"Seismicity and rates of ground deformation reintensified in October. The total number of caldera earthquakes for the month was 6749, and seismic energy released was [5.5 x 10¹⁸] ergs.

"The increased level of activity was due mainly to a seismic and ground deformation crisis on 18 October. The seismicity was concentrated in the Blanche Bay area and included four moderate-to-strong earthquakes (M_L 4.9, 3.6, 3.35, and 3.3). The seismic energy released during the crisis amounted to . . . about 90% of the month's total seismic energy. Tilt changes measured soon after the crisis indicated a deformation source immediately offshore (W) from Sulphur Point, at the N edge of Blanche Bay. The maximum measured tilt change was about 90 µrad. Using a point-source model, the deformation source was calculated to be about 1.2 km deep, and the volume change at the source about 1 x 10⁶ m³. The uplift at Sulphur Point was about 100 mm. The ground deformation associated with the crisis was very localized. At the SE coast of Matupit Island, about 1.5 km from the deformation source, the uplift was only 33 mm; at the N shore of Greet Harbour, about 2.5 km away, it was only 5 mm. No marked horizontal deformation took place in association with the crisis.

"In addition to the crisis on the 18th, there were a number of seismic swarms and a few moderate-to-strong discrete earthquakes. The most notable was a swarm at Greet Harbour on 8 October (maximum M_L 3.8), a moderate-to-strong earthquake (M_L 3.8) at the entrance to Blanche Bay about 10 hours after the crisis on the 18th, and seismic swarms from around the Vulcan headland on 24 (maximum M_L 2.8) and 26 October (maximum M_L 3.2).

"Most of the October ground deformation took place on the 18th, but tilting and uplift continued at a reduced rate around Greet Harbour for the remainder of the month. An offset of about 25 µrad was registered at one station on Vulcan after the seismic swarm on the 26th. The maximum ground deformation recorded for the month was 130 µrad of tilt and 100 mm of uplift at Sulphur Point. Horizontal deformation was mostly insignificant, although a distinct N-S dilation was evident at the mouth of Blanche Bay. This was due largely to a northward shift (about 50 mm) of Sulphur Point."

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.