Rabaul volcano, Papua New Guinea: seismic tomographic imaging of an active caldera

The recently active Rabaul volcano on the island of New Britain in eastern Papua New Guinea is associated with a subduction zone located near the triple junction formed by the Pacific, South Bismarck and Solomon lithospheric plates. Analysis of our 1997 seismic tomography survey of the Rabaul caldera reveals the P-wave velocity structure to a depth of about 12 km using both explosive and earthquake seismic sources. The Rabaul volcanic complex is formed by a series of caldera collapse structures and a group of basalt–andesite volcanic centres with two currently active dacitic intra-caldera cones, Tavurvur and Vulcan. The 1994 eruption of these intra-caldera cones caused major infrastructure damage and required the evacuation of Rabaul township. Ongoing minor activity of Tavurvur continues to the present. The 1997 three-dimensional seismic tomography imaging identifies a 30–35 km3 low-velocity region (?magma reservoir) at 3–6 km depth beneath the central Rabaul caldera, and gives an insight into the geological features of the caldera ‘plumbing’ system on scales of a few kilometres. The seismic survey highlights the heterogeneity in P-wave velocity both laterally and vertically within the Rabaul caldera, indicating significant complexity within quite a small area. The structural complexity is consistent with that observed in outcrop of Palaeozoic eroded calderas in other parts of the world. The low-velocity region at Rabaul (<5.0 km/s) is identified with the part of the caldera under Rabaul harbour thought to be associated with the current magma injection system. There are also high-velocity (>6.0 km/s) rock units around the caldera rims that are interpreted to indicate large volumes of mafic intrusive rock at shallow (<4 km) depths. In the St. Georges Channel just north of Rabaul, the Tavui caldera, although a spectacular bathymetric feature, does not have a prominent low-velocity zone at depth and is therefore regarded as inactive.