High-resolution upper crustal velocity structure of the Aleutian and Sumatran outer rises from traveltime tomography and elastic full waveform inversion applied downward continued long-offset multichannel seismic data.

The hydrological state of subducting oceanic crust influences subduction zone processes like seismic coupling at the megathrust interface. One control on the amount of subducted water is likely the degree of bending related faulting in the outer rise, where extensional forces are applied to the uppermost lithosphere. Elucidating the physical properties and composition of the subducting upper crust is thus critical for understanding possible controls on hazardous seismic activity and potential tsunamigenic events as mineral bound water is expected to be released at shallower depths than water stored in the lower crust and uppermost oceanic mantle.

Reduced P-wave mantle velocities inverted from wide-angle refraction data correspond with images of bending faults, suggesting the presence of serpentinized mantle peridotite; however, seismic estimates of hydration are nonunique and the nature of penetrating fluids is poorly understood. Moreover, conventional refraction data are generally low-resolution and multichannel seismic (MCS) profiles often lack the offset required to appropriately resolve upper crustal velocity-depth structure.

We present preliminary results of traveltime tomography (TT) followed by elastic full waveform inversion (FWI) applied to downward continued (DC) MCS data acquired orthogonal to aseismic gaps offshore Alaska and Sumatra using 8 and 15 km-long streamers, respectively. DC transforms pre-critical refractions into first arrivals, increasing the offset range useable for velocity inversion of high-fold MCS data. TT of refracted P-wave arrivals shows strong evidence for decreased upper crustal velocities that correspond with the presence of observable bending faults in both areas. Additionally, we observe a multitude of converted arrivals in the DC data, particularly offshore Sumatra, that are sufficient enough potentially allow for the inversion of S-wave traveltimes. We will present analysis of these converted arrivals along with preliminary updates after applying elastic FWI. These data provide an unprecedented look at mature oceanic crust in two different outer rises, allowing us to explore the relationships between bending faults, hydration, and seismicity in these areas.