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.