Shallow Seismicity and the Classification of Structures in the Lau Back‐Arc Basin

Abstract

AbstractBack‐arc basins open in response to subduction processes, which cause extension in the upper plate, usually along trench‐parallel spreading axes. However, global seismic databases reveal that the majority of seismic events in the Lau Basin occur along transcurrent (strike‐slip) rather than extensional faults. To better characterize active deformation in this region, we compared centroid moment tensors (CMTs), calculated for large (Mw > 5), shallow (<30 km) seismic events, to the orientations of seafloor lineaments mapped throughout the Lau Basin. Ship‐based multibeam and satellite altimetry were combined with vertical gravity gradient data to create the lineament map. By comparing the possible focal planes of the CMTs to the orientations of the lineaments, the most likely fault plane solutions were selected, thus classifying the faults and establishing the nature of the highly variable stress regimes in the basin. We resolved the strike, dip, and dip direction of 308 faults and classified 258 additional structures by fault type. The analysis highlights a stress regime that is dominated by a combination of left‐lateral and right‐lateral strike‐slip faults, large‐scale transcurrent motion along rigid crustal‐scale fault zones, and nonrigid diffuse deformation along preexisting seafloor structures, with extension mainly limited to the tips of propagating rifts and spreading centers. By resolving many of the uncertain motions on the mapped lineaments of the Lau Basin, the CMT analysis addresses a number of questions concerning basin‐scale stress regimes and microplate development, complementing GPS measurements, and providing a more complete picture of the complexities of back‐arc basin development.