Telegraph News, UK
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A strong magnitude-7.1 earthquake rattled the coast of New Zealand's North Island on Friday morning, triggering a small tsunami but not causing any major damage or injuries.
The earthquake is the biggest to hit the North Island of New Zealand since 1995 when a magnitude 7.2 quake struck in a similar spot.
Scientists are investigating the possibility that the seismic event may have been two earthquakes, according to news website stuff.co.nz.
The complexity made it hard to estimate how deep the quake was, and to identify whether there was any tsunami risk scientists said.
Concerns have also been raised about the time taken to issue a tsunami warning after the quake - reported as a magnitude 7.1, which struck at 4.38am about 125 kilometres from land and was felt as far as Christchurch in the South Island.
The response by the Ministry of Civil Defence and Emergency Management (MCDEM), which took 40 minutes to send the first national advisory after the quake hit and a further 20 minutes to issue a tsunami alert, will be reviewed said the New Zealand Herald.
An initial "potential" warning was upgraded to a tsunami after a wave about 24cm above the normal tide level was measured on an East Cape gauge. The tsunami warning was later lifted.
"Based on all available data, the greatest tsunami activity has now passed. However, coasts may still experience unusual, strong currents and sea level fluctuations lasting for several more hours," MCDEM said in a statement.
The warning covered the East Coast of the North Island and the upper South Island.
It was centred 167 kilometres (103 miles) from the nearest main town, Gisborne, which has a population of around 45,000.
The shallow tremor was estimated at a depth of around 30 kilometres (18 miles) off the coast, according to the US Geological Survey (USGS).
Ken Gledhill, head of monitor group GeoNet, said experts had trouble working out the depth of the "complex" quake.
"We think it's the complexity. We think it might have been two earthquakes on top of each other," Mr Gledhill said.
"By itself the earthquake didn't pass the criteria for a tsunami warning. That was the issue." It was not apparent until sea level measurements started coming in that something needed to be done.
"Had the earthquake been larger it would have been easier to issue a tsunami warning quickly.
"As it was we had to wait until we had some confirmation (of a tsunami). It wouldn't have really changed much if we had a different system in place," Mr Gledhill said.
While there were no reports of injured people, some buildings private water supplies appear to have been damaged in the Gisborne region.
Earthquakes in that area are not uncommon as it's one f the most seismically active areas in New Zealand, GNS seismologist John Ristau told the New Zealand Herald.
"But to have a 7.1 earthquake is unusual," he said.
"What you have in New Zealand is a you have the North Island sitting on the Australian tectonic plate and off the East coast you have the Pacific plate that's pushing beneath the North Island.
"That generally causes a lot of earthquakes along the East coast. What we call it is a plate interface boundary."
The boundary of the Australian and Pacific tectonic plates, known as the "Ring of Fire," experiences up to 15,000 tremors a year.
A 2011 earthquake in the South Island city of Christchurch killed 185 people and caused extensive damage.
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