitude 6-7 earthquakes.

Haeussler, who has been performing research for the geological survey in the Southcentral area for about 10 years, is worried about the potential that lies in the fault.

&#8220I think people should know where the fault is, number one,” he said. &#8220From an awareness standpoint, the borough is lacking. The fault is not platted on any of their maps, and a person could buy a lot on top of the fault and not know it. But if they bought land near the Matanuska River the borough would tell them if they are in a flood plain.”

Ron Swanson, Director of Community Development with the Mat-Su Borough, indicated that the borough is aware of the fault and the potential for avalanches and rockslides related to developing in the area.

&#8220We're not going to allow for any development of homes or anything else in an avalanche zone,” Swanson said. &#8220That's not going to happen.”

However he stopped short of making the same pledge in relation to developing in areas where an earthquake appears to be imminent.

&#8220This is Alaska, and we all know of the earthquake dangers,” Swanson said. &#8220The study said it could be 50 years out. It's not going to stop us from building in the area. We'll just have to engineer and design for the fact that the area is so close to the fault.”

Hauessler disagreed with Swanson on building in the area of a fault with the potential of generating a magnitude 7.0 quake.

&#8220I would want to be really careful if I were going to build something near there,” Hauessler said. &#8220Not knowing when the earthquake will occur is not reason enough to justify building in the area. It might be 50 years, and it might be tomorrow. The important thing to consider is not when it will happen, but rather the potential damage that could occur.

&#8220If the borough and the developer go in knowing where the fault is and make future property owners aware, they might be able to engineer accordingly,” Hauessler said. &#8220But you still don't know what the degree of damage might be if there is a major quake and how all the engineering will respond.”

Darrell L. Breese can be reached at 352-2267 or at darrell.breese@frontiersman.com.

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