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Report on Pagan (United States) — March 1993


Bulletin of the Global Volcanism Network, vol. 18, no. 3 (March 1993)
Managing Editor: Edward Venzke.

Pagan (United States) Banded tremor; increased ash eruptions

Please cite this report as:

Global Volcanism Program, 1993. Report on Pagan (United States) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199303-284170


United States

18.13°N, 145.8°E; summit elev. 570 m

All times are local (unless otherwise noted)

The number of reports of ash columns above North Pagan . . . has increased since the beginning of the year. A 168-km-deep Benioff-zone earthquake (Mb 5.7) occurred at 0118 GMT on 18 January with an epicentral location (18.38°N, 145.71°E) ~40 km N of Pagan. In January 1982, three earthquakes, 500-600 km deep, in the vicinity of Pagan preceded eruptive activity (8:2), and it is speculated that the January 1993 event affected the behavior of the volcano.

Columns rising to 2,000 m above the summit were observed in mid-January and on 17 March. It is thought that other, unobserved eruptive events have also occurred. A distinct change in seismic activity took place on about 13 February. Before then, the seismicity consisted of 12-24 long-period events/day, each lasting 30-60 seconds. The dominant frequency of the events was 2-5 Hz and their amplitude was 2-3x background. Occasional small ash columns to 30 m were observed. Since mid-February, there have been frequent reports of intermittent ash eruptions, and banded tremor has been recorded at 10x background levels. The tremor episodes last 30 minutes to an hour, and are separated by 1-2 hour intervals of relative quiet. Though battery problems at the solar-powered seismic station prevent recording of activity when there is insufficient sunlight, the banded tremor has been consistently recorded when the station was functioning. It is speculated that the frequent ash emissions are associated with the tremor episodes.

Geological Summary. Pagan Island, the largest and one of the most active of the Mariana Islands volcanoes, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7 and 4 km in diameter, respectively. North Pagan at the NE end of the island rises above the flat floor of the northern caldera, which may have formed less than 1,000 years ago. South Pagan is a stratovolcano with an elongated summit containing four distinct craters. Almost all of the recorded eruptions, which date back to the 17th century, have originated from North Pagan. The largest eruption during historical time took place in 1981 and prompted the evacuation of the sparsely populated island.

Information Contacts: R. Koyanagi, HVO; R. Chong, Disaster Control Office, Saipan; R. Moore, USGS.