<p>The extensional deformation of arc lithosphere is a critical stage in the evolution of subduction systems. Although the tectonic dynamics of well-developed back-arc regions have been widely studied, our understanding of nascent arc rifts is still limited. The Southwest Mariana Rift (SWMR), located near the southern Mariana Trench, is currently in its initial rifting stage and serves as an ideal region for studying the evolution of nascent arc rifts. Local ocean bottom seismometer experiments have delineated short-term seismic activities of small earthquakes in the SWMR. However, moderate-sized earthquakes, which reflect long-term predominant rupture behaviors of the rifting faults, are poorly documented. Teleseismic waveforms are the primary data source for studying these moderate-sized earthquakes in the SWMR. Due to the effects of complex 3-D Earth’s structures, significant errors can occur in teleseismic source parameter estimation. In this study, we use a novel method to compute 3-D teleseismic Green’s functions. By leveraging the sensitivity of teleseismic P coda waveforms to source-side 3-D structural complexities, we effectively improve the accuracy of source location for moderate-sized earthquakes in the SWMR. Centroid depths indicate brittle middle to lower crust, suggesting that the rift crust may be undergoing active initial rifting from shallow to deep. Horizontal locations exhibit cluster distribution characteristics, indicating significant spatial heterogeneities in extensional deformation. Both the middle part and the two tips of the rift show seismicity of moderate-sized earthquakes. Generally, there is no clear evidence of asymmetric extensional deformation. We propose that the rollback of the subducting plate, rather than the collision of the Caroline Plateau, is the main driving force for the opening and evolution of the SWMR. This study provides new evidence for understanding the tectonic evolution of the southwestern Mariana arc.</p>