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M6.7 Nenana Mountain, Alaska, Earthquake
Released: 10/25/2002

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Peter Haeussler 1-click interview
Phone: 907-786-7447

On October 23, 2002, a strong magnitude (M) 6.7 earthquake occurred at 3:27 AM Local time 85 miles (135 km) south of Fairbanks and 172 miles (276 km) north of Anchorage. The earthquake was felt broadly, from Fairbanks in the north to Anchorage in the south, a distance of 255 miles. Because of the remote location, however, there was very little damage.

The earthquake resulted from right-lateral slip (the area north of the fault moved to the east, relative to the region south of the fault) on the Denali fault. The Denali fault is one of the longest continental faults in the world, stretching for over 700 miles across Alaska and rivaling California’s San Andreas Fault in size. Infrequent large earthquakes characterize the seismic behavior of the Denali Fault. An overflight of the epicentral region on Wednesday afternoon by U.S. Geological Survey (USGS) scientists revealed numerous small rockslides and snowslides adjacent to the fault over a 25-mile long zone east of the town of Cantwell. Scientists from the University of Alaska Fairbanks are now in the epicentral region installing instruments to monitor the aftershock activity and measure the slip in the earthquake.

The Denali fault slices the rugged Alaska Range and bounds the precipitous north face of Mt. McKinley, the highest peak in North America. Streams and glacial moraines are offset by the fault and record the long-term displacement. Scientists believe the fault is capable of generating earthquakes as large as magnitude 8, but none that large have been recorded in historic time. During the last prehistoric earthquake the fault moved about 19 feet.

This M 6.7 shock is the largest earthquake on the Denali fault since 1912, when a M 7.2 earthquake occurred more than 50 miles to the east of Wednesday’s temblor. Since there were no seismographs operating in Alaska at that time and no reports of surface faulting in the remote Alaska Range, the location of the 1912 shock is poorly known. The second largest known earthquake on the Denali fault prior to today was a M 6.2 shock that occurred on August 31, 1958. That shock was centered well east of the Richardson Highway and the Trans-Alaska Pipeline, which is more than 100 miles to the east of the fault segment that broke on Wednesday.

Although the Denali Fault is one of the largest in Alaska, it is not the most dangerous. Subduction zone earthquakes are caused by the slippage of the North Pacific plate beneath the North American plate, and these very large events can strike coastal regions in southern Alaska and the Aleutian Islands, including the Kodiak - Anchorage - Valdez region. The biggest of these was a M9.2 earthquake, which occurred on Good Friday (March 28), 1964 and was the largest instrumentally recorded earthquake in the United States. The damage and loss of life from the Good Friday earthquake was extensive (125 killed, $311M in loss in 1964 dollars).

A smaller but nevertheless significant threat to the Anchorage region is a large (M 6 or 7) earthquake on a crustal fault near Anchorage. A study published in the October issue of Geological Society of America Bulletin noted the potential hazard from the Castle Mountain fault. The Castle Mountain fault comes to the Earth’s surface in the Anchorage region and could produce a magnitude 6 to 7 earthquake in the next 50 to 100 years, according to USGS geologist Peter Haeussler, the principal investigator of the study.

Fairbanks and Interior Alaska also face hazards from nearby faults. Several active seismic zones cut across the region north of the Denali fault, including one that passes right through Fairbanks. These faults generate abundant small earthquakes, and moderate to strong earthquakes occur every few years - although spread over a vast area. The most recent of these earthquakes was the October 1995 M 6.2 Tatalina River earthquake, which was located 35 miles northeast of Fairbanks, which caused only minor damage as it struck an unpopulated area.

Alaska is not taking this combined risk sitting down. In 1989 the University of Alaska Geophysical Institute set up the Alaska Earthquake Information Center (AEIC) under the management of the State Seismologist. In addition, proactive community groups, working with State and local officials have become involved to plan emergency response. AEIC and the West Coast/Alaska Tsunami Warning Center, with support from NOAA, have installed a network of modern broadband seismometers across much of southern Alaska. , In the Anchorage region, with funding from the Alaska Science and Technology Foundation, researchers from the UAF Geophysical Institute and collaborating institutions installed 22 modern free-field strong motion sensors in the Anchorage region. The State and Municipality of Anchorage seismically retrofitted the Office of Emergency Services - the nerve center for response efforts following a damaging event.

This effort has been complemented by a Federal effort led by the USGS to implement an Advanced National Seismic System (ANSS) across the United States. In 2001, an additional 14 new strong motion earthquake sensors were installed in the Anchorage region as part of ANSS. The existing Anchorage network has been merged with the new ANSS stations to provide improved coverage of the Anchorage region. In 2002, 11 of 20 planned new strong motion stations have been installed in Anchorage, Juneau, Valdez, and Kodiak. Ten ANSS stations in Fairbanks are being installed by the UAF Geophysical Institute this fall, and one of these was already in place and recorded the recent earthquake. Installation of the remaining Fairbanks stations is expected to be completed by mid-November.

The enhanced monitoring capability - still a fraction of what is envisioned under ANSS — will soon allow emergency responders and scientists to assess how intensely the ground shook in the region. Using color-coded displays known as ShakeMaps, emergency responders, facility managers, media, and the public will be able to assess within minutes where the shaking was greatest and what the possible extent of the damage might be. In Alaska, ShakeMap will combine the ANSS strong ground motion recordings with earthquake source information from the AEIC, which is the State of Alaska’s earthquake monitoring agency. This combination of data will allow the ShakeMap to reflect accurately both local and regional variations in ground shaking.

ShakeMap is operating in parts of California, Washington, and Utah, but is still under development in Alaska. Currently, 47 instruments contribute to ShakeMap in the Anchorage region. The ANSS plan calls for the eventual installation of 300 strong motion instruments and additional broadband seismometers in the State of Alaska, funding permitting, and will provide ShakeMap for all of the major urban areas of Alaska.

Related to ShakeMap is a new tool for individuals with internet-access to enter their observations of shaking on the USGS web site and contribute to a community intensity map (coded by zip code) across the region. Already 483 responders have entered their observations of Wednesday’s earthquake at this Did-You-Feel-It web site through http://www.aeic.alaska.edu and this information is helping researchers fill in the gaps in areas where instrumentation is limited. It also allows citizens to compare their experiences with those of their neighbors or cross-town friends.

Future involvement by the USGS ANSS program in Anchorage is anticipated to include seismic instrumentation of building structures. A proposal is currently being finalized for submission to ANSS for seismic instrumentation of the Atwood Building combined with down hole reference instrumentation. It is anticipated that instrumentation of buildings will ultimately provide a better understanding of their performance during earthquakes and lead to an improvement in building design and construction.

State and Federal agencies are working diligently to improve all aspects of seismic monitoring in Alaska, and to prepare for the next damaging event. Recently, the State of Alaska created an Alaska Seismic Hazards Safety Commission. This commission will address the need for a consistent policy framework and a means for continuing coordination of programs and public safety practices related to seismic hazards at all governmental levels and in the private sector.

This type of proactive leadership will enable Alaska to weather future shaking and design municipalities that are built to survive in earthquake country.

For further information on this earthquake, go to the following web sites: http://earthquake.usgs.gov/eqinthenews/uskqaz/index.html, http://www.aeic.alaska.eduhttp://earthquake.usgs.gov

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