Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

A powerful magnitude 7.9 earthquake struck Alaska on November 3, 2002, rupturing the Earth’s surface for 209 miles along the Susitna Glacier, Denali, and Totschunda Faults. Striking a sparsely populated region, it caused thousands of landslides but little structural damage and no deaths.

Although the Denali Fault shifted about 14 feet beneath the Trans-Alaska Oil Pipeline, the pipeline did not break, averting a major economic and environmental disaster. This was largely the result of stringent design specifications based on geologic studies done by the U.S. Geological Survey (USGS) and others 30 years earlier.

Studies of the Denali Fault and the 2002 earthquake will provide information vital to reducing losses in future earthquakes in Alaska, California, and elsewhere.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

Shortly after midday on November 3, 2002, a magnitude 7.9 earthquake ruptured the Denali Fault in the rugged Alaska Range, about 90 miles south of Fairbanks. Called the Denali Fault earthquake, this shock was the strongest ever recorded in the interior of Alaska. Although comparable in size and type to the quake that devastated San Francisco in 1906, the Denali Fault earthquake caused no deaths and little damage to structures because it struck a sparsely populated region of south-central Alaska.

The November 3, 2002, magnitude (M) 7.9 Denali Fault earthquake was the strongest ever recorded in the interior of Alaska. Like most earthquakes of its size, it was complex, consisting of several subevents. It started with thrust (upward) motion on a previously unknown fault, now called the Susitna Glacier Fault. This rupture continued on the Denali Fault, where largely horizontal “right-lateral” movement (in which the opposite side moves to the right, when you look across the fault) propagated eastward at more than 7,000 miles per hour. As the rupture propagated, it offset streams, glacial ice, frozen soil, and rock, opening some cracks so wide that they could engulf a bus. The rupture crossed beneath the Trans-Alaska Oil Pipeline and terminated on the Totschunda Fault, 184 miles east of the epicenter, about 90 seconds after the quake began. The maximum horizontal movement (fault offset) of about 29 feet occurred in the eastern part of the rupture, near subevent 3.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

This powerful shock may have been triggered by a magnitude 6.7 temblor, the Nenana Mountain earthquake, that occurred nearby on the same fault 10 days earlier. Like the Denali Fault quake, the Nenana Mountain shock caused only limited damage because of its remote location. In contrast, the 1994 Northridge, California, earthquake, which had the same magnitude, caused 67 deaths and $40 billion in damage when it struck the densely populated Los Angeles region.

Effects of the Denali Fault Quake

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

The Denali Fault earthquake ruptured the Earth’s surface for 209 miles, crossing beneath the vital Trans-Alaska Oil Pipeline, which carries 17% of the U.S. domestic oil supply. Although slightly damaged by movement on the fault and by intense shaking, the pipeline did not break in the quake, averting a major economic and environmental disaster. This success is a major achievement in U.S. efforts to reduce earthquake losses.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

Violent, prolonged shaking from the quake triggered thousands of landslides, especially on the steep slopes of the Alaska Range. Mountainsides gave way, burying the valleys and glaciers below in deposits of rock and ice as much as 15 feet thick. The majority of landslides clustered in a narrow band extending about 8 to 12 miles on either side of the rupture.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002
 
Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002
 
Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

One facility that was badly damaged by the earthquake was the runway at Northway Airport, 40 miles from the eastern part of the November 3, 2002, fault rupture. The runway was rendered unusable by lateral spreading, accompanied by sand boils. These effects were the result of a phenomenon called “liquefaction,” in which strong, prolonged earthquake shaking transforms loose, water-saturated sediments into a liquid slurry. Areas that experienced liquefaction during the earthquake include much of the Tanana River Valley north and east of the rupture and other locations near smaller rivers.

Like some other large earthquakes, the Denali Fault quake triggered small shocks as far as 2,000 miles away, mainly in volcanic areas.

Yellowstone National Park had the most energetic swarm of triggered earthquakes.

Following the Denali Fault earthquake, Lake Union in Seattle experienced an earthquake-induced seiche, or water sloshing, which knocked many houseboats off their moorings and caused minor damage. Seiches were far away as Lake Pontchartrain in Louisiana.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

A GPS crew visiting earthquake site on Nov.4 observed sand blows on a baseball field in Northway.

