The 2002 Denali Fault earthquake
Magnitude 7.9 near Denali National Park, AK
Sunday, November 3, 2002 at 22:12:41 (UTC)
Tectonic Setting(updated 11/12/02)
This M7.9 shock, one of the largest ever recorded on U.S. soil, occurred on the Denali-Totschunda fault system, which is one of the longest strike-slip fault systems in the world and rivals in size California's famed San Andreas strike-slip fault system that spawned the destructive M7.8 "San Francisco" earthquake in 1906.
Most of the seismic activity in Alaska results from interaction of the northwestward-moving Pacific plate with the corner of the North American plate that comprises Alaska. Along the southeastern panahndle of Alaska, this motion is accommodated by right-lateral strike-slip faulting, but from Yakutat Bay westward along the southern coast of Alaska and the Aleutian Islands, the Pacific plate is colliding with Alaska. West of Kayak Island, this collision results in subduction of the Pacific plate beneath continental Alaska and the Aleutian Islands. This underthrusting motion was the cause of the M9.2 Alaska earthquake of 1964. Between Kayak Island and Yakutat Bay, the Pacific plate is mostly coupled to southern Alaska. This coupling has resulted in the rapid, geologically-recent building of the coastal St. Elias Mountains and produces northwest-southeast oriented compressive stress that extends inland through the Alaskan crust to Fairbanks and seaward into the Gulf of Alaska. This compressive stress drives the right-lateral slip witnessed on the Denali-Totschunda fault system.
Geologic Evidence of Past Earthquakes on the Denali Fault
The Denali fault is a prominent topographic scar on the Alaksan landscape. Prior to the recent earthquake, the fault scarp between Denali National Park on the west and the Richardson Highway on the east appeared degraded, indicating that the last major earthquake was a few hundred years ago. In the epicentral area of the recent quake, USGS geologist George Plafker observed that the last major earthquake had 6-8 m of right-lateral offset. Slip rate on the Denali fault since the last glaciation (10,000 years before present) has averaged about 1 cm/year. Accordingly, a 6-m displacement would require 600 years of strain accumulation.
The Current Event(updated 11/10/02)
This is the largest earthquake on the Denali fault since at least 1912, when a M 7.2 earthquake occurred in the general vicinity of the fault, more than 50 miles to the east of the November 3rd epicenter. 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.
Fault rupture initiated about 25 km east the M6.7 Oct. 23 foreshock and continued eastward amd southeastward for about 300 km, crossing the Richardson and Glenn (Tok Cutoff) highways. Near Mentasta Lake the rupture branched from the Denali fault and continued for about 75 km along the Totschunda fault. This branching from the Denali to the Totschunda was expected based on earlier geologic fault investigations and is reflected in the USGS seismic hazard map of Alaska. The seismic radiation pattern determined by Harvard University seismologists is consistent with the observed right-lateral slip. Preliminary measurements of fault displacements in the field by geologists range from under a meter in some locations in the west to nearly 9 meters near Mentasta Lake.
As of this update, five aftershocks larger than magnitude 5 have occurred, with the most recent three on November 8.
ANSS Location Map (automatically generated)
You Feel It?
- 11/07/2002 USGS Press Release photos
- Alaska Science Center Reconnaissance on November 7
- Alaska Earthquake Information Center-Tok Cutoff Road
- Alaska Earthquake Information Center-Northway Road
- Shannon & Wilson Inc.-near Denali National Park
View of Denali Fault showing locations
of M7.9 and M6.7 earthquakes
- Road Offset, Richardson Highway
- Photos along the surface rupture of the Denali Fault - Univ. of Alaska
- Major earthquake shakes Alaska
- Major earthquake rocks Alaska
- Alaska hit by year's strongest quake
- BBC Article - Saturday, 9 November - Geologists dissect massive quake
Preliminary Earthquake Report
Alaska Earthquake Information Center
University of Alaska