"Seismicity increased markedly in April, when 1,769 events were recorded, almost an order of magnitude greater [than] March. It is the highest monthly total since March 1985 (2,042). This increase was not caused by seismic crises but by fairly consistently higher daily earthquake counts. The strongest event was a M_L 2.7 earthquake on 13 April.

"Another unusual feature of Rabaul's April seismicity was the average duration of the events, which increased progressively during the second half of the month. A considerable number of events were located near the centre of the caldera, rather than on the caldera fault zone; many events were shallow. The earthquakes were previously concentrated at depths of 1-3 km, but in April ~30% of located events were <1 km deep.

"Measurements of ground deformation indicated a slight resurgence of inflation in the Matupit Island-Greet Harbour area. Tilt stations in that area showed maximum changes of ~10-12 µrad for the month. The largest horizontal distance changes, ~10-15 microstrain, were across the mouth of Greet Harbour. Levelling measurements showed that the SE part of Matupit Island was elevated 16 mm between 11 April and 7 May.

"At the end of April there was no indication whether the rate of seismic and ground deformation activity was likely to increase or subside. The change in the pattern of seismicity may be significant."

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: P. Lowenstein, RVO.