Earthquake ’31 Page 6
Why the earth quakes
George Eiby, a retired superintendent of the Seismological Observatory of the former Department of Scientific and Industrial Research, Wellington, looks at the lessons of the 1931 earthquake in the light of a further half-century of research. Mr Eiby died in 1992.
Earthquakes are as inescapable as the weather. Even Mars: and the Moon have them.
Here on Earth, seismologists report that every year there are a couple of million of them strong enough to be felt, a thousand or so that could bring down chimneys, and a dozen potential disasters. It’s just as well that so much of our earth is ocean, mountain desert, or uninhabited forest so that our chances of being hit by a big one are small enough.
How is it then that we keep reading of distant disasters that kill a hundred times more people than all New Zealand shocks taken together? The answer is not bigger earthquakes, but poorer buildings!
One of the earliest lessons from the Hawke’s Bay earthquake was the need for strict building laws, and our engineers have come to be among the leaders in the art of designing earthquake resisting structures.
There has also been astonishing progress in understanding how and why earthquakes happen. Some seismologists are raising hopes that reliable earthquake forecasting is not far away. Others insist that forecasting is a side issue; that there is little point in predicting when cities will fall down, and that we should concentrate upon making them stand up.
Both schools of thought agree that we need a sound knowledge of the habits of earthquakes.
Piecing the earthquake story together has been a long business, and we have yet to reach the end of it. As long ago as the 18th century John Michelle realised that waves travelling through the solid Earth were responsible for shaking down buildings, and suggested ways of tracing them back to their source. By about 1900 recording instruments had been invented, and we had learnt a great deal more about the kinds of waves involved, and how they travelled.
We also knew that the shocks were a consequence of the processes that built mountains and shaped the face of the Earth. Rocks far underground were fractured and set off the seismic waves. If the shock was big enough and shallow enough, a great crack could break through to the surface and appear as what geologists call a fault.
After the big San Francisco earthquake in 1906, H F Reid examined the displacements of the ground near the San Adreas [Andreas] Fault, and realised that what had happened was an “elastic rebound”. A long, slow accumulation of strain had finally brought the rocks to breaking-point, and released the stored energy. The earthquake was not something sudden and abnormal, but a return to normal from a condition of strain.
That the strains are going on is clear enough from the folds and fractures that can be seen in rocks almost anywhere, but where do they come from? During the last few years, earth scientists have arrived at an explanation they call “plate tectonics”.
As with most important scientific theories, the basic ideas of plate tectonics are simple, but they turn out to explain most features of the earth’s surface, from the broad pattern of continent and oceans to earthquakes, volcanos, and the deep trenches in the ocean floors.
Great fractures have been found to divide the outermost layers of the earth into about seven large, rigid plates which float on the more plastic material of the mantle underneath. The Earth’s internal heat keeps this more plastic material in and moves outward toward the margins of the continents as it cools. There it descends into the mantle once more, and is melted and reabsorbed.
The plates are carried along by this movement, which brings their edges into collision, folding and fracturing them, and producing belts of earthquakes and volcanoes. Because they are rigid, the central parts of the plates are comparatively stable.
New Zealand lies at the meeting of two plates.
To the west, the Indian Plate carries both Australia and India, while to the east the Pacific Plate forms the North Island.
This thrusting accounts for the Hawke’s Bay earthquake of 1931 and, indeed, for all the earthquakes in the northern half of the South Island.
In Fiordland, the same forces are responsible, but there it is the Pacific Plate that is on top, and the Indian Plate that is being driven beneath it. The complexities of the transition are a subject of lively scientific argument.
Since European settlement began, New Zealand has had about 15 earthquakes within half a magnitude or so of the size of the Hawke’s Bay one, yet only the Murchison shock in 1929 produced casualties running to double figures, and none has caused comparable damage to property. Why was it so destructive?
The short answer is that its origin was unusually close to centres of population, and close to the surface. In places where a plate is being thrust back into the mantle, very deep shocks are often found. New Zealand’s deepest – under north Taranaki – were more than 600km down; but the great majority of shocks, including all the destructive ones, originate within the crust, about 30km thick.
In 1931, there were few good seismographs in New Zealand, and we had to get most of our information about the Hawke’s Bay earthquake from overseas recordings, so it would be rash to be too dogmatic about the position of the origin.
The best estimates put the epicentre (the point on the surface directly over the origin) between Rissington and Patoka, about 25km north-west of Napier, but this could be as much as 20km out in any direction. The depth was no more than 15 or 20km.
The magnitude of the shock was 7.9, about the same as that of Murchison earthquake two years earlier. It is not the country’s biggest shock, that distinction going to the south-west Wairarapa earthquake of 1855, which had a magnitude of at least 8. The Inangahua earthquake in 1968, the last of New Zealand’s really serious shocks, reached only 7.1.
