Scientists analyse impact of quake swarm

The Institute of Geological and Nuclear Sciences is looking at what a swarm of earthquakes that shook the lower North Island and upper South Island yesterday means for the regions.

Nine earthquakes were recorded within nine hours from yesterday afternoon, with the biggest reaching 5.3 on the Richter scale. Seven of the quakes were centred 40km southeast of Martinborough in Wairarapa.

Dr Martin Reyners, duty seismologist at the Institute of Geological and Nuclear Sciences told NZPA yesterday's events were uncommon but had happened before.

"It's uncommon but there was a similar sequence about 15 years ago near Cape Palliser in Southern Wairarapa ... (in which) a 5.3 magnitude event was the largest of them," he said.

"That's very similar to what we had last night."

Dr Reyners said there was no need for concern and the institute was investigating.

"In terms of energy release these are very small, but having said that we are looking into what they mean in terms of what the plates are doing under the Wellington region."

Today the institute would look at the mechanisms of the events, he said.

"Each earthquake is a little fault break and we can work out the mechanisms reasonably quickly -- so we can say whether the fault went up or down or sideways," he said.

After that the institute would look at the bigger picture.

"Every earthquake, when it happens, changes the stress field in the immediate vicinity and what that can do is move some nearby faults closer to failure and some away from failure," Dr Reyners said.

"So we're interested in what effect this might have on plate interface as a whole."

Dr Reyners said recent events in Asia -- notably Boxing Day's high magnitude quake and subsequent deadly tsunami -- had led to a heightened public interest in earthquakes.

This became clear after yesterday's first earthquake, he said.

"One thing we noticed yesterday is we had the event west of Porirua in the afternoon, and we have a facility now on the GeoNet website where people can give information on how they felt the earthquake, and I think in the first three hours we had over 500 people doing that," he said.

"There is definitely a heightened interest in earthquakes in terms of the public response that we see."

Small aftershocks were likely today, he said.

"People living on the Wairarapa coast may feel one or two, but just looking at the seismographs this morning they seem to be dying away."

The quakes:

The first, at 1.38pm, measured 4.4 on the Richter scale and was centred 30km northwest of Porirua at a depth of 50km.

The second, at 5.19pm, measured 3.6 and was centred 20km west of Hastings at a depth of 20km.

The next seven were centred 40km southeast of Martinborough and were at a depth of 15km to 25km:

* 6.54pm -- 3.8 magnitude

* 7.24pm -- 3.7 magnitude

* 9.36pm -- 5.3 magnitude

* 9.46pm -- 3.8 magnitude

* 9.49pm -- 2.8 magnitude

* 10.00pm -- 3.1 magnitude

* 10.26pm -- 5.2 magnitude

- NZPA