Transcript 130830_Andreanof

Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
A major magnitude 7.0 earthquake ( ) was felt strongly on nearby Adak
Island about 94 km west-northwest of the epicenter. No tsunami warnings
were issued. The earthquake occurred at a depth of 34 km. Fourteen hours
later, a magnitude 5.9
aftershock occurred closer
to Adak island (62 km SSW)
and was felt as light
shaking.
In 1986, a magnitude 7.9
earthquake in the same
area caused moderate
damage to structures on
Adak Island and
generated a tsunami.
Epicenter from U.S. Geological Survey
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
The USGS PAGER map shows the
population exposed to different Modified
Mercalli Intensity (MMI) levels.
According to news reports, it was strongly
felt in Atka, an Aleut community of 64
people, and the larger Aleutian town of
Adak, where 320 people live.
The color coded contour lines outline regions of MMI
intensity. The total population exposure to a given MMI
value is obtained by summing the population between
the contour lines. The estimated population exposure to
each MMI Intensity is shown in the table below..
Image courtesy of the US Geological Survey
USGS PAGER
Population Exposed to Earthquake Shaking
Adak Is.
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
The 34-km-deep
hypocenter for this
earthquake ( ) is
plotted on a map of
regional seismicity
greater than M 5
since 1990.
Two dozen M 6.5 or
larger earthquakes
have occurred within
250 km of this
earthquake over the
last century. On
average, Alaska
experiences one M 7
earthquake per year.
Text, epicenters, and rupture zones from U.S. Geological Survey
Earthquake and Historic Seismicity
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
In 1986, a magnitude 7.9
earthquake in the same
area caused moderate
damage to structures on
Adak Island and caused
a small tsunami recorded
throughout the Pacific
Ocean.
The estimated 1986
rupture margin is
indicated by dashed red
line (– – –). The tan areas
show rupture zones of
historic earthquakes
labeled with dates and
magnitudes of those
events
2013
Text, epicenters, and rupture zones from U.S. Geological Survey
Earthquake and Historic Seismicity
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
This earthquake occurred as the result of thrust faulting on or near the
subduction zone interface between the Pacific and North America plates. The
depth and mechanism of this earthquake are consistent with it occurring along
the megathrust interface between these two plates.
W
N
S
(Only major plates shown.)
Asterisks indicate earthquake locations.
Earthquakes occur within the brittle plates
due to deformation. Megathrust
earthquakes occur on the interface
between the plates when force overcomes
friction.
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
http://jules.unavco.org/Voyager]r/Earth
Rate and direction of motion of the Pacific Plate with respect to the
North American Plate.
Note change in the direction of the relative motion of the plates, from nearly
perpendicular to the plate boundary on the eastern end of the Aleutian trench
to nearly parallel at the western end.
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
The record of the earthquake on the University of Portland seismometer (UPOR) is illustrated
below. Portland is 3836 km (2382 miles, 34.56°) from the location of this earthquake.
Following the earthquake, it took 6 minutes and 46 seconds for the compressional P
waves to travel a curved path through the mantle from the earthquake to Portland,
Oregon.
S waves are shear waves that follow the same path through
the mantle as P waves. S waves took 12 minutes and
14 seconds to travel from the earthquake to Portland.
Surface waves, both Love and Rayleigh, traveled the 3836 km (2382 miles) along
the perimeter of the Earth from the earthquake to the recording station. The travel
time for surface waves was about 17 minutes.
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
This earthquake occurred on the boundary between
the Pacific and North American tectonic plates as a
result of thrust faulting. The Pacific Plate subducts
beneath the North America Plate along the Aleutian
Trench.
Given the regional plate tectonic setting of this
earthquake where the interplate megathrust fault
dips to the north-northwest, it is expected that the
fault plane is the north-northwest dipping nodal
plane.
Thrust Fault
Image courtesy of Richard Harwood,
Black Hawk College
USGS Centroid Moment Tensor Solution
Shaded areas show quadrants of the focal
sphere in which the P-wave first motions are
away from the source, and unshaded areas
show quadrants in which the P-wave first
motions are toward the source. The dots
represent the axis of maximum compressional
strain (in black, called the "P-axis") and the
axis of maximum extensional strain (in white,
called the "T-axis") resulting from the
earthquake.
An introduction to focal mechanisms can be found in the animation at the URL below:
http://www.iris.edu/hq/programs/education_and_outreach/animations/25
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
Seismic waves crossing the
US as recorded by the USArray
As earthquake waves reach
the surface of the Earth, they
cause the ground to move.
With the 400 earthquake
recording stations in
EarthScope’s Transportable
Array, the ground motions
can be captured and
displayed as a movie, using
the actual data recorded
from the
earthquake. http://www.iris.edu/sp
ud/gmv/1559866
Magnitude 7.0, Andreanof Islands, Aleutian Islands, Alaska
Friday, August 30, 2013 at 16:25:02 UTC
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