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.

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.