Microearthquakes and tectonics in an active back-arc basin: the Lau Basin

Abstract

Abstract An Ocean Bottom Seismograph (OBS) array was deployed for 20–22 days in late 1984 to investigate the precise locations of microearthquakes and their tectonic implications for active back-arc opening in the northern Lau Basin. Using P- and S-wave arrival times from four or more OBSs, the hypocenters of ∼ 300 shallow earthquakes were located with a high confidence level. The magnitudes of most OBS-located earthquakes were estimated to be less than four. In the northern half of the survey area, a narrow, linear zone of microearthquakes, trending NNW-SSE, has been identified. The northern part of the narrow seismic zone is within a central axial depression at the southern end of the Peggy Ridge. Further south, the trend of the seismic zone becomes more N-S. The narrow seismic zone seems to be composed of at least six seismic segments, offset by short aseismic zones. Most of the seismic segments trend NNW-SSE, suggesting a system of left-stepping en echelon spreading ridges, where the spreading ridge segment is seismically inactive and the transform fault is active. The spreading ridges appear to strike N-S or NNW-SSE, but the direction of the back-arc opening is considered to be NW-SE. No hypocenters were located with a high level of precision in the area south of latitude 18°S, except a small isolated zone of shallow earthquakes at the southeastern part of the survey area. We suggest that the shallow earthquakes in this isolated seismic zone were intraplate events in the Tonga platelet. This platelet is separated from the major Indo-Australian plate by the back-arc opening system in the Lau Basin.