Seismic and tilt phenomena preceding a Pelean type eruption from a basaltic volcano

Summary

Manam is an island volcano situated close to the north coast of New Guinea. The composite basaltic cone has a width of eleven kilometres and a height of approximately 1200 metres. Manam is the most active volcano in the Territory of Papua and New Guinea. During the short period of its incompletely known history dormancy has rarely exceeded 10 years.

A new cycle of active began in December, 1956 and has continued intermittently ever since. In December, 1957, the entire population of the island was evacuated six weeks before a series of powerful eruptions which produced massive avalanches of the nuée ardente type and extensively damaged areas of agricultural land.

Spasms of relatively mild intermittent explosive activity ensued.

In December, 1959, a new phase of eruptive activity began. Strombolean explosions of increasing intensity led up to a Pelean eruption in March, 1960, when massive nuées ardentes descended into a valley on the north eastern flank of the volcano. This explosive outburst was followed immediately by heavy lava outpouring which continued with only short intermissions until the end of May. In June the activity reverted to the explosive mode and a series of noisy eruptions, which produced very little ejecta, occurred during June, July and August. Strombolean explosions were resumed in September and continued intermittently during the remainder of the year.

Seismic activity gave warning of the approach of the March, 1960 eruption. An unusual number of felt shocks occurred during the three months before the event and continuous volcanic tremor reached an amplitude much higher than that which preceded the climactic eruptions of 1958.

The eruption was also preceded and accompanied by marked tilt movements which ranged from 8 to 18 seconds.

The pattern of events during the new phase of activity in 1960 tends to confirm earlier conclusions concerning the role of luni-solar forces in influencing the behaviour of the volcano. The important changes in eruptive behaviour occurred close to the solstice and equinox periods.

During the year before the eruption notable tectonic earthquakes occurred throughout the Territory and some of them occurred in structural areas related to Manam’s location. It is believed that these events supplied evidence of a regional stress condition which played some part in the eruptive events of 1960. This hypothesis in combination with the observation that gas content and gas pressures in the magma were low lead to the conclusion that the dominant mode of activity would be effusive in character. Risks to population were therefore low and evacuation of the island was not recommended.