Analysis of Seismic Events Recorded with a Slow
Motion Tape Recorder near Chateau Tongariro,
New Zealand during February 18, 1966
to December 31, 1966



Volcanic prediction using slow motion tape recorders suitably situated near a volcano, produce good correlations when frequency analysis techniques are used with total energy from semi-continuous tremors, apparent explosions determined to disregard tectonic events.


Near Chateau Tongariro, situated on the lower slopes of Mt. Ruapehu, a seismometer (0.8 c/s natural period) and a slow motion tape recording seismograph (DIBBLE, 1964) are installed recording the seismic events in this region. The seismometer was installed at the end of 1959 up to the end of 1964 about 15 metres ENE from the Volcanological Observatory building (E 303' 900-N 370 755). Owing to building activity this site had to be abandoned and a provisional site was established at the Ski Patrol hut (E 305 600 - N 370 400) during 1965. A new site about 60 metres ENE (E 304 500 - N 375 750) coinciding with the recording site of the Chateau station of the Seismological Volcanic Observatory, was occupied in the beginning of 1966. An analysis of the seismic events recorded at the last site from Feb. 12, 1966 to Dec. 31, 1966 is given in this report.

Analysis of Seismic Events

For analysing the seismic events the signals originally recorded with a speed of 0.2 mm/sec were played back with a speed of 9.5 cm/sec. The amplified signals have simultaneously been studied by ear, monitored on a display oscilloscope and reproduced on a pen chart recorder.
Seismic events identified by ear have been marked on the chart records. A detailed analysis of the seismic signals in the frequency spectrum from 0.5 c/s to 45 c/s was made, a Rayspan Frequency Analyser (continuous sweep presented on 35 mm film) was used. A schematic diagram of the recording and analysing systems is given in Fig. 1. An example of the pen chart record is given in Fig. 2.

FIG. 1 above - Schematic diagrams for record and analysis systems.

FIG. 2 above - Initial evaluation chart, period during 16 to 25 July, 1966.
Presentation of Results

In playing back the tapes at a speed 475 times faster than the recording speed, the seismic events become audible. The terminology describing the various events, was derived at from this practice and frequency analysis (see, Table 1). Besides counting the number of certain events during the time intervals of one day and presenting these figures in histographs (Fig. 3) we have also plotted the average amplitude of the tremor versus time as given by the pen chart record, together with the temperature of crater lake (on the top of Ruapehu) (Fig. 3). It is not clear if the number of local earthquakes (L and VL) can be correlated with the volcanic activity manifested by visible activity, temperature and acidity of the lake. Specific local seismic events classified as sharp bursts of tremor (b and Rb), tremor of mixed frequencies (Rf ), tremor. of wide frequency range (Rd) and x chuffing >> and harmonic tremors (Rc and Rh), however, seem to be related with volcanic activity. A correlation between Rb and the average amplitude of the tremor R is indicated in Fig. 3. Since the amplitude of R and the occurrence of Rb and Rf show also the same relationship with the volcanic activity (temperature of lake in Fig. 3 for example) the occurrence of Rd, Rc and Rh have been observed and are *marked by a dark line. It can be seen that Rd occurs more frequently in the relatively quiet periods of the volcanic activity, Whereas Rc and Rh have only been observed during the peak of volcanic activity, after the steam eruption of 24 July 1966. Since average amplitude of tremor R is related to the volcanic activity, it is justified to describe tremor as volcanic ground noise.

Seismic Events Related with Steam Eruption on 24 July 1966.

The most outstanding event of visible activity during 1966, was the steam eruption on 24 July 1966, about 10.00 am. NZST. A detailed frequency analysis of seismic events during the period from 16-25 July 1966 is given in Fig. 4. This figure, together with Fig. 2 which covers the same period, shows that nothing spectacular happened before and after the event. The tremor amplitude about two days before the steam eruption, was low and stopped for about one hour, immediately after the eruption, followed by about a period of 7 hours with dominant Rh tremor.

Continued on page 4 TABLE 1

Terms Used on Evaluation Charts, Diagrams and in the Text

b Apparent explosions or sharp bursts of tremor, not affecting tremor amplitudes
before or after event.
R Volcanic tremor, narrow frequency range 1 to 3 c/s.
Rb Apparent explosions or sharp burst of tremor simultaneous with start of tremor.
Rc Chuffing tremor, appears apparently during dying phase of activity.
Rd Volcanic tremor of wide frequency range, sometimes with rapidly varying power
Rf Volcanic tremor of mixed high (9 to 12 c/s) and low (1 to 3 c/s) frequencies,
occurring for short periods.
Rh Harmonic tremor, as (Wh) on all records made before 9-9-67 and known as
whistle phenomena. A high frequency harmonic of R, directly related to gas
emissions of the volcano concerned.
L Local earthquake distance between 30 and 1000 km, indicated by time interval
between P and S arrivals.
VI, Very local earthquake distance less than 30 km, indicated by time interval
between P and S arrivals.
P Deep earthquake, P sharp compared to S arrival.
W Wind and rain.
X Instrumental, not seismic.
m Microseisms, as for oceans and lakes.
A Artificial man made noises.

