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.