There is more to a big earthquake than an underground breaking of rocks and the radiation of a train of waves. The whole of the earth’s rust [crust] must readjust, with consequent uplifts and subsidences, and a sequence of aftershocks that may persist for a year or more. In Hawke’s Bay two common accompaniments of a large shock were missing – a tsunami (or seismic sea-wave) and visible displacements along a geological fault.
The curious disruption of the tides and the draining of the Ahuriri Lagoon were not the result of a tsunami, but of the regional uplift that was a permanent result of the earthquake. An area some 90km long and 15km wide was raised by a maximum of about 3 metres. There was also settlement of poorly compacted land near the mouth of the Tutaekuri River and on the Heretaunga Plains.
Could it happen again? So far as the geological story is concerned, the answer is yes, the movement of the plates is not going to stop.
Hawke’s Bay will have more earthquakes and a few of them will be as big or bigger than the one in 1931. What need not be repeated is the loss of life and property. We know how we should build, and where we should build to withstand even the strongest likely shocks.
Are we wise enough to apply our knowledge?
Maps text –
PLATES THAT MOVE THE EARTH’S CRUST
NORTH AMERICA PLATE
Record of death and destruction
Dates, locations and Richter-scale magnitudes of the major New Zealand earthquakes since 1843, with the death tolls in brackets in the nine cases where lives were lost:
1843, July 8, Wanganui, 7.5 (2)
1848, Oct 16, Marlborough, 7.1 (3)
1853, Jan 1, New Plymouth, 6.5.
1855, Jan 23, South Wairarapa, 8.1 (5).
1863, Feb 23, Hawke’s Bay, ?
1876, Feb 26, Oamaru, ?
1888, Sept 1, North Canterbury, 7.
1891, June 23, Waikato, 6.
1895, Aug 18, Taupo, 6.
1897, Dec 7, Wanganui, 7.
1901, Nov 17, Cheviot, 7 (1).
1904, Aug 9, off Cape Turnagain, 7.5.
1914, Oct 7, East Cape, 7-7.5, (1).
1914, Nov 22, East Cape, 6.5-7.
1917, Aug 6, Nth Wairarapa, 6.
1921, June 19, Hawke’s Bay, 7.
1922, March 9, Arthur’s Pass, 6.9.
1922, June 19, Taupo, ?
1929, June 16, Murchison, 7.8 (17).
1931, Feb 3, Hawke’s Bay, 7.9 (258).
1931, May 5, Poverty Bay, 6.
1932, Sept 16, Wairoa, 6.8.
1934, March 5, Pahiatua, 7.6, (1).
1942, June 24, Sth Wairarapa, 7.
1950, Feb 5, south of the South Island, 7.
1950, Aug 5, south of the South island, 7.3.
1953, Sept 29, Bay of Plenty, 7.1.
1955, June 12, Seaward Kaikouras, 5.1.
1958, Dec 10, Bay of Plenty, 6.9.
1960, May 24, Fiordland, 7.
1962, May 10, Westport, 5.9.
1963, Dec 23, Northland, 5.2.
1966, March 5, Gisborne, 6.2.
1966, April 23, Cook Strait, 6.1.
1968, May 24, Inangahua, 7 (3).
1972, Jan 8, Te Aroha, 5.1.
1974, April 9, Dunedin, 5.
1981, April 14, under Lake Taupo, 6.
1981, May 25, 400km sth-west of Stewart Island, 6.4.
1981, Nov 17, nth-west of White Island, 6.3.
1981, Dec 28, Cape Turnagain, 5.7.
1982, Oct 30, Taranaki, 5.7.
1983, Jan 27, Kermadec Islands, 7.3.
1984, March 8, north-west of Gisborne, 6.4.
1984, Dec 30, east of Coromandel, 6.3.
1986, Oct 27, Lake Taupo, 5.9.
1987, March 2, Edgecumbe, 6.3.
1987, March 22, near Mayor Island, 6.1.
1988, June 4, Te Anau, 6.6.
1989, May 31, Otago, 6.6.
1989, July 7, north of Whakatane, 5.8.
1989, Aug 8, Patea, 5.8.
1989, Nov 30, Gisborne, 5.0.
1990, Feb 10, Lake Tennyson, 5.9.
1990, Feb 19, Weber, 5.8.
1990, March 28, Rotorua, 5.9.
1990, May 13, Weber, 6.0.
1990, Aug 16, Weber, 5.2.
1990, Oct 5, Cape Palliser, 5.6.
1990, Oct 6, Cape Palliser, 5.6.
1990, Oct 20, Lake Tennyson, 5.5.
1991, Jan 28, Westport, 6.3.
1991, Jan 29, Westport, 5.9.
1991, Feb 15, Westport, 6.0.
1991, July 12, Taupo, 6.2.
1991, Sep 8, Bulls, 6.3.
1992, March 2, Porangahau, 5.8.
1992, May 27, Blenheim, 6.7.
1992, June 21, Bay of Plenty, 6.1.
1993, April 11, Te Hauke, 6.1.
1993, Aug 10, Gisborne, 6.7.