"On 3 July, a major regional earthquake (M_L 7.1) in S New Ireland (about 80 km from Rabaul) caused large local ground deformations in Rabaul caldera. The main effects were tilt offsets of up to several hundred µrad. Tilt changes were greatest at the mouth of Greet Harbour. At Sulphur Point and the SE end of Matupit Island, tilt on the day of the earthquake may have reached 330 and 120 µrad, respectively. The tilt vectors at these locations and those of much smaller deformations at the head of Greet Harbour appear to radiate outward from a point near the mouth of the harbour. Tilting also occured on the Vulcan headland in response to the 3 July earthquake. Tilts of about 45-50 µrad were registered at two stations in the W-central and SE parts of the headland, but other nearby tilt stations showed only minor changes, and no clear pattern of tilt was evident. Other ground deformation measurements (levelling and EDM) did not show any notable changes associated with the earthquake.

"It is uncertain what significance should be attached to the fact that the earthquake-induced tilting in the Greet Harbour area conforms to some extent with effects produced by processes purely internal to the caldera. Such large tilt changes might be expected to have been accompanied by caldera seismicity, but as far as we know the caldera did not respond seismically to the 3 July earthquake. This contrasts with the effects of the [10] May earthquake, which included a seismic swarm and minor tilt changes in the Vulcan area. The apparently aseismic nature of the deformation on 3 July may be an indication of merely local surface movements.

"Apart from the effects of the 3 July earthquake, a low level of activity prevailed in Rabaul caldera in July. The total number of caldera earthquakes for the month was 595, down from 639 in June. A decrease in the daily rate of earthquakes was noted, from about 25 per day during the first half of the month to about 8 per day in the last week of the month. Caldera earthquakes were concentrated in the Rapindik area, immediately N of Greet Harbour. There were no strong caldera earthquakes in July; the strongest events had magnitudes (M_L) of less than 2.

"Levelling measurements along the line from Rabaul town to Matupit Island in late July showed that uplift in the caldera was continuing at a low rate. The greatest change since the previous survey in May was 26 mm at the SE coast of Matupit Island. This change indicates a rate of uplift probably similar to the pre-crisis rate if some of the uplift was associated with the 3 July regional earthquake.

"Horizontal distance measurements indicated continuing slow dilation in the caldera, with the greatest changes (5-10 ppm per month) occurring in the Greet Harbour area.

"Tilt changes after the 3 July earthquake were evident in the Greet Harbour area and at one station on the Vulcan headland. The largest changes were about 43 µrad about 1 km SW of the Vulcan cone and about 23-33 µrad at the mouth of Greet Harbour. These tilt changes accumulated at an exponentially decreasing rate and are believed to be related to the earthquake-induced tilting on 3 July.

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.