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Computer model warned scientists week before big Feb quake
New paper confirms computer model warned scientists week before Feb 22 of big quake
27 January 2012
Scientists have confirmed in a new research paper on last year's disastrous February 22 Christchurch aftershock that computer modelling showed a week beforehand that there was a 25 percent chance of "a magnitude 6 or greater earthquake occurring in the general aftershock zone of the Darfield earthquake in the next year".
They wrote in the paper,(available here) that time-dependent earthquake forecast models such as the Short Term Earthquake Probability (STEP) model were implemented after the 7.1 magnitude September 2010 Darfield earthquake, and forecast an aftershock "of the size of the 22 February earthquake with a high probability".
STEP was created in 2005 by Dr Matt Gerstenberger
of GNS Science and his colleagues in the United States and
Switzerland, and research into operational earthquake forecasting is
continuing overseas.
During the Royal Commission inquiry
into the Canterbury earthquakes the issue was probed last October. GNS
scientist Dr Kelvin Berryman said that the GNS team had
grappled over the merits of making a public warning: "There
were five or six obvious scenarios where that future
earthquake might occur. The worse possible case was directly
under the city. "It wasn't really a reasonable approach
to try and do that at that time because of the range of
places where that magnitude six might occur. We didn't want
to alarm unnecessarily," he said in October 2011. After
September 4, GNS staff noted a "social science recommendation" to
provide basic numbers. But some residents were alarmed by a
subsequent magnitude-4.9 Boxing Day quake, and another GNS
Science researcher, Dr Terry Webb said last October that in
the first couple of weeks after that Boxing Day 2010 shake,
"social science advice was basically that we've got a
traumatised population and what can you do to help them cope
best, and that really was to get them coping with
aftershocks". After February 22, GNS Science did make
public statements warning of the possibility of a further
aftershock of one magnitude less than the 6.3 magnitude
shake which resulted in the deaths of 182 people Earlier
this year it gave a public briefing on aftershocks fading
away over as long as 30 years, and Dr Berryman has said scientists should have a better picture
by next week of when Canterbury's shakes will settle into a
"background" level. Today, the journal paper -- published
in the New Zealand Journal of Geology and Geophysics
with GNS Science seismologist Anna Kaiser as lead author
-- said that the Darfield tremor was a "large regional
quake" and the February 22 a "close-by moderate-sized
earthquake" with stronger shaking than was expected for a
quake of that size. "The ground motions were of very low
probability according to the national seismic hazard model,"
the authors said. "Ongoing efforts are focused on better
quantifying the factors that contributed to the high ground
motions in order to assess the possible implications for
future earthquakes in the Canterbury region, and by
extension other comparable areas in New Zealand and
worldwide." Key points included: • Horizontal
ground motions 1.7 times the force of gravity were larger
than expected. The Science Media
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evidence-based reporting on science and technology by
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centre, or to offer feedback, please email us at smc@sciencemediacentre.co.nz. ENDS
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• One reason for this was the rupture's
"directivity" towards Christchurch.
• A trampoline
effect enhanced by the "slapdown" of falling upper soil
layers hitting rising sub-soils.
• "Remarkably high"
levels of apparent stress: two possible reasons are
canvassed.
• High water tables trapped energy in the
top layers of soil in some areas, boosting
liquefaction.
• The Banks Peninsula volcanic outcrop
may have concentrated the stress field.
• Data from 129
of Geonet's sites created the most significant dataset since
strong-motion recording began 50 years ago.
• And 17
ground and structural records from within 10km of the fault
are prized: most Wellington is also within 10km of a
fault(s).
• Liquefaction caused the largest damage to
land and buildings, including many CBD
high-rises.
• Deep-seated landslides caused most damage
in the southern Port Hills.
• Critical structural
elements in buildings 1976-1992 failed, staircase damage was
severe in some post-1992 buildings.Science Media Centre NZ
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