An earthquake occurs when elastic waves are generated in the earth by some sudden disturbance. Natural earthquakes produce vibrations at the surface which are often detected instrumentally over very great distances; they are sometimes felt directly within a limited area, and occasionally cause serious local damage. The wave energy emanates from a small region called the focus or hypocentre, and is released so abruptly that the time of origin can be estimated to the nearest second. The focus is specified by three coordinates: the latitude and longitude of the epicentre (the point at the surface directly above the focus), together with the focal depth.
The scientific study of earthquakes is called seismology from the Greek seio (shake). A seismometer is an instrument for detecting earthquakes; or if a permanent record (seismogram) is produced, the apparatus is called a seismograph. The number and kind of earthquakes that occur in a given geographical region constitute its seismicity.
Most natural earthquakes originate in regions of the world which also display other types of disturbance, such as active volcanoes, recent mountain-building, deep off-shore trenches, and large anomalies in the gravitational field. These disturbed regions, of which New Zealand is one, are evidently the site of some fundamental process affecting the development of the earth's outer layers. Together they occupy only a small fraction of the earth's surface; about three-quarters of the world's present earthquake activity occurs on the perimeter of the Pacific Ocean. Little is yet known about the earth's internal processes, nor are the relations connecting different types of surface disturbance understood in any detail.
Earthquakes originate at depths down to about 450 miles, or one-ninth of the earth's radius. The great majority of the world's earthquakes are shallow, originating at depths of less than 40 miles, and in some seismic regions there are no foci at greater depth. About one-third of New Zealand earthquakes have foci below 40 miles depth, and usually two or three each year come into the category of deep-focus earthquakes, originating more than 190 miles deep. The two deepest earthquakes so far recorded in New Zealand occurred five minutes apart on 23 March 1960, with a common focus 370 miles under north Taranaki.
The cause of earthquakes has not yet been established, and there is no known method of prediction. Formerly earthquakes were attributed to volcanism, but it is now recognised that volcanic earthquakes occur only as minor shocks in the immediate vicinity of the volcanoes. In New Zealand tremors of this kind are experienced in the zone of active volcanism that extends from Mt. Ruapehu to White Island. The widely held belief that fault movement is the cause of earthquakes will be discussed in some detail below.
Earthquake magnitude is a measure of the amount of wave energy radiated from the source. The magnitude of a particular earthquake is estimated from the amplitude of ground motion as recorded on a standard seismograph, taking into account the distance of the focus. On the Richter magnitude scale, the greatest known earthquakes have been of magnitude 8.9, while an earthquake of magnitude less than 2.5 is not likely to be perceived anywhere without the help of instruments. The greatest New Zealand earthquake in historical times was the Wellington earthquake of 1855, with a magnitude of about 8. The disastrous Hawke's Bay earthquake of 3 February 1931 had a magnitude of 7¾.
The precise numerical relationship between magnitude and energy is still uncertain. A recent version is
log10E = 11.8 + 1.5 M
where M is the magnitude and E is the energy in ergs. According to this equation, the energy varies by a factor of about 33 for each unit of magnitude. The energy released in a magnitude 7 earthquake is about 2 × 10 22 ergs. Man is now able, by detonating a thermonuclear explosion equivalent to some 50 million tons of TNT, to release an amount of energy comparable to that in a major earthquake.
Small earthquakes are very much more frequent than large ones; even so, the largest earthquakes account for most of the total energy released. The following table gives the approximate numbers of earthquakes, in successive magnitude ranges, occurring in New Zealand in an average year:
| Magnitude | 4.0–4.9 | 5.0–5.9 | 6.0 + |
| Number per year | 100 | 20 | 3 |
An earthquake makes itself felt with different intensities in different places. One might expect the felt intensity to be highest at the epicentre and to diminish uniformly with increasing distance. Actual intensity patterns are rather more complicated, however, because of the influence of geological structure and type of ground. Scales of intensity have been designed as a measure of the effects produced on people and objects at any locality where the earthquake is noticed without the aid of instruments. The following scale, which is used in New Zealand, was formulated in California in 1931:
Not felt except by a very few under especially favourable circumstances.
Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognise it as an earthquake. Standing motorcars may rock slightly. Vibration like passing of truck. Duration estimated.
During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motorcars rock noticeably.
Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.
Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motorcars.
Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Disturbs persons driving motorcars.
Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifts sand and mud. Water splashed (slopped) over banks.
Few, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and landslips in soft ground. Rails bent greatly.
Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air.
