The Aleutian arc extends approximately 3,000 km from the Gulf of Alaska in the east to the Kamchatka
Peninsula in the west. It marks the region where the Pacific plate subducts into the mantle beneath the
North America plate. This subduction is responsible for the generation of the Aleutian Islands and the deep
offshore Aleutian Trench.
The curvature of the arc results in a westward transition of relative plate motion from trench-normal
(i.e., compressional) in the east to trench-parallel (i.e., translational) in the west, accompanied by westward
variations in seismic activity, volcanism, and overriding plate composition. The Aleutian arc is generally divided
into three regions: the western, central, and eastern Aleutians. Relative to a fixed North America plate, the
Pacific plate is moving northwest at a rate that increases from roughly 60 mm/yr at the arc's eastern edge to
76 mm/yr near its western terminus. The eastern Aleutian arc extends from the Alaska Peninsula in the east
to the Fox Islands in the west. Motion along this section of the arc is characterized by arc-perpendicular
convergence and Pacific plate subduction beneath thick continental lithosphere. This region exhibits intense
volcanic activity and has a history of megathrust earthquakes.
The central Aleutian arc extends from the Andreanof Islands in the east to the Rat Islands in the west. Here,
motion is characterized by westward-increasing oblique convergence and Pacific plate subduction beneath thin
oceanic lithosphere. Along this portion of the arc, the Wadati-Benioff zone is well defined to depths of
approximately 200 km. Despite the obliquity of convergence, active volcanism and megathrust earthquakes are
also present along this margin.
The western Aleutians, stretching from the western end of the Rat Islands in the east to the Komandorskiye Ostrova (Commander Islands),
Russia, in the west, is tectonically different from the central and eastern portions of the arc. The increasing
component of transform motion between the Pacific and North America plates is evidenced by diminishing active
volcanism; the last active volcano is located on Buldir Island, in the far western portion of the Rat Island chain.
Additionally, this portion of the subduction zone has not hosted large earthquakes or megathrust events in
recorded history. Instead, the largest earthquakes in this region are generally shallow, predominantly strike-slip
events with magnitudes between M5-6. Deeper earthquakes do occur, albeit rather scarcely and with small
magnitudes (M<4), down to approximately 50 km.
Most of the seismicity along the Aleutian arc results from thrust faulting that occurs along the interface between
the Pacific and North America plates, extending from near the base of the trench to depths of 40 to 60 km. Slip
along this interface is responsible for generating devastating earthquakes. Deformation also occurs within the
subducting slab in the form of intermediate-depth earthquakes that can reach depths of 250 km. Normal faulting
events occur in the outer rise region of the Aleutian arc resulting from the bending of the oceanic Pacific plate as
it enters the Aleutian trench. Additionally, deformation of the overriding North America plate generates shallow
crustal earthquakes.
The Aleutian arc is a seismically active region, evidenced by the many moderate to large earthquakes occurring each
year. Since 1900, this region has hosted twelve large earthquakes (M>7.5) including the May 7, 1986 M8.0 Andreanof
Islands, the June 10, 1996 M7.9 Andreanof Islands, and the November 17, 2003 M7.8 Rat Islands earthquakes. Six of
these great earthquakes (M8.3 or larger) have occurred along the Aleutian arc that together have ruptured almost the
entire shallow megathrust contact. The first of these major earthquakes occurred on August 17, 1906 near the island of
Amchitka (M8.3) in the western Aleutian arc. However, unlike the other megathrust earthquakes along the arc, this event
is thought to have been an intraplate event occurring in the shallow slab beneath the subduction zone interface.
The first megathrust event along the arc during the 20th century was the November 10, 1938 M8.6 Shumagin Island
earthquake. This event ruptured an approximately 300 km long stretch of the arc from the southern end of Kodiak
Island to the northern end of the Shumagin Islands and generated a small tsunami that was recorded as far south as Hawaii.
The April 1, 1946 M8.6 Unimak Island earthquake, located in the central Aleutian arc, was characterized by slow rupture
followed by a devastating Pacific-wide tsunami that was observed as far south as the shores of Antarctica. Although
damage from earthquake shaking was not severe locally, tsunami run-up heights were recorded as high as 42 m on
Unimak Island and tsunami waves in Hilo, Hawaii also resulted in casualties. The slow rupture of this event has made it
difficult to constrain the focal mechanism and depth of the earthquake, though it is thought to have been an interplate
thrust earthquake.
The next megathrust earthquake occurred along the central portion of the Aleutian arc near the Andreanof Islands on
March 9, 1957, with a magnitude of M8.6. The rupture length of this event was approximately 1200 km, making it the
longest observed aftershock zone of all the historic Aleutian arc events. Although only limited seismic data from this event
are still available, significant damage and tsunamis were observed on the islands of Adak and Unimak with tsunami heights
of approximately 13 m.
The easternmost megathrust earthquake was the March 28, 1964 M9.2 Prince William Sound earthquake, currently the
second largest recorded earthquake in the world. The event had a rupture length of roughly 700 km extending from Prince
William Sound in the northeast to the southern end of Kodiak Island in the southwest. Extensive damage was recorded in
Kenai, Moose Pass, and Kodiak but significant shaking was felt over a large region of Alaska, parts of western Yukon Territory,
and British Columbia, Canada. Property damage was the largest in Anchorage, as a result of both the main shock shaking and
the ensuing landslides. This megathrust earthquake also triggered a devastating tsunami that caused damage along the Gulf
of Alaska, the West Coast of the United States, and in Hawaii.
The westernmost Aleutians megathrust earthquake followed a year later on February 4, 1965. This M8.7 Rat Islands
earthquake was characterized by roughly 600 km of rupture. Although this event is quite large, damage was low owing
to the region's remote and sparsely inhabited location. A relatively small tsunami was recorded throughout the Pacific
Ocean with run-up heights up to 10.7 m on Shemya Island and flooding on Amchitka Island.
Although the Aleutian arc is highly active, seismicity is rather discontinuous, with two regions that have not experienced
a large (M>8.0) earthquake in the past century: the Komandorskiye Ostrova (Commander Islands) in the western Aleutians and the Shumagin
Islands in the east. Due to the dominantly transform motion along the western arc, there is potential that the Komandorskiye Ostrova (Commander
Islands) will rupture in a moderate to large strike-slip earthquake in the future. The Shumagin Islands region may also have
high potential for hosting a large rupture in the future, though it has been suggested that little strain is being accumulated
along this section of the subduction zone, and thus associated hazards may be reduced.
