Seismicity of New Zealand
The main characteristics of earthquake distribution in New Zealand are illustrated by diag. 3 and 4. The activity is largely concentrated in two apparently distinct areas. The greater, northern area lies roughly between latitudes 36½ S and 43½ S, while the southern area lies to the west of longitude 169½ E. Shallow earthquakes are widely scattered within these active areas; indeed no part of New Zealand, with the possible exception of the district north of Whangarei, can be regarded as wholly exempt from shallow activity. Deeper foci, as shown in diag. 3, are mostly confined to a narrow belt in the northern area, extending from the Bay of Plenty south-westwards to Tasman Bay. (NOTE—The Whangarei exception was removed by a recent series of earthquakes centred near Kaitaia, 70 miles north-west of Whangarei, which included a damaging shock on 23 December 1963, with magnitude 5.2 and intensity up to MM7.)
New Zealand is a region of moderate earthquake activity. It is difficult to compare the seismicity of New Zealand directly with that of other active regions, because of the many differences that arise in earthquake type and mode of occurrence. For the class of major shallow earthquakes occurring on land or within about 50 miles of the coast, we may quote the following statistics from lists published by Gutenberg and Richter:
|Numbers of Major Shallow Earthquakes 1918–52|
For the whole world the total number of such earthquakes was about 500.
The most complete data on the distribution of major and large shallow earthquakes in New Zealand are plotted in diag. 4. Altogether 15 major earthquakes (magnitude 7 or greater) are known to have occurred since the European settlement; such earthquakes can be recognised by their notable and widespread effects. By about 1940 the seismograph network was capable of locating all local earthquakes with magnitude 6 or greater; 23 shallow earthquakes with magnitudes in the range 6.0–6.9 occurred in the period 1940–60.
Earthquakes have not constituted a major hazard to life and property in New Zealand as compared with many other seismically active regions, or if we consider earthquakes in relation to other types of accident. The great Hawke's Bay earthquake of 1931 resulted directly or indirectly in 255 deaths; a further 29 deaths have been recorded as due to other earthquakes since the year 1848. As is only prudent, however, various precautions are taken against the effects of possible future earthquakes. Organisations ready to deal with earthquake and other kinds of disaster are sponsored in the main centres of population by the Ministry of Civil Defence. A model bylaw drawn up by the New Zealand Standards Institute regulates the design of structures to resist earthquake damage. The Earthquake and War Damage Commission operates a National Earthquake Insurance Fund which is maintained by a compulsory levy on all fire insurance. Special studies of strong-motion earthquake effects are carried out by the engineering seismology section of the Dominion Physical Laboratory, Department of Scientific and Industrial Research, Lower Hutt.
The pattern of seismicity displayed in diag. 3 and 4 shows that earthquake incidence has been substantially greater in some areas than in others, but the historical record in New Zealand is rather short to be accepted as an accurate guide to the distribution of future epicentres.