Seismologists scrambling to identify last December’s Sumatra earthquake,
which triggered a devastating tsunami across the Indian Ocean, labeled it as
a magnitude-8.9 event within hours of receiving the first seismometer measurements
from across the planet. Within a day, the size was revised to magnitude 9.0.
Now that researchers have had time to go back to the records, they are finding
indications that the earthquake released much more energy than they thought,
in the form of rare low-frequency seismic waves.
Preliminary results show that the Dec. 26 earthquake may have been almost three
times as large as originally calculated, according to Seth Stein and Emile Okal
of Northwestern University, in Evanston, Ill., bumping the magnitude-9.0 earthquake
to a magnitude-9.3 event (on a logarithmic scale). They cautiously hypothesize,
Stein says, that such a large release of strain could partially reduce the future
threat for a tsunami in the region.
The researchers, who posted their results on Stein’s Web
site in February, found the extra energy in underlying low-amplitude seismic
waves with very long periods; the slow motion took at least 600 seconds, or
about 10 minutes, to unfold. The short fast waves that were initially measured
— the noticeable jerk that triggered the tsunami — “is how we
think of an earthquake,” says Meredith Nettles, a graduate student at Harvard
University who works on the Centroid-Moment-Tensor project, a leading magnitude-determination
group. “But at the same time, you have a long, drawn-out process in the
background and continuing even longer,” as if two earthquakes were superimposed
on each other.
That “background” earthquake moved the fault slowly enough and such
a short distance — possibly half a meter over the entire fault in the course
of an hour — that it would have been unfelt by humans standing above it,
Nettles says. The shaking over several minutes at higher frequencies could have
amounted to about 20 meters of displacement on the southern third of the Sumatran
fault. She says that other researchers have studied the low-period oscillations
from seismometer recordings of the event, finding different amounts of energy
released.
“It’s still a moving target,” says Jeff Park of Yale University
in New Haven, Conn. “One difficulty is that I don’t think we know
precisely where all the slip occurred” and on what part of the fault, something
that will take further examination of the seismic records. However, Park notes
that the few large aftershocks so far (about magnitude 7) are 1,000 times smaller
than the main shock. “To have so few [large] aftershocks is not unique,
but it is surprising,” he says, and might support Stein and Okal’s
assertion of lower future risk of tsunamis from the upper part of the Sumatra
fault.
The few low-frequency earthquakes that have ever been documented seem to be
associated mostly with large earthquakes. Park and others say that seismometers
have improved dramatically, enough so that since some of the last large-magnitude
quakes, such as the 1964 magnitude-9.2 Alaska earthquake, the instruments have
been able to detect and accurately record such low periodicities. (Therefore,
the Alaska quake may have been even bigger than seismometers then could record,
Stein and Okal say.)
“We observe slow, low-frequency motion in other parts of the world,”
says Kelin Wang of the Geological Survey of Canada. Wang and co-workers measured
similar strain releases indicated by changing pressures on the Juan de Fuca
triple junction plate boundary, without earthquakes associated. Nettles notes
that anomalous long-period signals also were seen in a 1992 earthquake in Nicaragua,
which triggered a tsunami larger than expected, making it one of an elusive
class of “tsunami earthquakes.” Wang says that such low-frequency
waves might also be possible in the Pacific Northwest’s Cascadia subduction
zone, which is similar to the Sumatra fault zone.
Resizing the earthquake probably will have little impact on tsunami warning
systems, but Park says that a review of the performance of the global seismic
network in place was satisfactory. That system contributed to the speed of the
initial determination of the Sumatra earthquake’s size. Nevertheless, “there
are still a lot of questions being posed” about the earthquake, Park says,
and further results are expected to be published in peer-reviewed journals later
this year.
Naomi Lubick
Links:
Seth
Stein and Emile Okal's Web site
"Sumatra
seismic risk," Geotimes, March 28, 2005
"Hundreds
dead in magnitude-8.7 Sumatra earthquake," Geotimes, March 28,
2005
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