Geophysical study of a magma chamber near Mussau Island, Papua New Guinea

Analysis of a 24-channel seismic-reflection data collected near Mussau Island, Papua New Guinea, shows a high-amplitude, negative-polarity reflection that we believe is from the top of a magma chamber.  The reflecting horizon lies at a depth of about 4.4 s subbottom and can be traced laterally for 2.6 km.  On shot gathers, the reflection demonstrates normal moveout appropriate for an in-place event.  The frequency spectrum of the reflection shows a decrease in high-frequency content when compared to the sea floor reflection, as would be expected for a deep subsurface event.  The polarity of the reflection event is negative, suggesting that the reflection horizon is the top of a low-velocity zone.  Magnetic data indicate that the ridge containing the reflecting horizon is magnetic, and the geology of Massau Island suggests that the ridge is volcanic in its origin.  We speculate that the high-amplitude reflection is from the top of a magma chamber some 7-11 km deep.