Moderate Quake Rocks Otago Province
Mb 4.1, 30km west of Dunedin, South Island, New Zealand.
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| The Magnitude 4.1 Lee Stream Earthquake as seen on the New Zealand National Seismograph Network |
A magnitude 4.1 earthquake has struck a short distance away from the South Island's third city, Dunedin, gaining over 1200 felt reports and a maximum intensity within the city of MM6. The quake's effects were relatively localised, mainly due to the sparsity of major settlements, so that the quake was felt at Oamaru to the north, Balclutha to the south and Kingston to the west. A supposed felt report from Arthur's Point, Queenstown, is not to be believed due to the lack of reports from Queenstown and it's associated suburbs.
The earthquake struck at 6:44pm NZST, and had a focal depth of 4 kilometres. It was located near the small community of Lee Stream, and only 10 kilometres north-west of the town of Outram.
The Dunedin region is one of the most aseismic parts of New Zealand, with seismic events quite rare. A quick look into the archives finds that since 1980, there have only been 6 earthquakes over magnitude 4 recorded by GeoNet, three of them offshore (M4.2 in 1982, M4.3 in 1989 & M4.1 in 1991).
The other three quakes (including today's) are located west of Dunedin, the northernmost near Middlemarch (M4.1, 1986). The other two are in closer proximity and therefore it is likely they occurred on the same fault, giving a very rough recurrence interval for a quake of the same size as today's. These two quakes occurred near Lee Stream. In 1982 an M4.0 struck at a depth of 12km, only a short distance east of today's epicentre. Today's quake struck at a depth of 4km, and with the increase in seismograph stations across New Zealand since the early 1980s the location and depth are almost certainly much more accurate than the 1982 event.
The largest quake to strike the region in the past century was an offshore M5.0 event in April 1974. The earthquake struck at 7:50pm local time, about 10km south of central Dunedin, at a depth of about 12km. The earthquake caused chimney damage, masonry to crack and tiles to come loose in Dunedin and its immediate suburbs. The fact that there have been five offshore earthquakes over M4 between 1941 and 1991, all south of Dunedin, infers a fault system there which is possibly, but not necessarily, quite constrained in size.
In recent years there have been seven earthquakes over M3 within 30 kilometres of Dunedin. Three such earthquakes occurred south-west of the city (M3-3.5), two south of the city offshore (M3.2-M4.1), one to the north of the city (M3.1) and today's earthquake. There have been many smaller earthquake, particularly to the south-west of the city on the Akatore Fault (see below).
Possibly the scariest thing about this is how little the public knows of the threat to the eastern Otago region. Only in recent years has there been any sort of effort to analyse seismic hazards in this region, with three major faults discovered: the Akatore Fault, which runs parallel to the coast from the Taieri River mouth to about 30km south-west of Dunedin, the Titri Fault which runs for 60 kilometres parallel to the Akatore Fault, and the lesser known Green Island fault, which is probably responsible for the offshore quakes mentioned above. The Titri Fault or a related fault is most probably responsible for today's earthquake.
The warning here is rather disconcerting and worrisome - the Greendale & Port Hills Faults were unknown before the Darfield & Lyttelton Earthquakes in 2010-11, and the damage they caused to Christchurch and the wider Canterbury Plains was massive. Seismicity in the region was known, with at least three quakes known from the 19th century underneath the city itself, and a M5.6-5.8 quake known to have occurred underneath Lake Ellesmere in 1870. None of these previous quakes were in living memory, nor on the faults which ultimately ruptured.
Perhaps the same is true for Dunedin and the Otago region. Dunedin is primarily built atop a Miocene basalt bedrock, very similar to that which Lyttelton and Akaroa are built in on Bank Peninsula, and the type of bedrock which was undoubtedly home to the deadly Port Hills Fault. Maybe it should be seen that, much like in Christchurch, the people of Dunedin should be aware of and prepare for the threat of a damaging earthquake in the future.
Search Parameters for Dunedin Data
General Information on Dunedin Seismic Risk
This in-depth article was written on the 16th October 2014 by J H Gurney.
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