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.