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. Magnitude 7.9 earthquake
Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
Where was the earthquake felt?
Modified Mercalli Intensity Scale
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

3. Magnitude 7.9 earthquake
Aleutian Islands, U.S.
Monday 23 June 2014 at 20:53:09 UTC
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
1965 and a M 7.9 quake in 1996. The former generated a large tsunami across the Pacific Ocean. There
was also a M8.4 earthquake in 1906. 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.
Past earthquakes in the region

4. 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 (59 - 76 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.
Tectonic
setting of the
central
Aleutian
region. The 23
June
earthquake is
given by the
red star.
USGS focal mechanism of the
earthquake indicating oblique
normal faulting.
Schematic illustration of
oblique-normal faulting.
North America plate
Aleutian Trench
Pacific plate
For more information on Alaska and Aleutian Island 73 mm/yr
tectonics, see this YouTube video:
https://www.youtube.com/watch?v=2nMiVd0zo_Y

5. Magnitude 7.9 earthquake
Aleutian Islands, U.S.
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.
Map of aftershocks recorded
(orange circles).
M7. 9 mainshock
M6.6 aftershock

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:
http://earthquake.usgs.gov/earthquakes/eventpage/usc000rki5#summary
• Alaska earthquake center: http://
www.aeic.alaska.edu
• BGS (British Geological Survey) – seismology and earthquakes – frequently asked questions
http://www.earthquakes.bgs.ac.uk/education/faqs/faq_index.html
• IRIS (Incorporated Research Institutions for Seismology) – learning about earthquakes
http://www.iris.edu/hq/programs/education_and_outreach/students
• UK School Seismology Project – classroom activities, videos and support documents
http://www.bgs.ac.uk/schoolseismology/home.html
• USGS (United States Geological Survey) – FAQs, glossary, posters, animations
http://earthquake.usgs.gov/learn/
• EMSC (European Mediterranean Seismological Centre)
http://www.emsc-csem.org/