Microgravity Changes and Ground Deformation at Rabaul Caldera, 1973–1985

Abstract

A progressive gravity decrease of more than 410 µgal took place at Rabaul Caldera, Papua New Guinea, between August 1973 and July 1985. The gravity change correlates with ground deformation at the caldera, particularly with uplift of more than 1.8 m. The patterns of gravity, elevation, horizontal distance and tilt changes indicate a principal source of inflationary deformation beneath the east-central part of the caldera. These phenomena, together with increases in seismicity, are expressions of a new phase of caldera unrest which commenced in 1971 and intensified markedly during late 1983 and 1984.

The observed relationship between gravity and uplift is about -216 µgal m^−l. This relationship is consistent with an increase of sub-surface mass, presumed to be a magmatic intrusion. A simple model which accounts for the observed gravity and elevation changes is a “point” source at a depth of 1.8 km. The depth of the source remained constant suggesting that it is a well-established feature capable of accommodating the inferred physical change without becoming unstable, and/or that further ascent of magma was prevented by an overlying body of unyielding rock.

The scale of the monitored geophysical changes is generally indicative of a relatively small-scale intrusion. Calculations based on the gravity changes indicate a mass increase of 1.0 × 10⁸ t. The magnitude of this intrusive event is well within the range of magnitudes of previous eruptions at the post-caldera vents.