Seismicity to the South of the Louisville Ridge-Tonga Trench Collision Zone: New Insight Into Processes Controlling Seismic Gaps and Subduction Erosion

Abstract

The Louisville seismic gap associated with the subduction of the Louisville Ridge (LR) along the Tonga-Kermadec trench is a globally prominent feature. Due to the lack of near-field seismic monitoring, the earthquake potential and seismic behavior in this region have long been an enigma. In this study, we investigate the local earthquake activity of the Louisville seismic gap and subduction erosion using a local network of ocean-bottom seismometers. Over 6 months of offshore network deployment, our local catalog supports the existence of the Louisville seismic gap at magnitudes ranging from M_w ∼2.5 to 5.5 and reveals that the southern boundary of the seismic gap aligns well with the flexural moat of the LR. To the south of the seismic gap, seismicity distribution over the forearc shows a patchy characteristic dominated by three earthquake clusters that correspond well with morphological forearc depressions, and a deforming upper plate middle prism is revealed by upward migrated aftershock sequences, suggesting ongoing basal erosion. Seismicity reveals deformation of the outer rise along trench-parallel normal faults with focal depths ranging from 5 to 25 km, indicating a highly faulted and hydrated downgoing plate, which agrees with down-dip extensive intermediate-depth earthquakes.