© NewScientist

Today's fatal earthquake near Christchurch in New Zealand confirms that a country already riddled with major fault lines has gained another one, say seismologists."Christchurch has never been identified as a major earthquake zone, because no one knew this fault ran beneath," says Roger Musson, a seismologist at the British Geological Survey in Edinburgh.New Zealand experiences thousands of earthquakes each year, because it lies on the boundary between the Pacific and the Australian tectonic plates. To the north-east, the Pacific plate is subducting beneath New Zealand's North Island, and to the south-west, the Australian plate is subducting beneath the South Island. Between these two subduction zones lies the Alpine fault, running along the mountainous spine of the South Island.It now appears likely that the Christchurch quake resulted from activity on a fault extending directly eastward from the Alpine fault that remained unknown until last year, says Musson.The new fault first came to light last September when a stronger but less calamitous quake shook Darfield, 40 kilometres west of Christchurch. Musson says the latest quake probably resulted from an eastward continuation of activity on the same fault. "It has probably not moved for tens of thousands of years, so lots of strain built up," says Musson.Christchurch was understandably unprepared for activity on a fault that is only now making its presence known. But two factors made today's damage worse. The quake was just 5 kilometres down, limiting the amount of energy it dissipated before reaching Christchurch from its epicentre just 10 kilometres away. Also, the rock on either side of the fault accelerated almost three times as fast as in a typical quake, says Musson, so the shaking was extra violent - and significantly greater than the levels Christchurch's structures have been designed to withstand.