"Seismicity in Rabaul caldera showed a further intensification in February. Although the total number of volcanic earthquakes recorded (8339) was slightly lower than in January (8372), there were more stronger events in February. The entire caldera seismic zone was active during the month. However, seismicity was concentrated in the NE, N, and W parts of the caldera. Seismic crises took place on 1, 5, 7, 8, 11, 12, 13, 18, and 27 February, the most energetic on the 7th, 13th, and 18th, affecting the NE, N, and W parts of the caldera, respectively. The strongest caldera earthquake in these crises was a magnitude (M_L) 4.3 event on the 18th.

"An interesting development in February was the increase in rates of tilting around Vulcan cone on the W side of the caldera. The tilt change at 1 station immediately SW of Vulcan was about 40 µrad, about four times the tilting seen there in January. By contrast, tilting around Greet Harbour in the NE part of the caldera was reduced compared with that of January, and the pattern of tilts there was more complex. Relatively minor tilting occurred during seismic crises. The largest crisis-related tilts were about 15 µrad near Vulcan.

"EDM data over the last 3 months show that significant horizontal deformations are taking place at Rabaul and are most pronounced in the Greet Harbour area. Extension of about 60 µrad (change in length divided by original length x 10⁶) has occurred across Greet Harbour in this period. There is no evidence of widespread horizontal movements."

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: C. McKee, RVO.