An earthquake swarm is hitting the area of the South Pacific that in May generated tsunami fears which led to people fleeing their Gisborne and East Coast homes.
On Monday and Tuesday three shallow earthquakes – of magnitude 5.4, 5.1 and 5.2 – have been registered around 300 kilometres north east of Tonga .
The quakes have struck as South Pacific media report what they describe as a 32 percent increase in magnitude 5.5 or greater earthquakes over the last five years in the region.
On May 4 a magnitude 7.9 earthquake, the biggest so far this year and in the top 50 of recorded earthquakes anywhere, struck the same region. It generated a small tsunami but caused controversy in New Zealand when people fled their homes on the strength of international news reports. Civil Defence authorities, who realised a big tsunami was not on the way, made no statement until the drama was over.
With the latest swarm, reports in Fiji and Tonga warn of a looming ‘’big one’’ but scientists have downplayed this.
Geological and Nuclear Sciences seismologist Dr Warwick Smith told Fairfax that human memory was not long enough to determine earthquake trends which went over millions of years.
The Australia and Pacific tectonic plates had been moving together for a long time and it was regularly producing earthquakes, including the latest sequence.
‘’I would think of that as a swarm, rather than an aftershock sequence…. Not well understood why earthquakes behave in this way, but they do happen,’’ Dr Smith said.
‘’They come and go.’’

Deep below the South Pacific, in a line from Samoa to New Zealand , vast continental plates are setting such cracking speed records they are rattling the entire planet.
Near Tonga the vast Australia and Pacific plates are crashing together at a world record setting combined velocity of 24 centimetres a year.
As the Pacific plate slides under the Australia plate the snap, crackle and pop of rock triggers massive earthquakes. On a global seismic map the region is a vivid red gash marking its tremors; in Tonga its seen in mythical terms, a clash between Polynesian gods.
In May, one such clash, had Gisborne people fleeing their beds after international news reports warned a magnitude 7.9 earthquake was sending a tsunami their way.
As the big temblors continue, some wonder if the ‘’big one’’ is not far off, while calmer heads say it’s a case of scientists getting better at measuring what goes on.
Of interest is a chunk of the South Pacific between Latitude 15 to 25 South and Longitude 170 to 180 West. It takes in most of Tonga and extends toward Fiji , Samoa, Niue and down toward New Zealand . Its most obvious feature is the long gash in the seabed, the Kermadec-Tonga Trench. To the west is a ridge which has a major volcano every 20 to 30 kilometres, and runs from Tafahi in northern Tonga to New Zealand . Underwater the northern terminus is romantically labelled ‘’King’s Triple Junction’’.
Part way down are the nearly 40 islands of Tonga ’s Ha’apai group were 12,000 people live a subsistence life on farming and fishing. It has had two moments in history: a powerful chief, Taufa’ahau was born there and went on to create the Tongan royal family. And in 1789 the Royal Navy ship Bounty was sailing through Ha’apai waters as its crew mutinied.

At 39 seconds after 4.26am on Thursday May 4, just to the east of Ha’apai’s main village of Pangai , and 55 kilometres below the surface, the two plates slipped against each other, creating a magnitude 7.9 earthquake, the biggest so far this year and in the top 50 of recorded earthquakes anywhere.
A man was injured jumping out of a hotel building in Nuku’alofa, 160 kilometres from the epicentre, and across Ha’apai buildings and roads were cracked. The US Geological Survey said it was felt from Wellington to Samoa and across to the Cook Islands and a small tsunami was recorded across the Pacific.
Living at Earthquake Central, Tongans are used to it, but one ‘’Ofa-ki-Tonga’’, writing on the popular Matangi Tonga website, said recently earthquake numbers had increased 32 percent in the last five years.
‘’It should also be noted that the total number of earthquakes for the last few years keeps going up, and that has been the trend for the last 20 years,’’ he writes.
In the first six months of this year, there have been 807 earthquakes, 24 over 5.5 magnitude.
‘’What is really happening beneath Tonga along the volcanic ridge?”
Dr Warwick Smith of New Zealand’s Geological and Nuclear Sciences answered saying scientists are getting better at recording shakes.
‘’The usual problem that lies behind claims like this is that people just take the total number of earthquakes reported in catalogues, not realising that the detection threshold usually reduces with time, i.e. the network gets better and detects more small earthquakes.
Ofa-ki-Tonga’s claims spooked Tonga and their principal government geologist, Kelepi Mafi, warned that they had no rescue plan in place for the ‘’next big one’’ and a tsunami.
He told Matangi Tonga there was no tsunami warning system; ‘’If an earthquake hits Tonga, it will take me five minutes to drive down to the office to calculate the data automatically received at the office, and I'll spend an average of 15 minutes to half an hour calculating it before I pass it on to the National Disaster Committee to declare a public warning - far too slow.’’
Dr Tim Worthington of the Institute of Geoscience at Christian-Albrechts University in Kiel, Germany, co-authored one of the key studies on the region which, he says, has ‘’long intrigued geologists’’.
He calms fears of an impending giant quake.
‘’At the present time no-one can predict if or when a big quake will occur, or whether the current ‘unrest’ in Tonga will end with one or more big quakes or simply fizzle out when all the built up stress has gone,’’ he told Fairfax.
‘’A ‘fizzle’ is more likely in my opinion.’’
Quake activity in the region is higher but it was all a question of time scale.
‘’As this subduction takes place, the two plates rub against each other and stresses build up. These are released as earthquakes. On ‘geologic’ timescales of thousands to millions of years, all this takes place at a constant rate.
‘’But in human terms, there are periods when the stresses on the plates are mostly building up and periods when they are being released faster than normal. So we tend to see periods of ‘unrest’.
He says that the quakes could be loosening up the region which was heading for a major quake.
‘’But on the other hand, you could also argue that every small quake is releasing stress and making a big quake less probable. A couple of hundred magnitude 4 to 5 quakes release as much stress as a single devastating magnitude 8, and clearly we would all rather suffer the few hundred that might make us nervous but don't cause much damage.’’
University of the South Pacific Oceanic Geoscience Professor Patrick Nunn says while there has been an ‘’uncommon amount of seismic activity’’ in Tonga this year, it was just ‘’a blip in a natural cycle’’ and meant little.
‘’I would counsel against making too much of the story, perhaps instead focusing on the lamentable lack of tsunami preparedness of Tonga which is, after all, in much the same location as Aceh (Indonesia, worst hit by the 2004 Asian tsunami).’’