A series of powerful tremors rattled the quake-prone New Zealand city of Christchurch Monday, destroying a building and sending boulders tumbling down hillsides. Six people suffered minor injuries.New Zealand's GNS Institute put the strongest of the quakes at a magnitude of 6.0 at 2.20 p.m. The United States Geological Survey also pegged it at 6.0 -- with the epicentre four miles east south east of the city at a depth of 5.6 miles. Four tremors above 4.3 were recorded from 0029 GMT. Shopping malls and office buildings were evacuated across the city, which is still trying to recover almost four months after a magnitude 6.3 earthquake killed 181 people and caused extensive damage. A building in the city center collapsed and minor damage occurred in buildings affected in previous earthquakes, Christchurch Police spokesman Steven Hill told Reuters."The building has been checked and been given the all-clear," Hill said, meaning no one was trapped inside. The St John ambulance service said six people had been taken to hospital with moderately serious injuries, all caused by falling building materials.As with the initial quake, Monday's aftershock sent boulders on the city's Port Hills tumbling toward houses. Parts of the eastern city, which suffered the most damage in February's tremor, suffered from flooding and liquefaction - where solid ground is turned into liquid by the force of the quake. Power was cut to about 10,000 houses.Christchurch has experienced a number of strong earthquakes since a magnitude 7.1 quake struck the city on Sept 4 last year.Parts of the central city have been closed to the public since the Feb 22 quake, and one of the city's tallest buildings, the Grand Chancellor Hotel, has been declared unstable and is being prepared for demolition.The cost to rebuild Christchurch after the quakes has been estimated at around NZ$15 billion ($12.2 billion).