Wednesday, 27 August 2008

1855 Earthquake

Te Ara Report
n 1855 a magnitude 8.2 earthquake – the most powerful ever recorded in New Zealand – rocked the southern part of the North Island. Caused by movement along a fault in Palliser Bay, it altered the landscape of the Wellington region and affected its subsequent urban development.
Intensity of the earthquake
The evening of 23 January 1855 was the end of a two-day holiday, the 15th anniversary of Wellington’s founding. Shortly after 9 p.m. a violent earthquake began; in Wellington the main shock lasted for at least 50 seconds. People fled outdoors, where they remained for the night in tents and makeshift beds, as incessant aftershocks rocked the area – one person counted 250 in the first 11 hours. The aftershocks would continue for months. For the first day after the main quake, as far away as New Plymouth an almost continuous vibration could be felt by people sitting, or when leaning against walls.
After the 1848 Marlborough earthquake, many Wellington buildings had been rebuilt in wood. Some new commercial premises, however, were constructed of brick because of fire risk. The 1855 earthquake damaged many of these, including the jail and the bank. The local council chambers and adjoining government offices, both two-storey wooden buildings, collapsed. However, single-storey wooden houses survived: although many were damaged by falling brick chimneys, or shifted on their foundations, few collapsed.
The number of fatalities caused by the earthquake is estimated at five. The sole casualty in Wellington was Baron von Alzdorf, who died when a brick chimney in his hotel collapsed. Two people died in a fissure in the Manawatū. In the Wairarapa, several Māori (their reported number varies from two to six), were killed when a whare collapsed. Surprisingly few people were injured. The quake was fekt as far South as Christchurch & Kaikoura to New Plymouth and Wanganui in the west to as far North as Palmerston North, Hawke's Bay including Wairoa.

Effects on land and sea
In the Hutt Valley, slips blocked roads and large fissures opened up in the ground. Numerous landslides scarred the slopes of the Rimutaka Range. The earthquake caused a tsunami in Cook Strait and Wellington Harbour; some buildings on Lambton Quay near the shoreline were flooded by tsunami waves.

The shock was felt across almost the whole country, and was highly destructive in Wellington 8.2 and severely damaging in Wanganui and Kaikoura. Between seven and nine people were killed in the earthquake, and five others sustained injuries that required hospitalisation.
The earthquake originated on the Wairarapa Fault and caused extensive faulting and uplift in epicentral areas. The latter was most dramatic at Muka Muka, on the western side of Palliser Bay, where the ground was raised by 2.7 m. Significant uplift also occurred in Wellington city, most noticably around Wellington Harbour, altering the city's shoreline considerably. Today, Wellington's Basin Reserve sports ground sits on land lifted by this earthquake; the area had previously been part of a waterway that led into the harbour. The ground level at Pauatahanui, Lowry Bay, and to the east of Lake Wairarapa was also raised, but it is possible that this was caused by material being deposited, rather than tectonic uplift.
The earthquake triggered extensive landsliding on both faces of the Rimutaka Ranges, along the Kaikoura coast and in Wellington, where access to Petone was cut off when a large landslide containing ~300,000 m3 of material cascaded down to block the coastal track north. The slip is still visible today along the Hutt Road. The shaking also created numerous slump cracks in flat areas of Wellington, the Hutt Valley, Wairarapa, and in the Manawatu district. In these areas the earthquake also triggered sandblows and the eruption of groundwater at the surface, the result of massive pressure increases underground that were caused by the shaking.
The earthquake was followed by many aftershocks, some of which were very damaging. There is strong evidence that the earthquake generated a local tsunami and it is also possible that small tsunami accompanied some aftershocks.

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