Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 8, August 1995

Rabaul (Papua New Guinea)  Intracaldera cones quiet, but nearby
       earthquake triggers local seismicity

New Britain Island, Papua New Guinea
4.27 S, 152.20 E; summit elev. 688 m
All times are local (= GMT + 10 hours)

Although Rabaul remained quiet in August, the region was subjected
to a strong tectonic earthquake (Ms 7.8) followed by associated
aftershocks (see Earthquakes, below). The earthquake struck on 16
August centered ~260 km SE of Rabaul. In Rabaul the earthquake
struck at 2027 with a modified Mercalli (MM) intensity of V,
causing some minor landslides and the collapse of a few buildings.

The earthquake triggered noteworthy responses at the caldera. On 16
August there were 24 high-frequency events, and, as late as 23
August, 3-9 additional events/day; in July there were 7
high-frequency earthquakes (M <1). On 22 August, one event was felt
by residents. With an approximate magnitude of ML 2.8, this was the
first felt caldera earthquake since the early phase of Rabaul's
September 1994 eruption.

These high-frequency earthquakes occurred mostly in the NE part of
the caldera (Namanula Hill area). In contrast, throughout the
period of heightened seismicity the number of low-frequency
earthquakes remained low, peaking on 30-31 August at 10 and 26
events, respectively. For comparison, during July low-frequency
events occurred 11 times. The August low-frequency earthquakes
appeared to originate from within or just outside the N caldera

During the first half of the month ground deformation remained
below the measurable level. Following the Ms 7.8 earthquake,
electronic and water-tube tiltmeters recorded offsets as large as
90 microradians. The offsets suggested subsidence in the central
part of the caldera with the outer rim remaining stationary. As of
15 September, adequate elevation data were unavailable to confirm
the pattern of offset seen in the deformation data.

Information Contacts: Same as for Manam.