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Depth of quake meant Sumatra avoided tsunami
SMC NZ Alert
01/10/09
Depth of quake meant Sumatra avoided tsunami
Following the devastating earthquake and tsunami in the Samoan islands, an earthquake off the coast of Sumatra has also caused widespread damage and loss of life.
The Science Media Centre in the UK gathered these comments from experts on yesterday's seismic events.
Dr Dave Rothery, volcanologist and planetary scientist at the Open University in Britain comments on the earthquake in the Samoan islands:
"...this tsunami was caused by an undersea earthquake, at a subduction zone where one tectonic plate is being pushed down below another. In this example, the floor of the Pacific ocean is being pushed westwards below the Tonga island arc. Plates do not slide past each other smoothly. Instead, strain builds up until the deformation is relieved in a major jerk. In this case, the 'jerk' began at a relatively shallow depth of about 18 km, and the seafloor above it was probably jolted upwards by several metres. This sudden displacement caused a series of waves on the sea surface, which became higher and steeper when they ran ashore. Local reports speak of waves reaching more than 5 metres above sealevel and rushing 100 metres inland."
Dr
Rothery on the earthquake off Sumatra:
"The
earthquake offshore of Sumatra was powerful but fortunately
did not trigger a damaging tsunami, perhaps because it
happened about 50 miles (80 km) down. The nearby rupture
that caused the Indian Ocean tsunami on 26 December 2004 was
much shallower. Both were caused by the same process - the
floor of the Indian Ocean is being dragged down
northeastwards below Indonesia. The motion is not smooth,
but goes in fits and starts. When a sudden jerk happens,
that's what causes an earthquake, and sometimes a tsunami if
the epicentre is below the sea but not too
deep.
"Unfortunately however the ground-shaking in
Padang was enough to collapse the roofs of many buildings.
Probably if buildings, notably hospitals, had been better
constructed fewer people would have died. Often it is
schools - built at public expense but on the cheap - that
fall down. Maybe that is a story that will emerge from
Padang too, though because the quake was after 5pm local
time maybe the schools were empty by
then."
Professor Bill McGuire, Professor of
Geophysical & Climate Hazards and Director of the Aon
Benfield UCL Hazard Research Centre, comments on the tsunami
that hit Samoa:
"This most recent tsunami disaster
shows that there are lessons we yet need to learn from 2004.
Most critically, populations living close to faults capable
of producing earthquakes that trigger tsunamis must be
taught to self-evacuate when the ground shakes or the sea
recedes. Waiting for a warning from a central monitoring
station could mean the difference between life and
death"
And courtesy of the AusSMC:
Gary Gibson is Senior
Seismologist at Environmental Systems and Services, based in
Melbourne, and is Chairman of the Executive Committee of the
International Seismological Centre. He is an Honorary
Research Associate in the School of Geosciences at Monash
University:
"The earthquake south of Samoa
[yesterday] occurred at 06.48 AM Samoa time (03.48 AM
Eastern Australian Standard Time). It was located 200 km
south of Samoa, north of Tonga and northeast of Fiji, and
was at a shallow depth.
"The earthquake had a
moment magnitude of about 8.0, which corresponds to a
rupture within the earth that may be 200 to 300 km long,
with a depth of 100 to 200 kilometres, with one side of the
fault moving 4 to 7 metres relative to the other. This is
quite large enough to deform the sea floor and generate a
tsunami, but the size and pattern of the tsunami depend on
the amount and orientation of the sea-floor
deformation.
"The earthquake magnitude and
deformation were much less than in the rare huge earthquakes
of magnitude 9.0 and higher, such that off Chile in 1960,
and the Boxing Day 2004 earthquake off Sumatra. The fault
length for those was greater than 1000 kilometres, and the
plates moved 10 to 20 metres or more relative to each other.
For the Samoa earthquake, this meant that a devastating
tsunami could be produced locally, but it was unlikely to
seriously affect countries on the boundary of the
Pacific.
"The earthquake occurred at the north end
of a major north-south tectonic plate boundary, the
Tonga-Kermadec Trench, where the Pacific Plate is subducted
to the west under the Australian Plate.
"The USGS
has determined that the earthquake was due to northeast to
southwest tension, with the earth being stretched rather
than compressed. This leads to faulting that is oriented
southeast to northwest, with vertical motion that will
deform (and possibly rupture) the sea-floor, producing a
tsunami.
"This earthquake mechanism suggests that
the earthquake was not precisely along the plate boundary,
but was within the Pacific Plate where it has been bent over
to the west and southwest as it enters the subduction
zone.
"The mechanism and the aftershock
distribution suggest that the earthquake rupture propagated
from the epicentre at the southeast towards the northwest,
and not to the south on the much longer and larger
Tonga-Kermadec Trench. Beyond the rupture, the plate
boundary then gradually turns to the west and passes north
of Fiji.
"The orientation of the faulting would
have generated a tsunami with higher waves to the northeast
(towards Samoa) and to the southwest (towards Tonga, eastern
Fiji, and beyond towards New Zealand and the Tasman Sea).
This may well have seriously affected communities on
isolated islands, especially in Tonga and Fiji. Hopefully,
authorities will be urgently contacting these
people."
ends
Science Media Centre NZ
Our aim is to promote accurate, evidence-based reporting on science and technology by helping the media work more closely with the scientific community.
The Science Media Centre is New Zealand's only trusted, independent source of information for the media on all issues related to science. Thousands of news stories providing context from and quoting New Zealand researchers have been published as a direct result of our work.
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