Moderate Earthquake Rattles North Island

At 1:07pm on the 4th January 2016 a local magnitude 5.1 earthquake struck 20 kilometres east of Pongaroa on the lower North Island. The earthquake was given a focal depth of 25 kilometres (as is stated), which indicates it occurred within the Pacific Plate. With a relatively deep focus (a shallow depth compared to others, but deep for the area in which it struck) this earthquake was unsurprisingly well felt from Picton in the south to Okahune in the north. The maximum intensity reported was MM6 in the Palmerston North area; this seems a common place for highest reported intensities from earthquakes in this part of the lower North Island. The earthquake has had to 15 aftershocks in the 12 hours after the mainshock between M1.7 and M4.0. From herein this event shall be referred to as Akitio II due to its proximity to the township and river of the same name. This earthquake is remarkable as being one which I "predicted" - I shall explain this later on.

On the 12th October 2015 a moment magnitude (Mw) 5.7 (local magnitude 5.8) earthquake struck 15 kilometres east of Pongaroa on the lower North Island. The earthquake was given a focal depth of 23 kilometres, which also indicates it occurring within the Pacific Plate. Over 6000 felt reports were received by GeoNet, with reports from Auckland down to Dunedin. The earthquake had an intensity of MM7 (damaging) in Tiraumea and MM6 intensities from Ongaonga (near Waipukurau) in the north to Ohau (near Levin) in the south. Over 900 of the felt reports received were from the Palmerston North area. A GeoNet moment tensor solution (given to me via email) indicated that this earthquake was in fact shallower (14 kilometres deep) and a result of either steeply-dipping reverse or shallowly-dipping oblique reverse faulting. This would indicate that the earthquake actually struck within the lower part of the overlying Indo-Australian Plate. The earthquake was followed by a relatively strong aftershock sequence, with two strong aftershocks striking within two hours of the mainshock (local magnitude 4.2 and 4.4). From herein this earthquake will be referred to as Akitio I as it was also very close to the Akitio township.

The area afflicted by these events is no stranger to moderate or even large earthquakes. In March 1934 a massive earthquake (moment magnitude 7.2-7.4) struck Pahiatua, injuring one and causing large damage to the lower North Island. In February 1990 a local magnitude 6.1 earthquake near the township of Weber was followed by a moment magnitude 6.4 earthquake about 10 kilometres northeast in May. Two smaller earthquakes (magnitude 5.5) struck in August 1990 and March 1992 as aftershocks to the February event. In January 2014 a moment magnitude 6.2 earthquake struck 20km east of Eketahuna and was followed by two earthquakes of magnitude 5.2 and 5.5 in September 2014.

The Weber Earthquakes are undoubtedly the most helpful regarding the current Akitio Earthquakes. The Weber Earthquakes began with a local magnitude 6.1 earthquake about 10 kilometres north of Akitio I in February at 1990 (Weber I). This event, with a depth of approximately 20 kilometres, was a result of normal faulting on a steeply dipping fault within the subducting Pacific Plate. The May event (Weber II) occurred about 10 kilometres northeast of the February event and was shallower (about 10 kilometres deep), thus occurring within the overlying Indo-Australian Plate. This event was a result of oblique reverse faulting on a shallow fault, just as Akitio I likely was. This means that these sequences (the Weber Earthquakes of the 1990-1992 and the ongoing Akitio Earthquakes) are inextricably linked and similar in mechanism and cause. Stress increases caused by Weber I triggered the later Weber II earthquake.

Back to the fact I predicted yesterday's earthquake (Akitio II). As the aftershock sequence to Akitio I was occurring my mind was brought back to the Weber Earthquakes I described above. At the time I knew very little about the sequence other than its occurrence (I was born in 1996) so, owing to Akitio I occurring in the same area I decided to use it as a proxy for what might follow. The aftershock sequence, though strong enough, was not outstanding as past New Zealand sequences had been (such as Seddon in July and August 2013). This made me pose the question of whether further earthquakes of magnitude 5.0 or larger might occur; I surmised that it was more likely to be updip of Akitio I due to stress transfer perhaps preferentially following a line of weakness (the plate interface), thus coming to the location of yesterday's earthquake. Whether this is the exact cause of Akitio II or not is unclear, but stress transfer was certainly involved and this earthquake cannot be called an aftershock of Akitio I either (it is outside the initial aftershock zone from Akitio I and more than likely struck on a different fault, perhaps even a normal fault if indeed it occurred in the Pacific Plate).

All of this will need further analysis and exploration, but I believe this is most likely case for the earthquake of January 4th 2016. Further aftershocks to this earthquake may occur, but are unlikely to be too large. As always with any shallow New Zealand earthquake there is a very small chance that this could be a foreshock to a larger earthquake in the same area. The stress changes induced by Akitio II will have been smaller than those from Akitio I due to its smaller magnitude, but may increase stress on another fault where stress was already increased by Akitio I (or could just have easily nullified the earlier stress increase). A moment tensor from GeoNet will at least answer some of these questions.

Edit 11:08am NZDT 6th January: I have just received an email reply regarding the moment tensor solution for the Akitio II local magnitude 5.1 earthquake. This shows that the earthquake had a moment magnitude of 4.6 (a bit smaller than I suspected) and a focal depth of 30 kilometres. This places it within the subducting Pacific Plate and to correlate with what Weber I (February 1990) would infer it was the result of a moderately dipping normal fault, almost certainly the nodal plane dipping to the northwest at 52 degrees. This would explain why an earthquake of a smaller magnitude (4.6) was felt so widely - it was for the east coast of the lower North Island a relatively deep focus earthquake.

Written by J H Gurney, 2:26am NZDT, 5th January 2016.
Frkm Hampshire, United Kingdom.