Thorne Lay, UCSC: Two
Great Earthquakes Caused 2009 Samoa-Tonga Tsunami Disaster
August 19, 2010
Scientists studying the massive earthquake that struck the South Pacific
on September 29, 2009, have found that it actually involved two great
earthquakes: an initial one with magnitude 8.1, which then triggered
another magnitude 8 earthquake seconds later on a different fault. The
details of this rare event, called a "triggered doublet," are unlike
anything seismologists have seen before.
Thorne Lay, UCSC
"We know of no precedent for the Samoa triggered doublet," said Thorne
Lay, professor of Earth and planetary sciences at the University of
California, Santa Cruz, who led a seismological analysis of the event
published in the August 19 issue of Nature.
The earthquakes unleashed devastating tsunami waves that swept onto the
islands of Samoa, American Samoa, and Tonga, killing 192 people. It took
months, however, for seismologists to make sense of the confusing
seismic data and figure out exactly what happened in the Earth's crust
to cause this disaster.
Most great earthquakes (earthquakes of magnitude 8 or greater) occur in
subduction zones, where one plate of the Earth's crust dives beneath
another plate. The Tonga subduction zone in the South Pacific marks the
boundary where the Pacific plate is sinking under the Australian plate.
In the sequence of events on September 29, the first earthquake actually
occurred not at the subduction zone, but within the Pacific plate at a
site 50 to 100 kilometers (30 to 60 miles) east of the plate boundary.
The rupture occurred along an extensional or "pull-apart" fault in the
middle of the plate. Such large extensional faulting near a subduction
zone is rare, and this is the third largest such event recorded in the
110-year history of seismological monitoring.
According to Lay, these events seem to occur in areas where the plate
boundary is relatively weak and one plate slides easily under the other.
As the leading edge of the oceanic plate sinks into the mantle, it pulls
on the rest of the plate, bending it downward and causing it to break
along the extensional fault.
"We think the subducted slab is pulling on the Pacific plate, and that
'slab pull' is responsible for the bending and extension of the plate
manifested in the Samoa earthquake," he said.
As seismologists in several programs, including Lay's team, began to
study the event in more detail, they noticed some strange
inconsistencies. Aftershocks were spread over a huge area, including the
Tonga subduction zone, with relatively few along the fault in the
Pacific plate that had ruptured. In addition, when researchers used
different methods to calculate the fault geometry, they came up with
inconsistent solutions, which is rare for large earthquakes. Finally,
tsunami prediction models gave results that were not entirely consistent
with observations from ocean buoys and on the islands.
At a meeting of the American Geophysical Union in December, Chen Ji of
UC Santa Barbara, who was not a member of Lay's team, suggested that the
event involved two earthquakes on different faults with different
geometries. Lay said he immediately began exploring the idea proposed by
"The huge signals from the extensional faulting made it difficult to
resolve the triggered event, but we were able to confirm that secondary
faulting had occurred, that it involved thrust faulting in the Tonga
subduction zone, and that it released energy from about 50 to 130
seconds after the onset of extensional faulting within the Pacific
plate," Lay said.
The strong shaking from the initial fault rupture appears to have
triggered the second event, which involved two major subevents of
magnitude 7.8, with a total magnitude equal to 8.0. This second event
was followed by many aftershocks in the subduction zone. It also
produced a tsunami wave that interfered with and complicated the wave
pattern generated by the initial event. And it made the seismic wave
analysis very difficult.
"It's impressive that we could have a magnitude 8 earthquake and not
recognize it, but this one occurred so quickly after the first that it
was not observed by the normal procedures," Lay said.
According to Lay, there is one prior case, in the Kuril Islands, of a
pair of great earthquakes that included a subduction zone rupture and
extensional faulting in the oceanic plate. In that case, however, the
initial event occurred on a thrust fault in the Kuril subduction zone on
November 15, 2006 (magnitude 8.4). Two months later, on January 13,
2007, the Pacific plate ruptured in a magnitude 8.0 extensional event.
In that case, the slippage on the thrust fault put added stress on the
extensional fault, which eventually ruptured.
"That type of behavior has been observed many times, but usually the
late extensional faulting is much smaller than the thrusting event," Lay
In the Samoa-Tonga event, the extensional faulting occurred first and
triggered the thrust fault, with a delay of less than a minute. The
location of the thrusting event was not right next to the extensional
faulting, but farther south along the Tonga subduction zone.
"In order for slab-pull to have loaded the extensional stress within the
Pacific plate, the region of the subduction zone right next to the first
event had to have low friction," Lay said. "The sudden extensional
motion then produced strong vibrations that shook the plate boundary to
the south and caused the abrupt thrusting motion."
the transient high strains from the shaking of the first earthquake,
that region might have slipped slowly rather than abruptly. According to
Lay, there is no record of prior great thrusting earthquakes in this
region. "This is a state called conditional stability; the region would
normally have episodic slow sliding without earthquakes, but if
accelerated it becomes a normal fast-sliding zone," he said.
Earthquake triggering greatly complicates earthquake forecasting and
rapid-warning procedures, Lay said. "There may be far more interaction
between earthquakes than we have understood," he said. "Studying these
complexities is essential for improving our understanding of how
earthquakes rupture, how they interact, and how we can mitigate their
impact on humans."
In addition to Lay, the coauthors of the paper include Charles Ammon of
Pennsylvania State University; Hiroo Kanamori of the California
Institute of Technology; Luis Rivera of the University of Strasbourg,
France; Keith Koper of St. Louis University; and Alexander Hutko of the
U.S. Geological Survey. This research was supported by the National
Science Foundation and the U.S. Geological Survey.