Moderate year for earthquakes

Press, Volume CXVI, Issue 34044, 7 January 1976, Page 12

Moderate year for earthquakes

Neio Zealand Press Association)

WELLINGTON, January 6.

Last year was a moderate one for earthquakes but both Mount Ngaruhoe and Mount Ruapehu erupted spectacularly, said the superintendent of the Seismological Laboratory, in a review of the year.

No earthquake reached a magnitude of 6 on the Richter scale, but two had magnitudes of 5.9.

The largest shallow shock of the year occurred on the evening of June 10, 1975. Its magnitude was 5.9, and_ its epicentre was about 15km south of Dannevirke, where there was minor damage, including broken windows and disloged masonry. The earthquake was reported felt from the Bay of Plenty to Banks Peninsula.

The other earthquake of k magnitude 5.9 had a focus about 75km deep beneath Cook Strait, to the west of Kapiti Island. It occurred on the morning of January 5, and was felt extensively in the central parts of the country. Some crockery was knocked off shelves in coastal areas to the north of Wellington.

The deepest earthquake in New Zealand since 1960 occurred on Feburay 8, at a depth of 580 km beneath Taranaki. Its position was close to those of the three other earthquakes that have been at depths of about 600 km beneath New Zealand, in an area where earthquakes are generally no more than 300 km deep. Its moderate magnitude of 4.9, and its great depth prevented it from being felt. Four deep earthquakes, with magnitudes of about 5.75, occurred at more usual depths beneath the North Island. Those on April 30 and August 19 were about 140 km deep, beneath Lake Taupo, and were felt from the Bay of Plenty to the north of the South Island. Deeper shocks, 220 km under Lake Taupo on November 12, and 300 km beneath the Tauranga area on November 23, were not felt so strongly, although they were of similar magnitude. Shallow "quakes Shallow earthquakes of magnitude 5 that were felt extensively in the central parts of the country occurred on April 13, near Queen Charlotte Sound, and on July 12 and December 30 south of Nelson.

A shock of magnitude 5.2 occurred' in the Gisborne region on August 26, but because it was 50km off the coast, it was not felt strongly. Earthquakes that were widely felt in the southern parts of the South Island originated near Milford Sound on March 16 (magnitude 5.5) and June 5 (magnitude 4.9).

Two earthquakes in the mountains between Arthur’s Pass and Lewis Pass were felt on the West Coast and North Canterbury on April 28 and September 25. Their magnitudes were 4.7 and 4.8 respectively. Minor tremors

An earthquake of magnitude 4.4 was felt extensively in Northland on February 12. It was centred about 50km south-east of Whangarei, near the Hen and Chicken Islands. A further shock in this region occurred on August 17. about* 30km south

of Whangarei, and was felt at Paparoa and Ararua, although its magnitude was only 2.8. Other earthquakes in areas of less frequent activity occurred about 40km off the coast to the west of Raglan on March 14 (magnitude 3.7) and June 20 (magnitude 4.0), and 170 km to the south-east of Chatham Islands on July 26 (magnitude 4.6). None of these earthquakes was felt. Tremors off N.Z.

Among the large earthquakes in the active area to the north of New Zealand, that which attracted most attention was a shallow shock, of magnitude about 7.5, that occurred between Tonga and Samoa in the early morning of December 27. ’ Such earthquakes may cause a tsunami, and on this occasion a Civil Defence alert was called until it had been established that no tsunami had been generated. During the alert, the public were warned to stay away from beaches and low-lying areas. Both Mount Ngauruhoe and Mount Ruapehu erupted spectacularly during the year. On February 19, Mount Ngauruhoe erupted ash and steam, and large rocks were thrown several kilometres. This was the mountain’s most violent eruption since 1954. There was an ash eruption of Mount Ruapehu on April 24, which caused a greatly increased flow in nearby streams and rivers. Local tremors were reported at the time of these eruptions, but no significant earthquakes were recorded.