1. Magnitude 7.9 earthquake Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
A magnitude 7.9 earthquake occurred in the remote Aleutian Islands region of Alaska. The earthquake occurred at an intermediate depth of 108 km (67 miles). An initial tsunami advisory was given but later cancelled for coastal Alaska. After the initial tsunami warning, the town of Adak evacuated its 150 residents to a shelter. However, there are no initial reports of damage or injuries. The deep nature of the earthquake was probably not favorable for the generation of a large tsunami. The earthquake was followed by a number of large aftershocks.

2. Where was the earthquake felt?
Magnitude 7.9 earthquake
Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
Where was the earthquake felt?
I. Instrumental
Not felt by many people unless in favourable conditions.
II. Weak
Felt only by a few people at best, especially on the upper floors of buildings. Delicately suspended objects may swing.
III. Slight
Felt quite noticeably by people indoors, especially on the upper floors of buildings. Many to do not recognise it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated.
IV. Moderate
Felt indoors by many people, outdoors by a few people during the day. At night, some awakened.
V. Rather Strong
Felt outside by most, may not be felt by some people in non-favourable conditions. Dishes and windows may break and large bells will ring. Vibrations like train passing close to house.
VI. Strong
Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books fall off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight.
VII. Very Strong
Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by people driving motor cars.
VIII. Destructive
Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved.
IX. Violent
General panic; damage considerable in poorly designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Intense
Some well build wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent.
XI. Extreme
Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Cataclysmic
Total destruction – everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock move position. Landscape altered, or leveled by several meters. In some cases, even the routes of rivers are changed.
The Modified-Mercalli intensity scale indicates the severity of ground shaking.
Strong shaking (VII) would have been felt throughout the Rat Islands area of the Aleutian Islands, although due to the minimal population density in the region, this strong shaking was unlikely to have been felt by many people.
USGS estimated shaking intensity in the region of the M7.9 earthquake
Modified Mercalli Intensity Scale

3. Past earthquakes in the region
Magnitude 7.9 earthquake
Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
Past earthquakes in the region
The Aleutian Islands is a very seismically active region. Since 1900, 26 earthquakes with M7 or greater occurred within 250 km of the 2014 earthquake hypocentre. Significant events include an M 8.4 quake in and a M 7.9 quake in The former generated a large tsunami across the Pacific Ocean. There was also a M8.4 earthquake in Most of these large earthquakes occurred at shallower depths than the June 23 event.
Historic seismicity (since 1970) showing magnitudes of greater than 5.5 in the region. Earthquakes are coloured by their hypocentral depth. The Jun 2014 earthquake epicentre is indicated by the red star. Earthquake locations from the USGS catalogue.

4. What caused this earthquake to occur?
Magnitude 7.9 earthquake
Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
What caused this earthquake to occur?
The Aleutian Islands region is seismically active due to the northward subduction ( mm/yr) of the Pacific plate into the mantle beneath the North America plate along an arc-shaped plate boundary. This underthrusting process is responsible for the formation of the Aleutian Islands and the deep-sea Aleutian trench. The strongest earthquakes in subduction zones normally occur along the shallow megathrust fault due to slip along the interface that separates two plates. Based on its mechanism and depth, the 23 June earthquake likely occurred as a result of extension within the subducting Pacific slab.
USGS focal mechanism of the earthquake indicating oblique normal faulting.
Tectonic setting of the central Aleutian region. The 23 June earthquake is given by the red star.
North America plate
Schematic illustration of oblique-normal faulting.
Aleutian Trench
For more information on Alaska and Aleutian Island tectonics, see this YouTube video:
73 mm/yr
Pacific plate

5. Magnitude 7.9 earthquake Aleutian Islands, U.S. Aftershocks
Monday 23 June 2014 at 20:53:09 UTC
Aftershocks
So far, over 20 aftershocks with magnitude greater than 4.0 have been recorded by the USGS. Most of these have been small (M < 5), but the largest aftershock was a magnitude 6.6 earthquake that occurred around six hours after after the mainshock. Most aftershocks have so far occurred to the north-west of the mainshock epicentre. Further moderate-sized aftershocks can be expected.
M6.6 aftershock
M7. 9 mainshock
Map of aftershocks recorded (orange circles).

6. Magnitude 7.9 earthquake Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
Recordings of the earthquake from the British Geological Survey seismometer network
Time of earthquake
First P-wave arrivals
Surface wave arrivals
The first seismic waves took ~13 minutes to travel from Alaska and arrive in the UK

7. Magnitude 7.9 earthquake Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
Find out more …
USGS webpage for this earthquakes:
Alaska earthquake center:
BGS (British Geological Survey) – seismology and earthquakes – frequently asked questions
IRIS (Incorporated Research Institutions for Seismology) – learning about earthquakes
UK School Seismology Project – classroom activities, videos and support documents
USGS (United States Geological Survey) – FAQs, glossary, posters, animations
EMSC (European Mediterranean Seismological Centre)