In New Zealand about 100 earthquakes are reported felt in an average year. On the more important occasions — usually several times a year — a questionnaire is issued to a large number of voluntary observers, and the information so obtained is used to draw an isoseismal map, showing contours of equally felt intensity. Diag. 1 shows an isoseismal map for the great Hawke's Bay earthquake of 1931. It will be seen that the intensity reached MM10 or higher in an area measuring about 50 miles by 25 miles, and the average radius for MM7 was about 65 miles.
The Seismological Observatory, Wellington, operates a network of 13 seismograph stations in New Zealand and five stations overseas (diag 2). The Observatory is a section of the Geophysics Division, Department of Scientific and Industrial Research. Seismographs for recording distant earthquakes are installed at Afiamalu, Karapiro, Wellington, and Roxburgh, and at Hallett and Scott Base, Antarctica.
The New Zealand Seismological Report, which is published annually by the Observatory, contains the following information for the year: a diary of all significant earthquake waves recorded by the network; a table giving the origin time, epicentre, focal depth, and magnitude for local earthquakes that reached magnitude 4 or above, or that were reported felt; a summary of felt reports; and isoseismal maps for the more widely felt earthquakes. The Observatory exchanges earthquake readings with other countries, and contributes to the international agencies which analyse earthquakes of world importance.
Several types of seismometer are used, all incorporating some form of pendulum or suspended mass. Some instruments are made sensitive to vertical and others to horizontal movement; also the design may favour either the more rapid vibrations or the slower ones. The seismograph illustrated incorporates the vertical Willmore seismometer, a short-period electromagnetic instrument that is particularly suited to New Zealand conditions, and is capable of magnifying the ground vibration as much as 50,000 times.
Some of the elements of earthquakes analysis are illustrated in the accompanying photographs. Here are shown two New Zealand earthquakes recorded on the visual seismograph at Wellington. Unlike most seismographs, which record on photographic paper, this instrument uses a pen-and-ink recorder, and is also equipped with an alarm bell which sounds when the ground movement exceeds a certain value. The recording paper is wound on a drum which revolves once every 15 minutes, and also moves laterally so as to separate the traces drawn on successive revolutions. The pen is given a small impulse to mark each minute, and time pips sent out by the New Zealand Time Service are recorded to give absolute time. The record is completed in one day.
Two prominent groups of vibrations may be distinguished. The first arrival (reading from left to right) is the primary or P wave, which is known to travel at about 4½ miles per second. Later comes the secondary or S wave, which travels at about 2½ miles per second. P waves involve a vibratory motion of the ground in line with the direction of travel, as with sound waves in the air; while with S waves the motion is transverse to the direction of travel. Both P and S waves may travel in a solid medium but only P waves in a fluid. The existence of P and S waves in earthquake records was first demonstrated by the Englishman, R. D. Oldham, in 1897. Since both waves are generated at the same time, and travel directly from focus to recording station, the difference in arrival time indicates the distance of the focus. In this case the time interval is 18 seconds, indicating a distance of 100 miles from Wellington; then the maximum deflection on the record gives an earthquake magnitude of 5.2. With the help of other seismograph stations, this earthquake was found to have an epicentre about 40 miles south of Nelson, and a shallow focus.
As seen in the accompanying photograph, the P wave is comparatively small. The “S minus P” time interval is 115 seconds, indicating a distance of 720 miles from Wellington. This earthquake had an epicentre between East Cape and Kermadec Islands, a focal depth of about 260 miles, and a magnitude of 7½. Other waves which may be seen in this illustration have travelled by more complicated paths.
On feeling an earthquake one can sometimes distinguish the more rapid vibration of the initial P wave from the later, swaying motion of the S wave. The distance of the focus can then be roughly estimated by allowing 6 miles for each second of time separating the two arrivals.
The analysis of earthquakes recorded by the New Zealand seismograph network provides the Observatory with essential data for studying the seismicity and deep structure of the New Zealand region, and for investigating the major problem of earthquake mechanism.
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 | |
| Japan | 39 |
| Chile | 23 |
| New Zealand | 9 |
| California | 6 |
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.
Earthquakes have yielded much of our present knowledge of the earth's internal structure. Earthquake waves traverse every part of the earth's interior, and from the study of travel times one can reconstruct the wave paths and calculate the velocity of travel at each point. This study has led to the fundamental discoveries that the earth is an elastic body and that its elastic properties are to a high degree symmetrical about the centre.
