The September 29, 2009, quake in Samoa, American Samoa and Tonga was in fact a triple-quake.
Click on image for full size
Image Courtesy of Keith Koper, University of Utah
Deadly Tonga Earthquake Revealed as Three Big Quakes
Geologists have learned that a magnitude-8.1 earthquake and tsunami that killed 192 people on September 29, 2009 in Samoa, American Samoa and Tonga was in fact a triple-whammy. The 8.1 "great earthquake" concealed and triggered two major quakes of magnitude 7.8.
"At first, we thought it was one earthquake," says paper co-author Keith Koper, director of the University of Utah Seismograph Stations. "When we looked at the data, it turned out it wasn't just one great earthquake, but three large earthquakes that happened within two minutes of one another. The two quakes that were hidden were responsible for some of the damage and tsunami waves."
The earthquake created tsunami waves that varied in height depending upon where they struck. In some places the water reached more than 49 feet above sea level. It is the first time that scientists know that a large "normal" fault earthquake (the 8.1 quake) occurred on a sea-floor tectonic plate and then triggered major "thrust" quakes (the 7.8 quakes) in the "subduction zone" where the oceanic plate is diving or "subducting" beneath a continental plate of Earth's crust.
"This is the first time a large normal-faulting quake has been shown to trigger large thrust-faulting earthquakes," says Koper.
Scientists know of only three previous cases of great earthquakes--those measuring magnitude 8 or more--that happened due to pull-apart or normal faulting within a diving seafloor plate.
Shop Windows to the Universe Science Store!
Our online store
includes fun classroom activities
for you and your students. Issues of NESTA's quarterly journal, The Earth Scientist
are also full of classroom activities on different topics in Earth and space science!
You might also be interested in:
The expression "on solid ground" is often used to describe something as stable. Usually the solid ground underfoot seems very stable. But sometimes it is not. "The ground seemed to twist under us like...more
The main force that shapes our planet's surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics. This picture shows how the rigid outer layer of the Earth,...more
When two sections of the Earth's crust collide, one slab of crust can be forced back down into the deeper regions of the Earth, as shown in this diagram. This process is called subduction. The slab that...more
Scientists have learned that Mount Hood, Oregon's tallest mountain, has erupted in the past due to the mixing of two different types of magma. "The data will help give us a better road map to what a future...more
The Earth's mantle is a rocky, solid shell that is between the Earth's crust and the outer core, and makes up about 84 percent of the Earth's volume. The mantle is made up of many distinct portions or...more
Some geologic faults that appear strong and stable, slip and slide like weak faults, causing earthquakes. Scientists have been looking at one of these faults in a new way to figure out why. In theory,...more
The sun goes through cycles that last approximately 11 years. These solar cycle include phases with more magnetic activity, sunspots, and solar flares. They also include phases with less activity. The...more