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Is NZ getting more earthquakes?

TUESDAY MAY 22, 2012

NZ has averaged 15,000 recorded earthquake events per year since first records began with the thousands of earthquakes in Christchurch in 1869-70. So far, about 2 million earthquakes have been documented for NZ. They have not suddenly appeared. NZ gets the second greatest number of earthquakes in the world, behind Japan, which is why we are known as The Shaky Isles.

Nearly all countries have earthquakes but for followers of the news there is less appetite for historical analysis and more of an interest in records being broken. Successive generations like to think they live in a time when history is being made. It may spoil our party to learn that in 1994 there were more Canterbury earthquakes than in the last 18 months. Yet that is fact.

The difficulty in amassing global earthquake data is that technological improvements change, replace and increase instrumentation. As more seismographs are installed, more earthquakes can be and have been located. However, the number of larger earthquakes (magnitude 6.0 and greater) has stayed relatively constant. It is better to look at closer perigee years (moon closer to earth) and see if extreme weather has mainly occurred then, compared to lapse years between.

As Apollo astronauts found in 1969, when earth and moon get closer (perigee), there is more seismic activity on both. The first week of September 2010 perigee was the second closest for the whole year. March 2011 saw the closest perigee for 17-19 years (according to how it is calculated), which accounted for extensive destruction in Japan and Christchurch during Feb/March 2011.

Averaging distances over a year gives years when the moon has been averagely closer compared to other years. Looking at the 112 years from 1900 to present, 2011 was the 4th closest moon-year. All years in that top ten brought us unusually powerful earthquakes. In the 112 years, the order of 10 closest perigees has been 1905, 1918 , 1936, 2011, 1993, 1980, 1962, 2020, 1949, 1927 and 1931. In August of 1904 a 7mag, the 15th biggest earthquake ever in NZ occurred in the Wairarapa, causing chimney-falling damage over the whole lower NI a week after a big 6mag earthquake in Hawkes Bay. November 1918 saw a 6.8mag earthquake, the 30th in the biggest-ever scale, in the far south. The other years have also brought us destructive events.

Prior to 2010, 1994 was the last-biggest previous earthquake year in Canterbury. At Arthur's Pass on 18 June 1994 the 6.7M shake was the largest on NZ land since 1968. More than 10,600 aftershocks were recorded in 32 days following that quake. Altogether there were 12,000 earthquakes recorded in 1994 (vs 10,000 since Sept 2010). Arthurs Pass is only 80kms from the Christchurch CBD. Buildings swayed in Christchurch and many people fled into the streets. Media were not particularly interested because there was no loss of life, and today 1994’s activity is mostly forgotten.

Close to the 8th closest perigee year, 1947 saw large earthquakes in Canterbury, including a 7.1M earthquake in Poverty Bay on 26 March 1947 generating a tsunami scare. Another came in 17 May 1947, caused by another 7.1M earthquake in Tolaga Bay. And 1931 (11th closest perigee year), on 3 February was the Napier earthquake.

This type of number crunching can go on and on, and I will not bore the reader, but it puts into perspective 2011 as a perigee year. Whilst 2011 was the 4th closest perigee year in over a century, it is comforting that 2012 is only 57th, with 2013 a powerless 111th. It means we have 2-3 years relative quiet, ample time to erect a new city, but contractors will have to get cracking because after 2014 seismic risks could creep back. In the 110-year perigee-closeness list, 2014 lies at 62nd, 2015 at an untrustworthy 26th and 2016 an almost equally risky 29th.

So 2015-16 will again be close-moon years, with equivalents to these being 1997(27th) and 2002 (28th). The bad news is that 1997 was a hugely active year for global earthquakes and between 2001-2003, NZ had four 6M-7M earthquakes - in May 2001 a 6.5M (62nd) in Raglan; in August 2001, a 7.1M (10th biggest-ever in NZ) just east of the North Island; and two months later a 6.3M (99th biggest-ever) in the same area. And in August 2003 there was a 7M (21st biggest-ever) off Te Anau.

We can only look to what happened in the past to find a pattern. Earthquakes have always been part of our lives and it is time we realized that, just as they have been for many years in other countries on the Ring of Fire. They have always been a threat in NZ and it is unwise to ignore them for too long.

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