On the Late April 2015 Seddon & St Arnaud Earthquakes, New Zealand

This is just a short update on recent seismic activity, with an unsurprising focus at present on New Zealand.

On Wednesday night at 10:36pm BST a local magnitude 4.4 earthquake struck due east of Cape Campbell on the north-east coast of the South Island of New Zealand. This earthquake is a bit of a surprise – it has been 21 months since the first Mw 5.5 earthquake struck in the Cook Straits & began the Seddon sequence (18^th July 2013 in the UK). This Cape Campbell earthquake proved to be a foreshock to a larger local magnitude 5.1 earthquake, which struck about 7km north-east of Lake Grassmere.

This second Seddon earthquake is probably the more interesting; it struck on what I assumed in my Extended Project was the second fault to rupture with the 21^st July 2013 Mw 6.5 earthquake. This second fault was strike-slip, trending roughly 233° (NE-SW) with a dip of about 70-80°. Though GeoNet have not yet released a moment tensor solution for this earthquake, USGS did give one: Mw 4.65, nodal plane 1 trending 234° (NE-SW), dipping 62°. This matches what I would expect, and its location at the epicentre of the Mw 6.0 earthquake which struck on the 16^th August 2013 (after the second mainshock underneath Lake Grassmere on the same day) seems to infer that this earthquake was indeed on the second fault to rupture during the 2013 sequence.

This morning, at 4:36am BST a large earthquake struck underneath the Kaikoura Ranges in the north-east of New Zealand’s South Island, about 35km south-east of St Arnaud. This earthquake had a local magnitude of 6.2 (GeoNet) - moment magnitude of 6.0 (GEOFON) & 6.1 (USGS) and had a focal depth of 52km (GeoNet). The earthquake was a long one due to its location, depth and magnitude – most people who felt it in New Zealand reported on Twitter that shaking lasted at least 20 seconds. The earthquake was felt across the whole country – over 6500 felt reports were received by GeoNet, from Auckland in the north, to Dunedin in the south.

As soon as I saw it (I was unfortunately awake when the earthquake struck!) I could discern several things. First of all, there was unlikely to be any structural damage. If the earthquake had struck Christchurch or Wellington city centres (as the Port Hills quake did in February 2011) then there would have been severe damage and likely fatalities; however the quake struck in the sparsely populated Kaikoura Ranges between Kaikoura and St Arnaud, whilst the depth meant there was unlikely to be any damage regardless to modern structures in New Zealand (and probably only very minor damage to older buildings).

Secondly, the type and depth of earthquake indicated that there would be few aftershocks and those which did occur would not follow Båth’s Law. Generally the largest aftershock is expected to be one magnitude unit below that of the mainshock – in this case it would be about magnitude 5.0. However, moderate depth earthquakes rarely follow such a rule – generally the largest aftershock is 1.5-2 magnitudes below that of the mainshock. So far there have been three major aftershocks – a M3.7 (32km depth, 4:46am BST), a M3.8 (36km depth, 4:53am BST) & a M4.2 (96km depth, 3:35pm BST – not revised). I immediately wrote this tweets on my UKEQ account:

“M6.3 strikes upper South Island, NZ, ~70km depth. Large aftershock (M5+) are unlikely #eqnz”

Unsurprisingly, when the first detailed news article was put out by GeoNet (http://info.geonet.org.nz/display/quake/2015/04/24/Quake+hits+inland+Marlborough), they agreed with my assessment entirely. So far my assessment for aftershocks has proven correct and will very likely remain accurate.

The only remaining thing which I immediately asked was if there was a relationship between the Kaikoura Ranges earthquake & the Seddon earthquakes the day before. I concluded that though it was very unlikely the M4.4 & M5.1 earthquakes directly triggered the M6.2 Kaikoura Ranges earthquake, it was plausible that the Seddon earthquakes as a whole could have been a catalyst or contributor to this morning’s shake. This may well be investigated by seismologists in the next few months to years (perhaps a journal article will appear). Once again, the news article above corroborated with my own beliefs. It is therefore very evident that my experience with New Zealand seismicity – particularly South Island seismicity – has meant that my conclusions and assumptions are well founded from observations over the past 50 or so months.