The circum-Pacific region is dominated by subduction zone plate boundaries that not
only produce 80% of earthquakes worldwide, but has over 400 active volcanoes, earning
the title “Pacific Ring of Fire”. The earthquake belt sweeps through Alaska where
more earthquakes occur than in the other 49 States combined, and it is home to the second-largest
earthquake ever recorded. Pacific–North American plate interactions
dominate Alaskan tectonics. The Queen Charlotte transform fault, a right-lateral strike-slip
fault, accommodates the northwest motion of the oceanic plate. The megathrust boundary
between the Pacific and North American Plates results in both the 4,000-km-long Aleutian
Trench and in the arc of active volcanoes that lie subparallel to the trench. Note that
the North American Plate is oceanic beneath the western Bering Sea, and continental adjacent
to mainland Alaska. Lithospheric plates are spherical shells similar
to pieces of an eggshell resulting in changing rates and directions during movement on a
spherical surface. Along the immense length of the Aleutian Trench, the subduction rate
increases from 5.5 cm/yr in the Gulf of Alaska, to 7.8 cm/yr in the Near Islands region where
relative motion is highly oblique and the plate boundary is mostly right-lateral strike-slip.
In addition to the changing rate and direction of subduction, the angle of subduction changes
from steep in the west to shallow in the east. A seismic cross section through the Andreanov
Islands shows how earthquakes outline the subduction geometry. To 70 km depth, earthquakes
are concentrated along the plate boundary but also occur within both plates near the
boundary. Below 70 km depth, earthquakes occur only within the relatively cold, and therefore
brittle, subducting plate. Volcanoes are located above the point where the top of the Pacific
Plate has subducted to about 70 km depth about 170 km from the trench. Melting of
mantle rocks just above the subducting plate, not melting of the subducting plate itself,
generates the magma that supplies these volcanoes. The Pacific Plate subducts at a shallower
angle beneath Shumagin Bank and the distance from the trench to volcanic arc has increased
to 360 km. Beneath Cook Inlet, the subduction angle becomes very shallow and the distance
from the trench to the volcanic arc increases to 420 km, one of the largest in the world.
Since 1900, six Great megathrust earthquakes, above magnitude 8 , have occurred on the Aleutian
subduction zone producing destructive tsunamis. Red circles mark the epicenter above the site
where fault rupture began, whereas yellow shows the area on the megathrust fault that
ruptured. A great earthquake also occurred in 1949 on the Queen Charlotte transform fault.
Alaska has over 24,000 measured earthquakes every year with, on average, a major M7 event
every 2 years. By examining the pattern of seismic activity of all earthquakes greater
than magnitude 4 during the same 15-year period, we see the expected patterns of shallow to
deep earthquakes along the Aleutian subduction zone and shallow earthquakes along the Queen
Charlotte Transform Fault. In addition, there are hundreds of earthquakes far north of these
plate-boundary faults, especially in the mountainous south central part of Alaska. Accretion of
the Yakutat terrane is largely responsible for the tectonic and earthquake activity to
Fairbanks and even farther north. But what is geologic terrane and how does accretion
deform continental crust into mountains and break it into crustal blocks that grind past
each other on crustal faults? A terrane is an assemblage of rocks that were
transported far from where they formed and are distinct from surrounding rocks. To show
how accreted terranes form, we begin with a generic ocean-continent subduction zone.
As sediment is scraped off the top of the subducting oceanic plate, an accretionary
wedge builds on the continental plate margin. An oceanic plateau with thick and therefore
buoyant crust is traveling on the oceanic plate. When the plateau reaches the trench,
it cannot easily subduct and instead collides with and wedges beneath the continental margin.
Terrane collision deforms the continental margin into a new mountain range and slows
the subduction process with resulting decrease in magma supply to the volcanic arc. The
plateau may continue wedging beneath the continent for millions of years causing deformation
and uplift of an immense new mountain range. Although the following process is not occurring
in Alaska, eventually the oceanic plate, still driven by tectonic forces, could buckle and
break to form a new subduction zone outboard of the accreted terrane. In that case, magma
generation would resume to form a new volcanic arc.
The Yakutat terrane is an oceanic plateau that traveled northwest on the Pacific Plate.
It is bounded by the Queen Charlotte-Fairweather transform fault, the Chugach-St. Elias thrust
fault, the Aleutian megathrust, and the Transition fault on the south. About 25 million years
ago, the Yakutat terrane began colliding with and wedging beneath the continental margin.
The top of the subducting plate directly below Anchorage is only 50 km deep 400 km from the
trench. Because Yakutat crust is thicker and more buoyant than the Pacific oceanic crust
to the southwest, the Yakutat megathrust boundary is an interface of very high friction. Indeed
the M 9.2 1964 Great Alaska Earthquake rupture started on that thrust-fault boundary then
progressed along the Aleutian megathrust to the southwest.
GPS observations indicate that the Yakutat terrane is still moving northwest at about
5 cm per year. The ongoing collision is thickening the crust and uplifting the Chugach
and St. Elias ranges into some of the tallest and steepest mountains on Earth.
In addition to local mountain building, Yakutat terrane acts like a geologic snowplow as it
shoves immense blocks of crust into counterclockwise rotation. Because the effects of Yakutat collision
decrease to the north, crustal block rotation is faster between the Chugach-St. Elias thrust
fault and the Castle Mountain Fault, and slower between the Castle Mountain and the Denali
faults Right-lateral strike-slip faulting along the Castle Mountain and Denali faults
is caused by decreasing rate of block rotation. Both faults experience major earthquakes.
The 2002 magnitude 7.9, Denali Fault Earthquake rupture initiated on a previously unknown
thrust fault, produced up to 8 meters of right-lateral displacement on Denali fault, then branched
onto the Totschunda fault for a total rupture length of 340 km. Strong ground shaking caused
large rock avalanches from steep-sided valleys. This major earthquake triggered sequences
of small earthquakes up to 3600 km away in southern California and even affected geothermal
activity at Yellowstone caldera in Wyoming. In the earthquake hazard map for Alaska, regions
near the subduction zone and the Queen Charlotte Transform Fault are clearly high-risk zones.
In addition, the earthquake hazards are high in the region affected by Yakutat terrane
collision and resulting crustal faults to the north.