Magnitude 8.0 Samoa Islands Great Earthquake
Tuesday, September 29, 2009 at 17:48:11 UTC
10:48:11 AM Pacific Daylight Time
Epicenter: Latitude 15.558°S, 172.073°W (indicated by star on maps below).
Magnitude = 8.0
Depth: 18 kilometers.
A great earthquake occurred Thursday morning Portland time near the northern end of the
Tonga Trench where the Pacific Plate subducts below the India – Australia Plate (Map A).
This earthquake occurred about 250 km southwest of American Samoa (Map B). 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. Map B shows the historic seismicity near the
northern end of the Tonga Trench. 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. Shallow great earthquakes
in subduction zones often cause tsunamis because they offset the ocean floor. A 3-meter-high
local tsunami was produced by this earthquake and did cause major damage to some coastal
villages. Damage and injury reports are still being finalized but the death toll is at least 130.
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.
Map A
Map B
Western
Samoa
Upolu
American
Samoa
Tonga
Trench
Earthquake
India Australia
Plate
Pacific
Plate
The record of the Samoa earthquake on the University of Portland seismometer is illustrated
below. Portland is about 8400 km (5200 miles, 75 degrees) from the location of this
earthquake.
Following the earthquake, it took 11 minutes and 42 seconds for the P waves to travel from
Samoa to Portland. P waves are body waves, compressional waves that travel through the
Earth’s mantle. About 14 minutes 30 seconds after the earthquake occurred, the PP waves
arrived. PP is a compressional body wave that bounces once off the Earth’s surface between
the epicenter and Portland. For this earthquake, the PP phase is quite visible.
The S waves started arriving 21 minutes 18 seconds after the earthquake occurred. S waves
are also body waves, but they travel as shear waves through the Earth’s mantle. SS waves,
are S waves with a path that differs by a bounce off the Earth’s surface between the epicenter
and Portland. SS waves are expected to arrive 26 minutes 12 seconds after the earthquake.
On this recording, the SS phase does not stand out and would be difficult to pick.
The surface waves traveled from the earthquake to Portland around the perimeter of the Earth.
Because the distance around the perimeter is longer than the distance through Earth’s mantle
and the speed of surface waves is slower than body waves, it takes surface waves much
longer than body waves to travel from an earthquake to a distant seismic station. In this case,
the first surface waves from the Samoa earthquake started arriving at the University of
Portland about 31 minutes after the earthquake occurred.