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
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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