Magnitude 6.7 in Central Alaska

Tectonic Setting (updated 10/23/02 2:15 PDT)
Early this morning a 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 strike-slip 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.

This earthquake likely resulted from slip on the Denali fault or a closely related fault. No prior historical earthquake having the size of this earthquake had been definitely attributed to the section of the Denali fault that lies near the epicenter. However, the section of the fault near the epicenter had been thought capable of producing a major earthquake on the basis of geologic evidence and from the nearby occurrence of smaller earthquakes.

This M 6.7 shock 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 today’s 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 second largest known earthquake on the Denali fault prior to today 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.

On a broad-scale, the shallow earthquakes of Central Alaska are occurring within the North American tectonic plate, as a result of stresses generated by the plate's interaction with the Pacific tectonic plate. The Pacific plate is moving north-northwest with respect to the North American plate with a velocity of about 5.2 cm/year (about 2 inches per year). Much of this relative motion is accommodated at the principal boundary between the two plates, in the Gulf of Alaska, but there is also substantial crustal deformation and earthquake activity inland of the plate boundary, in Central Alaska. In addition to occurring on major mapped faults such as the Denali fault, shallow-focus earthquakes in Central Alaska also occur on lesser unmapped faults.

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