Occasional seismic swarms 1989-99; no eruptive activity

During August-October 2000 there were no reports of unusual volcanic activity occurring at Esa'ala (also called the Dawson Strait group). RVO had a 1960s-vintage seismic recorder at Esa'ala until 1994. Since then, maintenance and funding problems have meant it has neither functioned nor been replaced. Discussion with Professor Abe following a seismic survey in the area in the second part of 1999 revealed that he had seen continued seismicity at the Esa'ala base station.

The last notable seismic swarm at Esa'ala before the RVO instrument broke down was in November-December 1992. Another prior swarm of earthquakes took place in mid-December 1989 (BGVN 15:01). RVO maintains a part-time observer at Esa'ala who keeps track of felt earthquakes. He typically reports that no felt earthquakes have occurred.

General References. Davies, H.L., 1973, Fergusson Island, Papua New Guinea-1:250,000 Geological Series: Bur. Miner. Resour. Aust. explan. Notes, SC/56-5.

Smith, I.E.M., 1976, Peralkaline rhyolites from the D'Entrecasteaux Islands, Papua New Guinea, in Johnson, R.W., ed., Volcanism in Australasia: Elsevier, Amsterdam, p. 275-285.

Smith, I.E.M., 1981, Young volcanoes in eastern Papua in Johnson, R.W., ed., Cooke-Ravian Volume of Volcanological Papers: Geological Survey of Papua New Guinea Memoir 10, p. 257-265.

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

January 1990 (BGVN 15:01) Cite this Report

Felt swarm of A- and B-type earthquakes; divers film submarine vents and hear booming and roaring

"A swarm of earthquakes took place in the Esa'Ala area in mid-December 1989. Earthquakes were felt with intensities MM III-V by villagers at Esa'Ala, Dobu Island, and on SE Fergusson Island at Oiau, Deidei, and Numa Numa. The local volcano observatory at Esa'Ala noted felt events on the 10th (26 events), 11th (12), 12th (6), and the 13th (3).

"Reports received at RVO on 11 and 13 December prompted a 5-day on-site inspection and local seismic recording by three RVO officers, 13-18 December. When local recording resumed at Esa'Ala Volcano Observatory (on the 13th), 30 sharp volcano-tectonic earthquakes (A-type with S-P

Table 1. Number of A- and B-type events recorded at the Esa 'Ala Observatory, 13-17 December 1989.

"The deployment of two portable seismographs at Numa Numa and Waiope Island, together with the seismograph at the Esa'Ala Volcano Observatory, allowed location of some of the larger earthquakes. Of the five earthquakes located, three occurred in the Dobu passage, another under Dobu Island, and the 5th between Dobu Island and Neumara Island (figure 1). These locations lie [near the outline] of an inferred submerged caldera.

Figure 1. Map of the Dawson Strait Group area, showing volcanic vents, the volcano observatory (with permanent seismograph), sites of two portable seismographs deployed 13-18 December 1989, and the five earthquakes located during that period. Courtesy of RVO.

"The strongest earthquake caused minor landslides from 4-30-m cliffs of pyroclastic deposits along the N and E shores of Dobu Island, and one landslide inland, from a steep gulley on the E flank of Dobu. Ground cracks, 1-4 m inland and 0.5-2 cm wide, reported at Kenaie village on the NE side of Dobu Island were related to the collapse of marine cliffs parallel to their edges.

"Off the N coast of Dobu Island on the afternoon of the 11th, a team of professional divers filmed and photographed very turbulent springs (28°C) and degassing vents through sand holes up to 1 m in diameter, at a depth of 6 m of water. They also reported subcontinuous booming and roaring underwater sounds (at an estimated 30-40 db level).

"The December 1989 earthquake swarm was the first episode of unrest [in this area] since April 1969. At that time (1969), over a period of a few days, 5-7 shocks were felt daily, accompanied by noises. This prompted spontaneous evacuation of villagers on Dobu Island and around Oiau Volcano. Recording at the time was from one station only (ESA). On the basis of the S-P interval (1-1.4 seconds) and reports of earthquake intensities, the swarm was presumed to have originated from the Dobu Passage."

Information Contacts: I. Itikarai and P. de Saint-Ours, RVO.

December 2000 (BGVN 25:12) Cite this Report

Occasional seismic swarms 1989-99; no eruptive activity

During August-October 2000 there were no reports of unusual volcanic activity occurring at Esa'ala (also called the Dawson Strait group). RVO had a 1960s-vintage seismic recorder at Esa'ala until 1994. Since then, maintenance and funding problems have meant it has neither functioned nor been replaced. Discussion with Professor Abe following a seismic survey in the area in the second part of 1999 revealed that he had seen continued seismicity at the Esa'ala base station.

The last notable seismic swarm at Esa'ala before the RVO instrument broke down was in November-December 1992. Another prior swarm of earthquakes took place in mid-December 1989 (BGVN 15:01). RVO maintains a part-time observer at Esa'ala who keeps track of felt earthquakes. He typically reports that no felt earthquakes have occurred.

General References. Davies, H.L., 1973, Fergusson Island, Papua New Guinea-1:250,000 Geological Series: Bur. Miner. Resour. Aust. explan. Notes, SC/56-5.

Smith, I.E.M., 1976, Peralkaline rhyolites from the D'Entrecasteaux Islands, Papua New Guinea, in Johnson, R.W., ed., Volcanism in Australasia: Elsevier, Amsterdam, p. 275-285.

Smith, I.E.M., 1981, Young volcanoes in eastern Papua in Johnson, R.W., ed., Cooke-Ravian Volume of Volcanological Papers: Geological Survey of Papua New Guinea Memoir 10, p. 257-265.

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.