The Richter Scale was devised in 1935, so magnitudes have only been verifiable since then.
Looking at NZ’s earthquake records can show which areas have been more active, enabling patterns to be roughly worked out as a predictive tool.
As we are mostly interested in destructive events, the following analysis covers earthquakes that have been above 5M, from the 1930s to present.
In our earthquake recorded history, Cape Reinga to Bombay has shown the least activity with only 3 verified above 5Ms, the last in 1948.
Otago-Southland is another inert region, with only 2 above 5Ms verified, the last in 1998. Taranaki's big shakes have numbered 15, but all no greater than the 5.6M that occurred in 1957. Taranaki's last above 5M was in 1998.
In Marlborough there have been over 20 above 5Ms, the largest a 5.5M in 1950, but none have exceeded 5.5M. The large tracts of flat land in the district may have much to do with the lowered activity.
In the Central North Island there have been nearly 30 exceeding 5M but nearly all have been below 5.5M.
However, earthquakes here can set off dangerous lahars or avalanches.
For Bay of Plenty west, over 30 above 5Ms have been recorded, with 2 above 6M (1987 and 1992).
The last above 5M was in 2010. The most active year for above 5Ms was 1974 with 8 in March of that year.
In Buller, only about 40 above 5Ms have been recorded.
The last above 6M was in 1991 but with around 25% of all above-5M events resulting in either 6M or 7M events, there is significant risk whenever rumbling starts.
As the last above 5M was in 2007, the region appears currently less active, but stays on alert for slips causing road blockages.
Wellington has had over 50 above 5Ms, with maybe 2 above 7M (1904 and 1934) and 4 above 6M. The most active year was 1934. The last above 6M was in 1961 and the last above 5M in 2011.
The southeast corner of the North Island has seen over 70 above 5Ms.
Although the area is fairly active, it has not delivered many calamitous events.
The main dangers may be to mudslides and loss of stock.
For Canterbury there have been just under a 100 above 5Ms, only one above 7M (2010) verified, and 10 that have been 6-7M.
However, half of all the above 5Ms have been during 2010-2011, and are now winding down.
It is unlikely that a major series could suddenly start up again.
Nevertheless the area remains active, with potential hazards from already loosened rocks and collapsing hillsides, especially after heavy rains.
It is not expected for more buildings to fall down, and most dwellings still standing should be now out of danger and safe to live in. Most activity appears to cease at the Rakaia river.
In Eastern BoP there have been over 100 above 5Ms; 6 above 6M, the last above 6M in 1994, and the last above 5M in 2010. But nearly 40% of this region’s larger quakes were confined to 1984-85.
The southwest of the country, containing Te Anau and Fiordland has been the second most active in NZ with over 160 above 5Ms.
The most active month saw 37 during July 2009, including a 7.8M in Fiordland. That quake twisted the South Island, with Puysegur Point moving 30 cm closer to Australia.
In the unpopulated ocean area of the NE of the North Island there have been over 200 above 5Ms, on average nearly three above 5Ms per year, making this region the most active.
There has been one above 7M (1995), 12 above 6Ms, the last above 6M in 2001 and the last above 5M in July of this year. The most potent year was 1995, with over 70 above 5Ms, all but 3 during the February.
The moon cycle of 18-19 years suggests that 2013/14 may be the next active time.
For the whole North Island about 400 above-5M events have been recorded. Of these, 3 may have been above 7M (1904, 1931 and 1934), with 17 above 6M. The most in a month was 84 in February 1995 north of East Cape, with high numbers also in 1947, 1956, 1984 and 1985.
For the whole South Is, 330 above 5Ms comes in less than for the North Island. Two have been above 7M (2009 and 2010) and 17 above 6M.
The month with greatest magnitude numbers has been September 2010, with 21 events recorded.
So it seems that the far SW and the far NE of the country are the greatest ongoing risk regions and fortunately they are non-populated. Logically one would expect the southern part of the Southern Alps to be more mobile, because high mountains represent greater land-mass, resulting in a larger daily earth/ land tide.
The smallest land area is the thin flat strip from Cape Reinga to Bombay, and this receives least seismic movement.
It is notable that in the 11-yr sunspot cycle, solar minimum years seem to have brought more big shakes on average to the North Island, and solar maximum years have brought them more to the South Island.
From this we can suggest that the upcoming solar maximum brings more large events to the far southwest over the next 2 years, and the next solar minimum year around 2016 may bring destructive earthquake activity further north.
Earthquake clusters that have involved more than one region have all been around the start of decades only.
This 10-11 yr cycle correlates with sun and moon cycles and the 10yr return of Jupiter/Saturn conjunctions and oppositions.
It is therefore unlikely that we will see, before 2019, quakes affecting more than one region at once.
The above is merely what anyone can access from our public records.
There seems no reason for immediate increased anxiety, but equally no cause for complacency long-term.Ken Ring of www.predictweather.com is a longrange forecaster for Australia's Channel Seven.
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