Earthquake Summary Poster

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 includes Alaska. Along the southeastern panhandle 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.

The Denali fault is a prominent topographic scar on the Alaskan 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 meters 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 meter displacement would require 600 years of strain accumulation.

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 3 November 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 of the M6.7 23 October foreshock and continued eastward and 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.