Seismic Events Interpreted as of Volcanic Origin with Relation to Frequency Spectrum

1 c/s to 3 c/s Volcanic tremor (R), dominant frequency 1 c/s to 2 c/s.
1 c/s to 5 c/s Apparent explosions (b and Rb), dominant frequency 1 c/s to 2 c/s.
1 c/s to 7 c/s Volcanic tremor (Rd), dominant frequencies between 1 c/s and 3 c/s.
9 c/s to 12 c/s Volcanic tremor of high frequency (Rf) normally superimposed on
tremor (R).

Average Amplitude of Volcanic Tremor (R)
Amplitude ranges of (R) mentioned in text and in Figures 2, 3 and 5, are readily taken from the initial evaluation charts. The approximate equivalent amplitude of the ground motion at 2 c/s from February 20, 1966.

HIGH 0.6 microns and above.
Medium 0.4 to 0.6 microns.
Low up to 0.4 microns.

These levels were determined by test signals between 40 and 400 microvolts fed into the tape seismograph input, and the corresponding ground amplitudes have been taken from the calibration charts as shown in the manual for the Willmore seismometer.

FIG. 3 - Histograms of tape seismic events.

But if we consider the seismic events throughout the whole year (see Fig. 3) there is an identification of build-up - of seismic activity, preceding the steam eruption. In February and March R was prominent but not accompanied by a great number of N = b + Rb events (b + Rb = 7) although the temperature of the lake increased up to 47 C on 20-3-1966. Later the temperature dropped again, and the number N was 5 up to 12-6-1966 when N suddenly increased, reaching a peak value N = 11 on 17 July 1966 and dropped sharply to N = 3 between 19 and 21-7-1966; the occurrence of b + Rb reached a second peak on 23-7-1966 (N = 11) shortly after which the steam eruption took place. The lake temperature had also risen to 53 C on 25-7-1966.' During the steam eruption lake seisching was observed and a few fragments of fresh lava were thrown out of the lake (W. T. LLOYD, pers, comm.).

Comparison of Seismic Events (Rb + b and R) during 1964 with Those during 1966.

Visible volcanic activity similar to that during 1966 was observed during May and June 1964, although in 1964 apparently no steam eruption took place, and the visible activity was more gradual.

FIG. 4 - Rayspan frequency analysis of events direct from tape 0 to 5 c/s, period from 16 to 25-7-66.

In order to find out if the seismic events during the periods of visible activity in 1964 and in 1966 show a relation, the number of Rb + b and the average tremor amplitude R are plotted in a histogram (Fig. 5). It can be seen that in 1964 the occurrence of Rb + b was less N = 3 before and after visible activity, than in 1.966. Tremor before the visible activity during April 1964 is also more continuous than in 1966, and is not accompanied by tremor explosions. The number of Rb + b in 1964 also peak values N = 10 during the visible activity. In 1964 the lake also heated up to 54 C on the 26th of April


An analysis of seismic events, recorded with a slow motion tape recorder near Chateau Tongariro, on the lower slopes of Ruapehu during the period 18-2 to 31-12-1966 has shown that certain local seismic events are related to the visible volcanic activity of Ruapehu. The number of sharp bursts of so called tremors Rb + b and the average amplitude of the tremors seem to be directly related to the visible activity.

FIG. 6 - A comparison of the average frequency spectra as applied to the stations (co-ordinates stated in introduction), for the arrivals of all tectonic and volcanic events *.

A comparison of the histogram of Rb + b events during 1966 with that of 1964 when visible volcanic activity occurred, also plots have shown that a potential eruption is indicated if there is a sharp build up of Rb + b events, and N = 10 during a period of less than one week, which is associated with tremor R = high. It is recommended that if such a situation occurs in the future a warning should be given to the Tongariro National Park.

* Figure 6 shows the frequency determination, for the division of tectonic and volcanic events that are used in the histogram, from all observed apparent explosions and earthquakes.


The author wishes to thank Dr. T. Hatherton, and R. R. Dibble for advice. Messrs J. Healy, G. Jenkins, D. Dickinson, R. Ford, J. Scobie, and Members of the Tongariro National Park Rangers who assisted in obtaining the observed and recorded information. Dr. Glover for the temperature information. Dr. H. Taziell and Dr. R. W. Decker for their encouragement.


DiBBLE, R. R., 1964, A Slow Motion Tape Recording Seismograph for Geophysical Purposes. N. Z. J. Geol. & Geophys., 7, No. 3.

FINCH, R. H., 1949, Volcanic Tremor, Part 1. Bull. Seism. Soc. Amer., 39, No. 2, p. 73-78.
Manuscript received Jan., 1971

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