Moderate Rotorua Quake Rocks North Island

At 1:14pm NZDT on the 11th January 2016 a local magnitude 4.6 (moment magnitude 4.8) earthquake struck 25 kilometres east of Rotorua in the north of the North Island, just south of the Bay of Plenty. The earthquake was given a focal depth of 150 kilometres, consistent with subduction related earthquakes beneath the Taupo Volcanic Zone. This earthquake was likely near the plate interface as the Pacific Plate subducts beneath the Indo-Australian Plate below the North Island. The earthquake was very well felt, with over 750 felt reports received by GeoNet, from Te Kaha in the north to Blenheim on the South Island. The highest intensities were received from eastern locations on the North Island, namely Te Aro & Tawa (Wellington area), Hastings and Mangapapa (Gisborne area), all at MM5 (moderate) intensity.

This is the first moderate felt earthquake in New Zealand since the local magnitude 4.7 earthquake near Opotiki on the 5th January. As with that earthquake, this earthquake occurred at an intermediate depth, although owing to the fact it was further west than the Opotiki event it struck at a greater depth. This is because the Pacific Plate is sinking beneath the North Island from off the east coast at the Hikurangi Trench.This it the second most felt earthquake so far this year after the local magnitude 5.1 Akitio earthquake on the 4th January (which received over 1100 felt reports).

Yesterday's Rotorua earthquake was a result of oblique reverse faulting on a vertical fault plane at a depth of 150 kilometres. Whether this puts it within the Pacific or Indo-Australian plates is impossible to say with certainty. Nevertheless, with a moment magnitude of 4.8 this is the largest New Zealand earthquake so far this year.

Written by J H Gurney, 12:05am NZDT, 12th January 2016.
From Devon, United Kingdom.

Deep Earthquake Strikes Bay of Plenty

At 12:57pm NZDT on the 5th January 2016 a local magnitude 4.7 earthquake struck 15 kilometres southeast of Opotiki in the north of the North Island, just south of the Bay of Plenty. The earthquake was given a focal depth of 78 kilometres, consistent with most quakes in this area (although shallow seismicity does occur as well). This certainly places it near the plate interface as the Pacific Plate dives very deep below New Zealand and the Indo-Australian Plate at this point. The earthquake was felt relatively well with more than 300 felt reports received by GeoNet, from Te Kaha in the north to Wellington in the south. The highest intensities were received from more epicentral population centres, Whakatane, Opotiki and Opape at MM5 (moderate).

This is the third earthquake so far this year in New Zealand which has had a magnitude greater than 4.0. The 4th January had two such events: a moment magnitude 4.6 (local magnitude 5.1) near Akitio which was the result of Pacific in-slab normal faulting and a moment magnitude 4.2 (local magnitude 4.0) near Akitio which was (with very low confidence from GeoNet*) was a result of Pacific in-slab reverse faulting at 36 kilometres depth. This latest event near Opotiki was likely oblique normal or normal faulting, although I could not say in which plate regardless of any focal depth given by a moment tensor solution.

*The Akitio Earthquakes on the 4th January 2016 occurred an hour after the moment magnitude 6.7 earthquake in Manipur Province, India - this earthquake killed at least 23 people. The low frequency body waves from the India Earthquake masked the local signal from the Akitio Earthquake, making any moment tensor solution questionable. I have been assured that the moment tensor for the local magnitude 5.1 earthquake is quite reliable, but owing to its smaller magnitude the later local magnitude 4.0 event has a far poorer confidence in the solution given (personal email, John Ristau, GeoNet).

Written by J H Gurney, 10:44pm NZDT, 5th January 2016.
From Hampshire, United Kingdom.

First Moderate Seddon Aftershock of 2016

At 9:04am NZDT on the 5th January 2016 a local magnitude 4.2 earthquake struck 15 kilometres east of Seddon. This earthquake is the first moderate aftershock to the Seddon Sequence of July and August 2013. Currently GeoNet has received more than 160 felt reports, primarily from Blenheim and Wellington, but also with reports from Picton and Seddon. The maximum felt intensity was MM5 (moderate) in Blenheim, a usual location for maximum intensities from Seddon earthquakes.

This earthquake has not yet been revised, so it's location needs to treated with caution. Currently it is located beneath Cape Campbell at a depth of 37 kilometres, placing it deep within the Pacific Plate. Whilst the location is very plausible, the depth is more questionable.

More to follow...

Written by J H Gurney, 9:35am NZDT, 5th January 2016.
From Hampshire, United Kingdom.

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.

Eketahuna Aftershocks Still Strong

At 5:35am NZDT on the 3rd January 2016 a local magnitude 3.7 earthquake struck about 5 kilometres south-east of Eketahuna in the Lower North Island. The earthquake was felt locally, with reports from Mauriceville, Eketahuna, Pahiatua and Palmerston North. Felt intensities did not exceed MM4 (surprising considering the proximity to Eketahuna and the depth being shallower than previous Eketahuna events at only 10km).

This earthquake is the first moderate earthquake in New Zealand this year. A M4.0 near St Arnaud did not actually occur, it being one of three mislocated earthquakes sourced from wave arrivals in New Zealand from a moment magnitude 5.6 earthquake near Raoul Island, ~1200 kilometres north-north-east of the North Island. There will be many more of these events this year - I simply refer to them as Kermadec earthquakes (along the Kermadec Trench, which marks the plate boundary between the Pacific and Indo-Australian Plates to the north of New Zealand (with the Pacific Plate diving, or subducting, westwards beneath the Indo-Australian Plate).

The Eketahuna earthquake is most interesting as it struck outside the aftershock zone from the January 2014 Mw 6.2 earthquake. It is also much shallower (the aftershock zone from the 2014 quake had depths of between 20 and 35 kilometres). This is likely a result of stess changes in the crust - when an earthquake strikes, the rock around the rupture zone from it is further stressed (or less stressful) as a result. This is what caused the February 2011 Christchurch Earthquake after the Darfield M7.0 Earthquake in September 2010 and the August 2013 Lake Grassmere Earthquake after the July 2013 M6.6 Cook Straits Earthquake.

Last Wednesday (30th December 2015) a magnitude 4.7 earthquake struck 20km west of Pongaroa, ~30km north-east of today's Eketahuna earthquake. This event was definitely a result of stress triggering by the 2014 earthquake and was very well felt with over 500 reports to GeoNet and a maximum intensity of MM5. It was also the right depth for it to possibly be related to the main Eketahuna earthquake at 22km (or a similar neighbouring fault). A moment tensor solution, which will show which type of fault was responsible for the 30th December event, should be released by GeoNet in the coming days and will confirm if it was indeed on the same fault (or give supporting evidence to such a hypothesis).

I expect many more earthquakes in New Zealand this year and this blog shall continue to report on them as they occur. Posts will only be made for well felt earthquakes or swarms and not for all, or for large events north or south of New Zealand (on the Kermadec Trench and Macquarie Ridge respectively).

For more information on my Eketahuna research, see here.

Written by J H Gurney, 10:16 NZDT, 3rd January 2016.
From Hampshire, United Kingdom.