". . . Rabaul Caldera was quiet again throughout June. Routine leveling on 27 June showed that uplift of ~20 mm had taken place at the S end of Matupit Island since the previous survey on 27 May.

"There were 220 detected caldera earthquakes in June, compared to 694 in May and 397 in April. Days of higher activity (>10 earthquakes) occurred on 13, 15, 16, 19, and 23 June. On all of these days except the 16th, small swarms of earthquakes were recorded. None of these earthquakes were felt widely, although the largest, on the 13th, had a magnitude of 3.0. Only 23 earthquakes were located, 14 with location errors of <1 km. Most of the activity was located in the NE part of the caldera seismic zone. However, the swarms on the 13th and 15th included some earthquakes that appear to have originated from the SE part of the zone, although the location errors were large.

"On 23 and 24 June, the seismic station on Rabalanakaia (RAL) showed a number of unusual signals. Three types of signals were seen: brief high-frequency (~5 Hz) harmonic signals, low-frequency harmonic signals (~1 Hz) that lasted for up to a minute, and a non-harmonic tremor-like signal. The last two were man-made 'noise,' but no cause has yet been found for the high-frequency harmonic signals."

Geological Summary. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: D. Lolok, R. Stewart, I. Itikarai, P. de Saint-Ours, and C. McKee, RVO.