Magnitude 8.0 SAMOA ISLANDS REGION

Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
A magnitude 8.0 earthquake in the Samoa Islands region,
has killed 39 people and left thousands homeless.
While there was minimal damage from the earthquake, most
of the damage and all of the loss of life from this
earthquake was from the resulting tsunami.
There is widespread damage to infrastructure,
communications, and power. There is additional
damage on the islands from landslides.
A main road in the downtown area of Fagatogo,
American Samoa is flooded by water
AP Photo/Fili Sagapolutele
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
Residents in both Samoa and American Samoa reported being shaken awake by the quake,
which lasted two to three minutes.
Sulili Dusi told New Zealand's National Radio that "everything dropped on the floor and we
thought the house was going to go down as well. Thank God, it didn't." Along with neighbors,
they fled to high ground.
There were no immediate reports of injury or serious damage from local emergency services, but
people reported cracks in some homes and items tossed from shelves.
Cars and people were swept out to sea by the fast-churning water as survivors fled to high
ground, where they remained huddled hours later. Signs of devastation were everywhere, with
a giant boat getting washed ashore and coming to rest on the edge
of a highway and floodwaters swallowing up cars and homes. AP
Tsunami waves
that swept ashore
on Samoa and
American Samoa
early Tuesday
flattened villages
Terri Chynoweth
A car is seen
pushed up against
a bridge after
strong sea water
from the tsunami
filled a small stream
AP Photo/Fili Sagapolutele
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
This great earthquake occurred near the northern end of the Tonga Trench where the
Pacific Plate subducts below the India – Australia Plate. This earthquake occurred about
200 km south of the island of Upolu. The rate of convergence at the Tonga Trench is about
80 mm/yr (8 cm/yr) and this is one of the most seismically active subduction zones on
Earth.
Western
Samoa
Upolu
American
Samoa
Tonga
Trench
Earthquake
India Australia
Plate
Pacific
Plate
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
Notice that the September 29 event occurred
slightly east of the axis of the Tonga Trench. We
often think of shallow subduction zone earthquakes
being caused by compressional (or thrust) faulting
on the interface between the two converging
plates. However, this earthquake resulted from
extensional (or normal) faulting within the top
portion of the Pacific Plate where it bends into the
Tonga Trench.
Normal Faulting
USGS Centroid Moment Tensor Solution
The tension axis (T) reflects the minimum
compressive stress direction. The
pressure axis (P) reflects the maximum
compressive stress direction.
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
Shallow great earthquakes in subduction zones
often cause tsunamis when they offset the ocean
floor. This offset generates tsunami waves. A 3meter-high local tsunami was produced by this
earthquake which arrived to the islands of Samoa
within 20 minutes.
"Some of the areas there are
only a few feet above sea
level, so you can imagine the
devastation," Eni
Faleomavaega, the territory's
delegate to the US Congress
Apia
Pago Pago
“American Samoa is a small island and most of the residents are
around the coast line. There are very few villages inland…There
was no warning or anything at all. By the time the alert was out of
the Pacific Tsunami Warning Center, it had already hit.” Filipo Ilaoa,
the deputy director of the Honolulu office of Gov. Togiola Tulafono
of American Samoa
MERAIAH FOLEY, NYTimes
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
Tsunami monitoring
systems have been
strategically deployed
near regions with a history
of tsunami generation, to
ensure measurement of
the waves as they
propagate towards the US
coastal communities and
to acquire data critical to
real-time forecasts.
Locations of the 6
tsunameters comprising
the operational network
are shown on the
accompanying map.
A DART® system consists of a seafloor bottom pressure
recording (BPR) system capable of detecting tsunamis as
small as 1 cm, and a moored surface buoy for real-time
communications. An acoustic link is used to transmit data
from the BPR on the seafloor to the surface buoy. The data
are then relayed via a GOES satellite link to ground stations,
which demodulate the signals for immediate dissemination
to NOAA's Tsunami Warning Centers.
Pacific Tsunami Monitoring
NOAA
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
These instruments assist the Pacific Tsunami Warning Center in monitoring tsunami
propagation and predicting the travel times for tsunami waves throughout the Pacific.
For this earthquake, the damage
from the tsunami was most
significant in Samoa and American
Samoa, but other regional islands
including Tonga reported damage.
The Pacific Tsunami Warning
Center issued a general alert for
the South Pacific region but a
tsunami warning for Hawaii and the
US West Coast was soon
cancelled.
A tsunami map shows projected travel
times for the Pacific Ocean. This map
indicates forecasted times only, not
that a wave traveling those distances
has actually been generated.
NOAA
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
Quick Time Required
Animation of the generalized
path of seismic waves
traveling from the Samoa
earthquake to a
seismometer in Portland,
Oregon
Jenda Johnson
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
The record of the Tonga earthquake on the University of Portland AS-1 seismometer is
illustrated below. Portland is about 8400 km (5200 miles, 75 degrees) from the location of this
earthquake.
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
The earthquake was well recorded across the
Seismographs in Schools Network. Seismic waves
generated from this earthquake were recorded for
hours at stations around the country.
NRMA
CHCA
CAOK
Magnitude 8.0 SAMOA ISLANDS REGION
Tuesday, September 29, 2009 at 17:48:11 UTC
DIAS
PAUL