article: Nov 30 2005 - Rare Auckland Quakes Probably Relieving Tension

Seismologists are describing a series of small earthquakes under Waiheke Island today as rare but not unexpected.

All of the earthquakes, three of which have been felt, were centred 30 km east of Auckland. The first two, both magnitude 2.5 and 8 km deep, occurred at 11:35 pm and 11:54 pm on Tuesday. At 11:08 am today a magnitude 3.3 quake occurred at the same location and at a similar depth.

Duty seismologist at GNS Science, Ken Gledhill, said there was no known fault where the earthquakes occurred. There had been earthquakes under the Hauraki Gulf before, but it was most unusual for three to occur in one day. It seemed unlikely the quakes were a signal that Auckland’s volcanic field was re-awakening, Dr Gledhill said.

"If magma started rising from deep within the earth, we would expect it to start causing earthquakes much deeper than 8 kilometres. The most likely explanation is that there is a knot of tension in the earth’s crust which is gradually being relieved."

"It is possible there could be more earthquakes in this sequence. We will keep a watch on this area simply because of its proximity to a volcanic field and a major population centre."

Dr Gledhill said the biggest of the three quakes was recorded on five seismic instruments in the Auckland area, and by more distant instruments at Morrinsville, Whitianga, and at Waipu Caves, south of Whangarei.

"The only reason the earthquakes were felt is because they were located directly under Waiheke Island. If they had been further out in the Hauraki Gulf, it is unlikely anyone would have felt them."

Dr Gledhill said the area north of Auckland had a history of clusters of small shallow earthquakes occurring on rare occasions. The Waiheke Island quake sequence seemed consistent with this type of activity.