M 7.9 Denali Fault earthquake of November 3, 2002


Overview

The largest inland earthquake in North America in almost 150 years struck Alaska on November 3, 2002. It ruptured three different faults ending with a total rupture length of ~330 km. It started on the previously unrecognized Susitna Glacier Thrust fault, a splay fault south of the McKinley strand of the Denali fault system (DFS). Then the rupture transferred onto the main strand of the DFS and continued as a right-lateral strike-slip event for ~220 km until it reached the Totschunda fault near 143oW longitude. At that point, it right-stepped onto the more south-easterly trending Totschunda fault and stopped after rupturing nearly 70 km of it. A team of geologists surveyed the total length of the ruptured faults and reported maximum vertical offsets on the Susitna Glacier Thrust of 4 m and maximum horizontal offsets of 8.8 m west of the Denali and Totschunda fault junction. The estimated magnitude of this earthquake ranges from the body wave magnitude mb of 7.0 to the moment magnitude MW of 7.9 to the surface wave magnitude MS of 8.5. While the fault rupture lasted for approximately 100 sec from its initiation to the arrest, its distal effects were felt for many days. Of the population centers, the hardest hit were the villages of Mentasta and Northway, located at the eastern end of the rupture zone. This event caused significant damage to the transportation systems in central Alaska. The Trans-Alaska Pipeline suffered some damage, but no oil spills occurred. Multiple land slides and rock avalanches occurred in the Alaska Range with the largest slide on the Black Rapids Glacier. The Denali Fault event was felt as far as Washington and caused seiches in pools and lakes as far as Texas and Louisiana. There were reports of triggered seismicity in volcanic and geothermal centers in Washington and California and regional seismicity in Utah. The M 7.9 Denali Fault event was preceded by the magnitude 6.7 Nenana Mountain event on October 23, 2002. Its epicenter was located on the Denali fault 22 km east of the M 7.9 event epicenter. In response to the magnitude 6.7 and 7.9 events, the Alaska Earthquake Information Center (AEIC) staff installed a network of temporary instruments for the aftershock monitoring. The temporary network was dismantled in June, 2003.

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This high-altitude view shows the approximate locations of the earthquake epicenters. The view is eastward along the main strand of the Denali fault, which is marked here by a prominent linear valley along the southern edge of the Alaska Range. In the left foreground, the Nenana River marks the eastern boundary of Denali National Park. The Parks Highway, which connects Anchorage and Fairbanks, lies immediately east of the river north of the fault, but diverges from the river to the south. The epicenter of the November 3 earthquake was approximately 42 miles (68 km) east of the highway. In the distance, the rugged peaks of Mts. Deborah and Hess rise to about 12,000 feet (3650 m) just north of the epicenters and the bend in the Denali fault. (Photo and interpretation by Wesley K. Wallace, Geophysical Institute, University of Alaska Fairbanks)


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Earthquake parameters:

Origin Time (UT):   11/03/2002 22:12:41.52
Latitude:   63.5141N
Longitude:   147.4529W
Depth:   4.20 km
Magnitude:   MW 7.9, MS 8.5, ML 7.2, mb 7.0

First motions focal mechanism:

Paz=335; Ppl=0; Taz=244; Tpl=72
strike1=262; dip1=48; rake1=115; strike2=47; dip2=48; rake2=65


Mainshocks and aftershocks, M6.7 Nenana Mtn. and M7.9 Denali Fault events

The Nenana Mountain and Denali Fault earthquakes generated a vigorous aftershock sequence. The AEIC located over 1,000 aftershocks of the M 6.7 event prior to the M 7.9 mainshock and over 35,000 aftershocks through the end of 2004. Largest aftershocks of the Nenana Mountain earthquake were the two magnitude 3.8 events. A magnitude 4.4 foreshock preceded the Denali Fault mainshock by 3.5 hours. The largest aftershock (M 5.8) occurred 20 minutes after the main shock and was located 95 km east of the mainshock's epicenter (~10 km east of the Richardson highway crossing). Following the Denali Fault earthquake, the analyst processing load increased from an average of 40 events per day in September to an average of over 400 events per day in November. The onslaught of the data has created processing delays. Additional delays have been caused by the necessity of reviewing the earthquake locations a second time when the data from the temporary Denali network were brought back from the field and merged with the bulk of AEIC data. Four month of data remain unprocessed at this time (January-April, 2003). The magnitude of completeness mc of the aftershock catalog varies along the rupture zone. While on average for October-December data mc is 1.4, it is as low as 1.1 at the western end of the rupture and as high as 2.2 at the eastern end .

The map below shows  aftershock locations though the end of 2004.

Map of M6.7 and M7.9 aftershocks
white line - mapped rupture;
red lines - fault traces; 

dashed black line - Trans-Alaska Pipeline;
black lines - roads;
blue lines - major rivers

Photos


Science

In December, 2002, scientists gathered for a special session on the Denali Fault earthquake sequence at the Fall Meeting of the American Geophyscial Union in San Francisco, CA. Fourty three papers were presented regarding different aspects of the M7.9 earthquake.

Seismological Society of America published BSSA Dedicated Issue: The 2002 Denali Fault Earthquake Sequence, V.94, N.6B, 2004.

Selected publications:


Additional information



Page created by N.Ruppert.
Updated: May 2008