Alaska – 7 Earthquakes , 4.5 to 2.5 Magnitude May 25th – 28th, 2013 : Total of 172 EQ’s in the last 41 days

Earth Watch Report  –  Earthquakes

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M4.4 – 108km S of Coldfoot, Alaska

 2013-05-25 08:29:29 UTC

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Contributed by Alaska Earthquake Information Center

Instrumental Intensity

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M2.9 – 60km WSW of Anchor Point, Alaska

 2013-05-25 14:52:43 UTC

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M3.3 – 79km E of Old Iliamna, Alaska

 2013-05-27 00:06:59 UTC

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M2.5 – 37km S of Tanaga Volcano, Alaska

2013-05-27 02:33:48 UTC

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M4.5 – 111km W of Craig, Alaska

 2013-05-27 03:50:57 UTC

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Contributed by Alaska Earthquake Information Center

Instrumental Intensity

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M4.3 – 80km SE of Nikolski, Alaska

 2013-05-27 06:57:23 UTC

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M3.2 – 86km NE of Fort Yukon, Alaska

 2013-05-28 18:00:24 UTC

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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.

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.

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