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

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 micro-earthquake activity of the Louisville seismic gap and its southern erosive area using a local network of ocean bottom seismometers. Over 6 months of offshore network deployment, our local catalog reaffirms the existence of the Louisville seismic gap at magnitudes ranging from Mw ~2.5 to 5.5. Furthermore, the width of the seismic gap revealed by our local catalog is much wider than the subducting seamount itself but aligns well with the flexural moat of the LR, indicating that additional features than just topographic relief control the occurrence of seismic gaps. To the south of the seismic gap, seismicity distribution over the forearc is not evenly distributed but 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. Given the widespread small relief highs in the subducting plate, we link the patchy seismicity to the occurrence of topographic anomalies, which might enhance fracturing along the base of the upper plate and ultimately contribute to basal erosion. Additionally, seismicity reveals deformation of the outer rise along trench-parallel normal faults with depths ranging from 5 to 25 km, indicating a highly faulted and hydrated downgoing plate, nurturing down-dip extensive intermediate-depth earthquakes, outlining a double seismic zone that is controlled by dehydration embrittlement.