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| USGS earthquake locations. |
April 24, 2015 - ALASKA, UNITED STATES - Two moderate earthquakes of magnitude 5.3 (mg/mb) was reported on Thursday and Friday in Alaska.
The first one struck at 74 kilometers (46 miles) from Buldir Island, with a depth of 56km.
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| USGS earthquake location for the tremor near to Buldir Island. |
A tsunami warning has been issued near Buldir Island in Alaska, according to the U.S. Geological Survey.
The second at 80 kilometers (50 miles) from Atka, with a depth of 49km.
Tectonic Summary - Seismotectonics of Alaska
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.
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| USGS plate tectonics for the region. |
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 (Magnitude less than 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 (Magnitude greater than 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 (Magnitude greater than 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
-
USGS.
