'When will this thing end?'

Cantabrians might be subjected to frightening earthquake aftershocks for weeks yet.

A large, shallow 5.1-magnitude aftershock, which hit at 7.49am yesterday, left residents shaken and caused more structural damage. It was the largest of more than 15 aftershocks in the region yesterday.

• Geologists map rupture 

• More on the Canterbury Earthquake...

"My guts is just churning up here. When will this thing end? It is like living in a maelstrom," Christchurch Mayor Bob Parker said.

"We were starting to think, maybe, just maybe, we are over the worst of this, and now we have had this shocking event. This is a hammer-blow to the spirit of a lot of people."

Yesterday, too, came news that Treasury has doubled its estimate of the Canterbury earthquake damage cost - to $4 billion.

Treasury gave an estimate of $2 billion at the weekend, but its secretary, John Whitehead, told media that was in relation to Earthquake Commission (EQC) liabilities only.

"The cost faced by the EQC, individuals, and insurance costs will probably be of the order of $4 billion," Mr Whitehead said.

The violent jolt yesterday occurred near Lyttelton Harbour, forcing the closure of the Lyttelton to Christchurch tunnel until 11.30am, after structural checks determined that only superficial damage had occurred.

Families in suburbs near Lyttelton Harbour were evacuated from their homes, which were worse hit yesterday than by the main quake on Saturday.

Hazel O'Leary (94) left the home she built in 1950 and is unlikely to return.

The Art Deco home, in Cashmere, was not damaged in Saturday's quake, but cracks split open her walls yesterday. A fracture rings the house along its foundations.

Her son, Jim O'Leary, who lives next door, said the cracks painted a clear picture of what had happened.

"It was huge, just a big bang up and down, boom boom. It was over in five seconds. The house was jolted up and fell back down again," Mr O'Leary said.

"This was the scariest one. We were just getting over it and this happens again. Saturday's went on rolling for a long time, but this was violent."