Government earthquake experts held back forecasts of a massive aftershock for Christchurch following the magnitude 4.9 Boxing Day jolt because they didn't want to alarm the already traumatised population.
The admission came as GNS scientists gave evidence at the close of the third day of hearings of the Royal Commission inquiry into the Canterbury earthquakes.
Under cross-examination by counsel for earthquake victims' families this afternoon, GNS scientist Dr Kelvin Berryman admitted information was withheld from the public, on advice from social scientists scared for the beleaguered city's collective mental health.
Christchurch lawyer Marcus Elliott, counsel assisting the interests of victims' families at the hearings, said that after February 22, GNS made public statements warning of the possibility of an aftershock of one magnitude less than the 6.3 magnitude which resulted in the deaths of 182 people.
He then asked the panel of four scientists if they considered issuing the same warning after the December 26 shake.
Mr Elliott asked: "If someone had come to GNS after Boxing Day and said, `we want to give people the Christchurch a people of understanding if this one less magnitude earthquake occurred under the city', could GNS have calculated the likely ground forces and given people an understanding of what they might expect?''
The panel confirmed they could provide such information.
But Dr Berryman admitted the GNS team were "very aware'' of the issue and had grappled over the merits of making such a public warning.
He said: "I think this line of questioning is very valuable. In that period of December and early January we were very aware of where would this magnitude six occur if it was to occur.
"There were five or six obvious scenarios where that future earthquake might occur. The worse possible case was directly under the city. Other possibilities were out in the southwest around Wigram, or Lincoln, and areas at Rolleston, and way out at Hororata.
"But that's not particularly helpful in terms of advice going forward. You can be particularly optimistic or particularly pessimistic.
"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.''
His colleague at GNS Science, Dr Terry Webb said the idea of a warning was deemed "unhelpful''.
Dr Webb admitted: "In the first couple of weeks (after the Boxing Day 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.
"They had a need and a right to know, and that's why we readily talked about the possibility of a six, so that certainly wasn't hidden.
"But what was thought of as unhelpful at the time, was talking about the possibility of an event bigger than Darfield (magnitude 7.1 on September 4) itself, in terms of magnitude.''
The first of 11 hearings in the inquiry, sitting at St Teresa's Parish church hall in Riccarton, Christchurch, continues and is due to conclude tomorrow (Thursday).
Christchurch rebuild plans 'unrealistic'
A Californian scientist has warned that future buildings erected across New Zealand will never be earthquake-proof and labelled Christchurch rebuild plans "unrealistic."
University of California professor Norman Abrahamson gave the stark warning during the third day of hearings.
During a live video conference, the international earthquakes expert questioned several aspects of a key government report produced after the tragedy.
The adjunct professor of civil engineering at Berkeley said GNS Science plans to make buildings across New Zealand safer during earthquakes go much further than those in his home state of California, which sits astride the infamous San Andreas Fault.
He said that New Zealand society must debate how much they are willing to pay to make buildings as safe as possible.
Professor Abrahamson said: "In the end, it all comes down to risk, and what is acceptable risk. In the earthquake business we don't do anything where we design for the worst case and give people zero risk. There will always be a risk."
He criticised the section on minimising risk in the GNS Science report, 'The Canterbury Earthquake Sequence and Implications for Seismic Design Levels', in his international peer review.
The expert asked: "What are the acceptable risk values? I think the (GNS) guidance document is brief on this - and I did get some additional information from Dr McVerry (GNS principal scientist) on where the risk numbers come from. But in my view, these risk numbers are unachievable as written in the report. Your goal is a bit too low and very difficult to obtain."
He said that California - especially San Francisco, which was devastated by the magnitude 7.9 earthquake of 1906 which killed up to 3000 people - and a 6.9 magnitude shake in 1989 - had a much more "realistic" building code.
Pointing to the high aversion to risk of loss of life in the GNS report, Abrahamson said: "We are nowhere near those numbers in California. We're at double the high end of that range."
He added: "In trying to think what is appropriate we have to start to think what is realistic numbers.
"It's not for me to decide, it's a broader social issue - do you want to have more safety at a higher cost, or do you accept the kind of risk numbers that are acceptable around the world?"
Prof. Abrahamson said New Zealand's existing building code was consistent with international standards.
He said the government needed to ask if they wanted to go above that level for added safety to try to cover more of these earthquakes on unidentified faults in the future.
After his evidence, Abrahamson took part in a panel discussion with GNS scientists.
The first of 11 hearings in the inquiry, sitting at St Teresa's Parish church hall in Riccarton, Christchurch, was due to be completed today, but is expected to run over into tomorrow (THURS.).
The commission is set to hear evidence until next March before presenting its findings to the government by April 11.
Evidence on the collapse of the PGC building, where 18 people died, is scheduled to start on November 28, while the CTV building, which killed 115 people, is down to be heard in March.