The M6.7 Nenana Mountain earthquake of October 23, 2002

The Alaska Earthquake Information Center located a strong earthquake that occurred on Wednesday, October 23rd at 3:27 AM AKDT in the central region of Alaska. This event has been felt strongly from Anchorage to Fairbanks (Community Internet Intensity map). This event occurred on the Denali fault. The source is typical for events in this area and indicates right-lateral strike-slip motion along the fault. This event was a foreshock of the M 7.9 Denali Fault earthquake of November 3, 2002, which was located just east of the aftershock zone associated with the 6.7 event.

Denail Fault Seismicity

The Nenana Mountain magnitude M 6.7 earthquake ruptured a segment of the Denali fault in interior Alaska, east of the Parks Highway and the community of Cantwell. The Denali fault is a major, seismically active fault that arcs through Alaska, slicing the rugged Alaska Range and bounding the preciptious north face of Mt. McKinley, the highest peak in North America. The streams and glacial morraines crossing the fault have been offset and record the long-term displacement of the fault. Scientists believe the fault is capable of generating earthquakes as large as magnitude class 8, but none that large have been recorded in historic time, since the beginning of the last century.

The Nenana Mountain M 6.7 shock is the largest earthquake on the Denali fault since 1932 when three magnitude 6.0, 6.9 and 6.0 shocks occurred within a 5-month time period. The 1932 events were located at the western end of the Denali fault (lat 62.5N and lon ranging from 152.5W to 153.3W). The largest recorded event that can be attributed to the Denali fault was in 1912, when a M 7.2 earthquake occurred more than 50 miles to the east of the October 23 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 not well-constrained. The most recent known earthquake on the Denali fault prior to the Nenana Mountain event is 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, more than 100 miles to the east of the fault segment that broke today.

The seismic behavior of the Denali fault, like the northern and southern segments of the San Andreas fault in California, is characterized by infrequent large earthquakes. This behavior contrasts to the segment of the San Andreas in central California where frequent small earthquakes and continuous fault slip gradually releases the strain in the Earth's crust caused by global plate tectonic motion.

AEIC located 1,067 aftershocks prior to the M7.9 event on November 3. Largest afershocks were the two ML 3.8 events on 10/23 and 10/26. The magnitude of completeness of the aftershock catalog is about 1.1.

A close up map of the aftershock locations and focal mechanisms.

smaller star - location of the M 6.7 shock

larger star - location of the M 7.9 shock of November 3

orange squares - reviewed locations of the aftershocks

white squares - towns and cities

yellow triangles - permanent seismic stations

red lines - mapped fault traces

black lines - roads

blue lines - major rivers

Earthquake parameters:

Origin Time (UT): 10/23/2002 11:27:19.43

Latitude: 63.5144N

Longitude: 147.9116W

Depth: 4.2 km

Magnitude: mb 6.7, MS 6.7, MW 6.7

Distance to nearby locations:

44 km ( 28 miles) E of Denali Park

50 km ( 31 miles) ENE of Cantwell

59 km ( 37 miles) SE of Healy

105 km ( 66 miles) SSE of Clear

112 km ( 70 miles) W of Pump Station #10

116 km ( 72 miles) SSW of Salcha

119 km ( 74 miles) WSW of Pump Station #9

123 km ( 77 miles) WSW of Fort Greely

146 km ( 91 miles) S of Fairbanks

277 km ( 172 miles) NNE of Anchorage

First motions focal mechanism:

Paz=308; Ppl=4; Taz=218; Tpl=4

strike1=253; dip1=90; rake1=175; strike2=353; dip2=85; rake2=0

The E-W oriented focal plane is consistent with the aftershock distribution.

Follow this link to see the focal mechanism plot.

Helicoder plot of station MCK recording (vertical channel) for 12 hours following the M6.7 shock. The station is located 56 km (30 miles) NW of the epicenter. Note decrease in size and frequency of aftershocks with time.

Field observations

These are the field observations provided by Peter Hauessler (USGS, Anchorage) who flew over the earthquake source area on the morning of October 23rd:

We found no evidence of surface faulting along the Denali fault in the epicentral region of the earthquake. We looked for surface expression of another fault both north and south of the Holocene fault scarp, but did not find any. However, there was a ~42 km long region straddling the epicenter of the earthquake along the north side of the fault in which rockfall and snow avalanches were particularly abundant. Sackung features were observed on one hillside adjacent to the the fault. All these mass wasting features were more abundant closer to the trace of the fault. We observed rockfall and snow avalanches within 20 km of the Denali fault along a 95 km length of the fault. We also observed shattered ice on lakes and ponds in the epicentral region. All these features point to high ground accelerations related to movement along the Denali fault.

A GPS crew visiting earthquake site on Oct.23rd observed fresh multiple cracks in the Denali Hwy roadbed. Follow this link to find out more details and view some photos.

Photo of the Denali fault and location of the earthquake

This is a high-altitude view 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. The view shows the approximate location of the epicenter of the magnitude 6.7 earthquake that occurred in the early morning of October 23, 2002. The Denali fault is a major fault that follows an arcuate trace across southern Alaska, following the Alaska Range for much of its length. The eastern part of the fault shows about 400 km of right-lateral strike-slip displacement since early Tertiary time. Offset surficial deposits and seismicity indicate that the fault has remained active to the present, but the October 23 earthquake is the largest seismic event so far recorded on the fault. The Nenana River, in the foreground, marks the eastern boundary of Denali National Park, and the Parks Highway, which connects Anchorage and Fairbanks lies immediately east of the river. The epicenter was approximately 25 miles (40 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 bend in the Denali fault. (Photo and interpretation by Wesley K. Wallace, Geophysical Institute, University of Alaska Fairbanks)

Additional Resources:

Information compiled by N.Ruppert

Last Modified: September 2005