Earthquake evidence for fore-arc extension at the southernmost Mariana subduction zone

The upper-plate deformation in the southernmost Marianas is suggested to be different from that in the central Mariana due to either increasing curvature of the Mariana system with time or oblique convergence. A complex upper-plate deformation is suggested by earthquake focal mechanisms in central Mariana, including arc-parallel extension near volcanic cross chains, arc-normal extension along the frontal arc, and arc-parallel compression farther into the back arc near the Mariana Trough. In addition, arc-parallel extension has been documented by GPS data, indicating a separation rate of approximately 5 mm/a between Saipan and Guam. However, the upper-plate deformation is unclear due to absence of GPS measurements south of Guam. To investigate upper-plate deformation at the southernmost Mariana, we determine focal mechanisms of local earthquakes via waveform inversion, using the first Ocean Bottom Seismograph (OBS) experiment (2016.12-2017.06) near the Challenger Deep. We first estimate the sensor orientations with flipping polarities of OBS data through polarization analysis of seismic waves and waveform modelling. We then conduct waveform inversion to derive earthquake focal mechanisms within or near the OBS network. For the events near the Southwest Mariana Rift (SWMR), both our results and historical GCMT solutions show normal faulting mechanisms. In the forearc block, normal faulting and strike-slip faulting are the dominant deformation mechanisms in our results, with focal depths shallower than 20 km. Seismicity distribution and focal mechanisms indicate arc-perpendicular extension over the SWMR and the fore-arc region, placing constraints on the present-day deformation of the southern Mariana subduction zone.