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 Alaskan 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 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 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 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.
East of the Aleutian arc along the Gulf of
Alaska, crustal earthquakes occur as a result transmitted deformation
and stress associated with the northwestward convergence of the Pacific
plate that collides a block of oceanic and continental material into
the North America plate. In 2002, the Denali Fault ruptured in a
sequence of earthquakes that commenced with the October 23 M6.7 Nenana
Mountain right-lateral strike-slip earthquake and culminated with the
November 3, M7.9 Denali earthquake which started as a thrust earthquake
along a then unrecognized fault and continued with a larger
right-lateral strike-slip event along the Denali and Totschunda Faults.
More information on regional seismicity and tectonics
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