Cellphone calls reveal Christchurch exodus

Destruction in Manchester Street in Christchurch's CBD. Photo / Brett Phibbs

Cellphone records show that about a fifth of all Christchurch residents fled the city after the February 22 earthquake.

A novel study by Statistics NZ, based on records from two of the country's three cellphone networks, has found that 20 per cent of the cellphones that were used in Christchurch City in the 10 days before the earthquake were used outside the city in the week after the quake.

Many people fled only to the surrounding parts of Canterbury, but between 8 and 10 per cent of those who left made cellphone calls in Otago and about the same proportion in Auckland, with Wellington a close third.

Only 2 per cent of the callers went to Marlborough and about the same number to the West Coast.

The study is unusual internationally because cellphone companies in most countries have kept their calling data secret. Kirsten Nissen, a Christchurch-based Statistics NZ analyst who co-ordinated the study, said the two local networks understood the public value of their data.

The third network, which she did not name, refused to co-operate.

The two networks provided records of all cellphone calls in February, March and April last year and this year, allowing a comparison of post-earthquake calls with the pattern in a normal year.

In most weeks last year about 5 per cent of Christchurch-based cellphones were used outside the city.

This year the number jumped to 20 per cent straight after February 22, but then fell back to about 7 per cent by April, showing that most people who left returned to the city within two months.

However, the numbers were still notably higher than at the same time last year in Auckland and Otago, showing that some have still not returned.

Statistics NZ estimated last month that Christchurch's usually resident population fell by 8900 people, or 2.4 per cent, in the year to June.

Project manager Adele Quinn said that estimate was based on data on births and deaths, international migration, enrolment in schools and primary healthcare organisations, electoral rolls, electricity consumption, Earthquake Commission data and tax-based employment data, as well as the cellphone study.

Dr David Johnston, director of the Joint Centre for Disaster Research run by GNS Science and Massey University, said 12,000 children from schools in Christchurch, Selwyn and Waimakariri districts re-enrolled in other schools immediately after the quake.

But 6600 of those children returned to their original schools by mid-September, leaving 5400 still enrolled elsewhere.

Wellington researcher James Newell said the Christchurch population had been growing by about 3000 a year before the earthquake, so the net effect of the quake was to reduce the population by about 12,000.

Phone figures

* 20 per cent of cellphones used in Christchurch before February 22 were used outside the city in the weeks after the quake.

* Auckland, Wellington and Otago had the biggest concentrations of exiles.

* In most weeks last year about 5 per cent of Christchurch-based cellphones were used outside the city.

* This year the number jumped to 20 per cent straight after February 22, but then fell back to about 7 per cent by April, showing that most people who left returned to the city within two months.