The July 1998 Papua New Guinea Earthquake: Mechanism and Quantification of Unusual Tsunami Generation

— The unusual tsunami generated by the July 17, 1998 Papua New Guinea earthquake was investigated on the basis of various geophysical observations, including seismological data, tsunami waveform records, and on-land and submarine surveys. The tsunami source models were constructed for seismological high-angle and low-angle faults, splay fault, and submarine slumps. Far-field and near-field tsunamis computed from these models were compared with the recorded waveforms in and around Japan and the measured heights along the coast around Sissano Lagoon, respectively. In order to reproduce the far-field tsunami waveforms, small sources such as splay fault or submarine slump alone were not enough, and a seismological fault model was required. Relocated aftershock distribution and observed coastal subsidence were preferable for the low-angle fault, but the low-angle fault alone could not reproduce the large near-field tsunamis. The low-angle fault with additional source, possibly a submarine slump, is the most likely source of the 1998 tsunami, although other possibilities cannot be excluded. Computations from different source models showed that the far-field tsunami amplitudes are proportional to the displaced water volume at the source, and the comparison with the observed tsunami amplitudes indicated that the displaced water volume at the 1998 tsunami source was ～0.6 km³. The near-filed tsunami heights, on the other hand, are determined by the potential energy of displaced water, and the comparison with the observed heights showed that the potential energy was ～2 × 10¹² J.