On Friday, 19 July, a magnitude 5.7 earthquake located in Cook Strait was felt strongly throughout the lower North Island and upper South Island. This quake was 25 km east of Seddon and at a depth of 16 km, and was felt from Auckland to Otago, with reports of minor damage such as items falling from shelves reported in Marlborough, Nelson and Wellington. This was the first of a complex sequence of earthquakes that continued to shake the area for the following several weeks, and for which smaller aftershocks still continue. It was followed on the morning of Sunday, 21 July, by a similarly located event of magnitude 5.8, and then 10 hours later, at 5:09 pm, by another of magnitude 6.5.
This quake, known as the Seddon earthquake, resulted in varying degrees of damage to thirty-five buildings within the Wellington CBD, with glass from broken windows falling onto the main thoroughfare. Instances of moderate to serious building damage were also reported throughout the Wellington region, and in the Seddon area. Four people received minor injuries, and it was reported felt from Northland to Otago. Four aftershocks of magnitude 5.0 to 5.5 and about 70 of magnitude 4.0 to 4.9 followed in the period to 15 August.
At 2:31 pm on 16 August, a magnitude 6.6 earthquake centred at Lake Grassmere, 10 km south-east of Seddon, occurred. This was at a depth of 8 km, and caused significant damage in the epicentral region, including land damage and landslips that blocked several roads, and serious damage to buildings. In Wellington, moderate damage was caused to some buildings and some had to be evacuated. Suburban train services were cancelled and there were traffic jams on major roads. It was reported felt from Auckland to Invercargill. Four minor injuries and one serious medical condition were reported. In the period to 5 September, there were 16 aftershocks of magnitude 5.0 or greater, with one of these of magnitude 6.0 occurring on 16 August. The last aftershock to have been strongly felt was a magnitude 4.7 event on 6 December.
Other significant earthquakes were:
- On 1 January, a shallow magnitude 5.0 centred 30 km north-west of Opunake, was felt from Auckland to Nelson, most strongly in the Taranaki region. Minor damage was reported in Stratford. A magnitude 4.3 aftershock shortly later was also felt strongly.
- On 16 February, a magnitude 6.3 centred 200 km north of Te Araroa, at a depth of 290 km, was felt from Bay of Plenty to Otago, most strongly in the north-east of the North Island.
- On 23 February, a magnitude 4.8 centred 40 km south-east of Wairoa at a depth of 70 km was felt in the Gisborne – Hawkes Bay region.
- On 17 March, a magnitude 3.9 earthquake centred 15 km north-east of Auckland, near Motutapu Island, and at a depth of 6 km was felt strongly throughout the Auckland region. There were several reports of minor damage such as items falling from shelves. Close to 14,000 felt reports were submitted for this event. It was preceded a few minutes earlier by a similarly located magnitude 3.1 that was also widely felt in Auckland.
- On 4 July, a magnitude 5.3 at a depth of 5 km and centred 25 km north-west of Milford Sound was felt from Southland to the West Coast, most strongly in Queenstown and Oamaru.
- On 8 July, a shallow magnitude 4.9 centred 25 km north-east of Waipukurau was felt from East Cape to Christchurch.
- On 29 July, a 9 km deep magnitude 4.7 centred 20 km north-west of Culverdon in North Canterbury was felt from Taranaki to Otago.
- On 17 December, a magnitude 6.2 centred 30 km off the south-west coast of Fiordland at a depth of 25 km was felt from Stewart Island to Wellington. No damage was reported.
Two earthquakes centred in the Kermadec Islands region were felt widely in New Zealand. On 12 August, a magnitude 6.1 quake at a depth of 340 km was felt from Auckland to Invercargill, most strongly in Wellington, and on 12 October, a magnitude 6.2 quake at a depth of 150 km was felt from Auckland to Marlborough, mainly along the east coast of the North Island.
Christchurch activity continued at a low level in 2013, with three aftershocks of magnitude 4.6 being the largest recorded. The latest on 18 November caused a few instances of minor damage.
No significant swarm activity occurred during 2013.
The GeoNet Duty Team responded to a tsunami alert following the Santa Cruz Island (Solomon Islands) earthquake on 6 February. A marine threat was in place for some coastal area of New Zealand for around eight hours and maximum wave amplitudes of around 20 mm were measure at a number of tsunami gauge sites.
In 2013, there were four earthquakes of magnitude 6.0 or greater, and 66 of magnitude 5.0 to 5.9. About 2,500 earthquakes were reported felt.