The 1977 Three Kings Ridge earthquake ( Ms = 6.7): broad-band aspect of the source rupture

Abstract

The 1977 Three Kings Ridge earthquake ( M s = 6.7) is an isolated event which took place in the South Fiji Basin. The centroid moment tensor (CMT) solution by the Harvard group shows a left-lateral strike-slip motion with a dip angle of 55° while the P-wave first motions require a nearly vertical fault plane. The centroid depth 17.4 km is deeper than the bottom of ordinary oceanic crust. Using long-period P and SH waveforms from the Worldwide Standardized Seismograph Network, we model the entire source rupture process with three subevents which span a 42 s duration. The best-fit double-couple focal mechanism obtained from the tensorial sum of the subevents is primarily a strike-slip with a nearly vertical fault plane subparallel to the Cook Fracture zone. The total moment release is (2.2–2.3) × 10 26 dyn cm, which is about 40% lower than the CMT estimate. The subevents' depth distribution is similar to the centroid depth (17 km), which is deeper than a typical oceanic Moho. Precise modeling of the waveform is also incompatible with a standard oceanic crustal structure, lending support to previous interpretations of the Three Kings Ridge as a fossil island arc. The discrepancy between our body waveform moment tensor solution and the Harvard CMT solution is dominated by the tensor components M rθ and M rθ . In the case of a shallow source, these components are notoriously unresolvable because of their vanishing excitation of seismic waves in the low-frequency range ( T ⩾ 45 s). However, the low-pass filtered body waves can be better modeled with a dip angle similar to that of the CMT solution. This observation suggests that the source process may include some level of evolution of the rupture mechanism with time. We interpret the earthquake as expressing a moderate level of internal deformation of the Australian plate, in the context of significantly different spreading rates in the Lau and Havre Basins.