We were jolted awake this morning in Wellington by a severe quake – magnitude 5.8. The intensity in our house was IV on the Modified Mercalli ‘felt intensity’ scale.
As I write this the national seismograph network is still picking up aftershocks – every few minutes. That followed Friday’s succession of jolts which included a 5.7 magnitude shock – felt intensity in Wellington, again, was IV. They are from a fault complex in the northern South Island, near the town of Seddon – a line stretching north towards Wellington.
Quakes are a fact of life in New Zealand. It’s a part of living on the joint between two crustal plates. The other issue is volcanoes, which I covered last week. Quakes are the more immediate risk. They’ve killed more Kiwis than volcanoes. The two worst were the magnitude 7.8 Hawke’s Bay quake of February 1931, which killed 258 and seriously injured over 400; and the magnitude 6.3 quake of February 2011 that killed 185.
A word of explanation . The ‘magnitude’ – what used to be the Richter scale – is a measure of energy. What counts on human level is ‘felt intensity’, a subjective measure of the energy delivered to a particular place. That varies, depending on the ground the shock wave has to travel through. In the case of the lethal Christchurch quake, bedrock reflected some of the waves back under Christchurch city, where the felt intensity was VIII – ”destructive’ to IX – ‘violent’. Peak ground accelerations were estimated at up to two gravities (19.4 m/sec/sec).
None of these came close to the quakes that shattered infant Wellington in 1848 and 1855. After the 1848 shock one settler observed that : ‘Only 1 bakers oven was left intact . . . A brick wall fell and killed Sgt. Lovell and 2 children. Medical hall kept by Dr Dorset became a scene to be imagined with bare shelves and the contents broken and badly mixed . . . A number of land slips occurred on the wooded hills between Wellington and Wairarapa and in one instance a house was shaken off the piles supporting it.‘
Some settlers blamed poor mortar. ‘Sand and water is not very sticky,’ Charlotte Godley explained in a letter to her mother. The quake was centred on the Wairau Valley and later estimated to have a magnitude of 7.1, with a strength in Wellington of about VIII on the Modified Mercalli Scale. Wellington swayed to another tremor in May 1850.
The proverbial ‘big one’ hit in late January 1855. This was catastrophic, a major failure of the Wairarapa fault with an estimated magnitude of 8.1 or 8.2, and a peak felt intensity in Wellington of X. Destruction spread from Wellington to Wanganui, and the quake was felt as far north as Wairoa. The shelved land brought up by this quake is still visible today – in fact, roads run directly along the edge of it.
Two quakes in quick succession like that was unprecedented,
The frightening part is that some seismologists theorise they were linked. What happens, the theory goes, is that a rupture of one fault sets up tensions in adjoining rocks – setting up the next fault to break a little later.
There has been suggestion that the Christchurch quake swarm that began in September 2010 and continued into 2013 – effectively shaking the city to pieces, slo-mo style – was set up by a massive quake that hit Fijordland in 2007. As for the current quakes near Seddon…well, they’re at the other end of the Christchurch complex and…uh…the next one up are the two big Wellington faults.
Scientifically speaking, the jury’s out, but I’d hate to find out the theory’s right the hard way.
Copyright © Matthew Wright 2013