In 1906 Oldham showed that the earth has a central core with radius 2,200 miles (compared with 4,000 miles for the whole earth). Both P and S waves travel in the surrounding mantle, which makes up about five-sixths of the earth's volume, but no S waves have been found to travel in the core. Thus the core appears to be fluid; it is commonly supposed to be made of molten iron and nickel. At the very centre there is a small inner core with radius 800 miles. This was discovered in 1936 by the Danish seismologist, Miss Lehmann, from studies of two New Zealand earthquakes recorded in Europe. The velocity of P waves appears to increase suddenly on entering the inner core, and the New Zealander, K. E. Bullen, has recently found evidence that the inner core may be solid. Some of the types of path followed by earthquake waves in the earth's interior are sketched in diag. 5.
The symmetry of the mantle and core does not extend to the thin outer crust. The patchwork of continent and ocean that we see at the surface connotes important differences in the crust itself. Beneath the oceans the solid crust has a thickness of 3–6 miles, while the continental crust is on an average 20 miles thick. The crust-mantle boundary is thus about 15 miles deeper under the continents; this boundary is called the Mohorovicic discontinuity after the Croatian seismologist who discovered it in 1909.
Although the crust makes up only about 1 per cent of the earth's volume it naturally holds a special interest for mankind. Here, too, our chief knowledge has come from the study of earthquake waves, though natural earthquakes have been supplemented by artificial ones generated by means of gelignite explosions.
Part of the energy of large shallow earthquakes becomes trapped within the crust and travels in the form of guided waves, known as Love waves and Rayleigh waves. The character of these waves is influenced by the crustal wave guide, and they can be used to find the thickness of the crust through which they have travelled. New Zealand seismologists, applying this technique to earthquakes recorded at Hallett Station and Scott Base, have proved that the Antarctic ice cap is underlain by continental land in eastern Antarctica, whereas western Antarctica is not fully continental. Similar studies have shown very recently that New Zealand itself has a continental crust — a discovery of much scientific interest in view of the relatively small area of New Zealand.
New Zealand provided one of the earliest well-substantiated occurrences of movement on a geological fault accompanying an earthquake, when the Wairarapa Fault moved at the time of the large Wellington earthquake of 1855. Several dozen other coincidences of this kind, in various countries, have since been reported, and hence has arisen the idea that it is fault movement that causes earthquakes. This hypothesis has been widely publicised, especially by English and American writers, but the difficulties that confront its acceptance are often overlooked.
Movement on the great San Andreas Fault in California has been the subject of much careful study. At the time of the disastrous San Francisco earthquake of 1906, movement of up to 15 ft occurred on this fault, over a distance of some 200 miles. A comparison of triangulation surveys made before and after the event showed that elastic strain had been released over a wide area of ground. It is commonly supposed that this strain release supplied the energy that was radiated as earthquake waves, but we still know too little about either class of phenomenon to be sure that the strain energy would be sufficient. Sudden fault movements may yet turn out to be but part of the damage wrought by large earthquakes. The most serious effects of the 1906 disaster did not occur in the neighbourhood of the fault.
The New Zealand Alpine Fault, which extends from Milford Sound to Lake Rotoiti, where it joins the Wairau Fault, is comparable to the San Andreas Fault in length, and from geological evidence appears to have undergone lateral movement totalling 300 miles. This movement has continued into geologically recent times, yet the Alpine Fault is not marked by earthquake activity, and in central Westland it traverses a region where even small earthquakes seldom occur.
Recent observations on the San Andreas Fault have revealed a gradual creep movement of about ½ in. a year. This discovery appears to invalidate the commonly held view that all displacement on geological faults is evidence of past earthquake activity. A similar rate of creep would be sufficient to have given the total movement of the Alpine Fault, and it seems possible that gradual creep, which would not ordinarily be noticed, may be a regular mode of fault movement.
Physicists have pointed out that fault movement as usually conceived, involving shear fracture with sudden release of elastic strain, cannot be presumed to occur at depths greater than a few miles, because of the inhibiting effect of frictional forces. There is, moreover, no compelling geological evidence that fault movement extends to great depths. Thus it is of interest that the study of earthquake waves has given no suggestion that the shallowest earthquakes might have a different source mechanism from deeper ones.
The relation between earthquakes and faults is part of the larger problem of earthquake mechanism, the solution of which may eventually lead to methods of predicting earthquakes, perhaps even of controlling them. New Zealand as a region of moderate though varied earthquake activity has much to offer towards the solution of these challenging problems.
by Frank Foster Evison, M.A., B.SC., PH.D.(LOND.), D.I.C., Director, Geophysics Division, Department of Scientific and Industrial Research, Wellington.