"Seismicity remained at a low level in March. The total number of caldera earthquakes for the month was 140. Daily totals ranged between 0 and 9. All events were of small magnitude (M_L <1.0). None were large enough to be computer-located, but the patterns of station registration indicated that most of the events were from the NE part of the caldera seismic zone, with a few from the E and NW parts.

"Tide gauge measurements indicated there was little or no net elevation change in March in the central part of the caldera (Matupit Island area). There was, however, a progressive rise of ~10 mm from the beginning of the month until the 17th, followed by rapid subsidence of about the same magnitude until the 24th."

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 688-m-high asymmetrical pyroclastic 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 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western 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: C. McKee, RVO.