Real Scientist – by Alice Klein
New Zealand is continuing to shake after being hit by a magnitude 7.5 earthquake, and there are fears the underground stress could set off even bigger tremors nearby.
“We currently calculate a 12 per cent probability of a magnitude 7 or larger earthquake within the next day, and 32 per cent within the next 30 days,” says John Ristau at GNS Science, New Zealand’s geoscience research and consultancy firm. “An earthquake like this can increase the risk of a major earthquake nearby, although it can also decrease stress on a nearby fault and lessen the risk.”
The latest earthquake struck 90 kilometres north-east of Christchurch in the South Island on Monday just after midnight local time, killing two people and tearing up buildings and roads in the rural area.
The quake triggered a tsunami warning that was later cancelled. Waves measuring 2.5 metres hit the coast soon afterwards, but further, larger waves of up to 5 metres that were feared didn’t happen.
Preliminary data show the earthquake occurred on a previously unknown fault near the interface of the Australian and Pacific tectonic plates. The South Island was thrust up over the Pacific plate with some sideways slip.
Stress change
The stress change could trigger a powerful earthquake at the interface between the Australian and Pacific plates, known as the Alpine Fault, says Kevin McCue at Central Queensland University. The giant Alpine Fault borders the South Island and splays into many faults through the North Island, he says.
If the 600-kilometre fault ruptures, it will produce one of the biggest earthquakes in New Zealand since European settlement, says GNS Science. It last ruptured in 1717.
However, Ristau believes Monday’s earthquake is probably too far away from the Alpine Fault to have a direct effect. Nevertheless, it could set off the closer Hope Fault, which branches off the Alpine Fault, he says.
The region has already experienced more than 300 aftershocks, half of which have been magnitude 4 or greater. The strongest aftershock recorded so far was magnitude 6.3.
New Zealand sits on the seismically active “Ring of Fire” around the Pacific Ocean, where about 90 per cent of the world’s earthquakes occur.
A magnitude 6.3 earthquake struck 10 kilometres from Christchurch’s central business district in 2011, killing 185 people and injuring thousands more.
“…could trigger a mega-quake”
or relieved enough tectonic pressure thus preventing one…