Documenting the Quake

The locations of the Nenana Mountain and Denali Fault earthquakes and their aftershocks were determined by the Alaska Earthquake Information Center (AEIC) at the University of Alaska Fairbanks. AEIC receives data from more than 370 seismic stations, integrating all seismic networks in Alaska. A few of these stations are part of the new Advanced National Seismic System (ANSS) being deployed by the USGS and cooperators. After the Nenana Mountain earthquake, AEIC installed several temporary seismographs, including some ANSS instruments. When the Denali Fault earthquake struck a few days later, these stations helped to provide crucial data. Additional instruments were deployed after the Denali Fault quake, and as of December 2002, a total of 26 temporary seismic stations were gathering data on the quake’s aftershocks.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002
 
Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002
 
Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

During the 10 days following the Denali Fault earthquake, geologists from the USGS and Alaska Division of Geological and Geophysical Surveys, as well as several universities, mapped and measured the earthquake rupture on the ground and using aircraft. They identified the previously unknown Susitna Glacier Fault in the area where the quake began and showed that the rest of the rupture exactly followed an older rupture that geologists had documented in the 1970’s. They also located major landslides caused by the quake. The pattern of landsliding may help to better estimate levels of shaking along the length of the fault, especially because of the sparsity of seismic instruments in this rugged mountainous region.

Implications for Future Quakes Elsewhere

Because the 2002 Denali Fault earthquake occurred on a “strike-slip” fault, like the San Andreas Fault, it offers a realistic example of effects likely to accompany the next major earthquake in California. The Denali Fault quake is similar to three earthquakes that ruptured the San Andreas Fault in the past few centuries. These include the magnitude 7.8 San Francisco earthquake in 1906, the magnitude 7.9 Fort Tejon earthquake in 1857 north of Los Angeles, and a quake that struck east of what is now Los Angeles in about 1685. Evidence of the 1685 earthquake was only discovered in the past 20 years.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002
 
Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

The 1857 California and 2002 Alaska earthquakes struck far from major cities, causing little or no loss of life. However, the 1906 earthquake near San Francisco killed at least 700 people (the actual death toll was probably 3 to 4 times greater). Many geologists who study evidence of ancient earthquakes in deposits and landforms along the southernmost San Andreas Fault, where the 1685 earthquake occurred, have concluded that a major quake on this segment of the fault is likely to happen again in the near future. Should such a quake occur today, San Bernardino, Los Angeles, and other populations centers in southern California could suffer heavy damage and loss of life.

Lessons Learned and Future Opportunities

The survival of the Trans-Alaska Oil Pipeline in the 2002 Denali Fault earthquake demonstrates the value of combining careful geologic studies of earthquake hazards and creative engineering in designing and protecting such important structures and lifelines. Instrumental recordings of ground motion near earthquakes like the Denali Fault quake are critical for improving engineering design, but such quakes do not occur often. Following the Denali Fault earthquake, adjacent fault segments have been stressed, increasing the likelihood of additional earthquakes on those segments. This presents a rare opportunity to catch a major earthquake in the act. However, full ANSS instrumentation on either end of the 2002 rupture is critical if this goal is to be achieved.

Rupture in South-Central Alaska—The Denali Fault Earthquake of 2002

By studying earthquakes like the 2002 Denali Fault earthquake, scientists and engineers gain the knowledge necessary to reduce the vulnerability of buildings and other structures to damage in these inevitable and terrifying events. USGS studies of the Denali Fault earthquake are part of the National Earthquake Hazard Reduction Program’s ongoing efforts to safeguard lives and property from the future quakes that are certain to strike in Alaska, California, and elsewhere in the United States.




Mary Greeley News
www.marygreeley.com

credit: https://pubs.usgs.gov/fs/old.2003/fs014-03/

https://www.fhwa.dot.gov/publications/publicroads/03nov/05.cfm
https://pubs.usgs.gov/fs/2003/fs014-03/fs014-03.pdf

https://pubs.usgs.gov/fs/old.2003/fs014-03/