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 (18th 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 21st 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 16th 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.
