More earthquakes in Hawke’s Bay, but it’s not a swarm

A sudden surge in the frequency of earthquakes being recorded in Hawke’s Bay and nearby areas will come as a surprise to many.

Earthquake preparedness exercise Shakeout in action at a Waipukurau school last year. Photo / NZME

Most of about 120 shakes recorded since 11am on Tuesday were “unnoticeable”, according to seismic agency Geonet. However, internet watchers will have noted the series, which surround a shake of 5.0 magnitude centred around Seddon, in Marlborough, at 5.49pm on Monday, and a 6.6 shake about 2400km away and south of Tonga at 9.09am on Wednesday.

The majority were in eastern regions from Marlborough to the East Cape, with 10-15% in the Hawke’s Bay-East Coast region, where the last major earthquake involving significant damage was the Mother’s Day earthquake in May 1990.

While Geonet records thousands of tremors each year, the 70 from 11am Tuesday to 11am Wednesday appear to be about double per day of an average reported to be about 14,000 a year.

They are, however, not of “swarm” status, according to Geonet, which by comparison points to the 600 in the Kawerau area on the weekend of March 18-19 last year. There were almost 11,000 nationwide in January-June that year.

“Earthquake swarms are quite common in New Zealand,” Geonet said at the time. “While they may sound scary, swarms are just a collection of quakes about the same size, happening in a localised area, usually over a short time period [hours to days to weeks]. Swarms usually don’t have a mainshock or larger quake that starts off a sequence.”

Seismic duty officer Elizabeth Abbott told Hawke’s Bay Today: “The activity along the East Coast and across Cook Strait in Hawke’s Bay and Seddon does not appear unusual for seismicity in New Zealand.

“When we get a larger event, we usually expect to see more earthquakes immediately after a large earthquake [an aftershock sequence], which would very much increase the number of earthquakes we would locate over a period of a few days in that specific region,” she said.

“Although we are seeing more earthquakes in the past few days, it is just a more active time, but the activity around the North Island does not appear to be swarms, or driven by anything specific.

“These events are usual for seismicity in New Zealand.”

However, the earthquakes were “a good reminder that we live in the shaky isles, and so we should be prepared for earthquakes anywhere at any time”.

Statistically, some are “deleted” from the count when further information rates them as “ghost” quakes.

Abbott said that when the seismic waves of a distant event were “rolling through our network”, the detection system did not recognise the wave arrivals as coming from one local earthquake.

Geohazards analysts manually review such earthquakes and delete them. She said “ghost” quakes were an “unfortunate side effect” of getting information out as quickly as possible.

A national earthquake drill and tsunami hīkoi staged by the National Emergency Management Agency (Nema) will be held on October 24, at 9.30am.

Nema’s Civil Defence Emergency Management director John Price encouraged schools, businesses, community groups and whānau to sign up if they had not already.

“All of New Zealand is at risk of earthquakes and tsunamis at any time. Shake Out gives us the chance to practise, exercise and test the correct actions to take during an earthquake and tsunami.

“A large earthquake such as an Alpine Fault rupture will be a defining event for all of New Zealand. We need to be ready.

“By practising Shake Out each year, knowing what to do to stay safe in an earthquake and tsunami will become second nature. Exercising regularly improves our ability to cope on the day and keep ourselves and our communities safe.”