On Late Tuesday night, there were dual small earthquakes of M1.9 and M2.7 in the northern area of the Hauraki gulf associated with the ongoing earthquake swarm off Whangarei. That now brings the total number of quakes in the sequence to 119 over the last 41 months. Before the swarm began, there was no known fault line in that location, 35km east of Whangarei Heads. The nearest known fault is the Whangarei Harbour fault, 25km to the south west running through Bream Bay.
The swarm is also in the same location as a major New Zealand international communication cable and the Niagara shipwreck. The communication cable is the newly commissioned Southern Cross NEXT which will be the largest capacity data link between Sydney, Auckland, and Los Angeles at 16,148 km. The Niagara ship was sunk on 19 June 1940 by a German marine mine and is still believed to be carrying up to 1000 to 1500 tonnes of oil and bars of gold.
The sequence of earthquakes started on 26th December 2018 with peak intensity and number of quakes between February and April 2019. This included with three magnitude +4 quakes and ten magnitude +3 quakes during that period. The main shock so far was a magnitude 4.1 on 10th February 2019. Since then, the numbers and size of the earthquakes have decreased but they still continue until the the time of writing. For reference, GeoNet’s official statement on the earthquake swarm released on 10 February 2019 when the count stood at 39 quakes.
For comparison, the Hauraki Gulf region next closet sized swarm that has been recorded by GeoNet was in 2007 with 14 shallow earthquakes over a 4 month period.