Quantifying amount of free water stored in incoming upper oceanic crust outboard of the Alaska Peninsula and Sumatra Trenches

Water stored in the incoming oceanic plate is thought to exert strong controls on subduction zone processes including megathrust properties and arc magmatism. However, very little constraints exist on amount of water carried by the uppermost oceanic crust near subduction zones as typical ocean bottom seismometer refraction data lack sampling and resolution within this layer. Here we present three high-resolution seismic velocity models of the upper oceanic crust based on downward continuation of long-offset streamer data followed by traveltime tomography. Seismic velocities with the upper crust are then converted into upper crustal porosities using differential effective medium theory. Two of the profiles are located offshore the Alaska Peninsula and the third one is located offshore Sumatra. All profiles are at a distance >70 km from the trench axis and were shot parallel or slightly oblique to it. The three profiles exhibit along strike variations in seismic velocities and in the amount of free water in the uppermost crust. In Alaska, outboard of the Shumagin segment, velocities increase from 3.75-4.2 km/s at the top to 5.6-6.2 km/s at 1.1 km below the basement and conversely, porosities decrease from 8-12% at the basement top to 3-4% at 0.5 km below the basement. The lowest velocities in the uppermost crust correspond to the boundary between crusts accreted at two different spreading centers. Outboard of the Semidi segment, top basement is rough and is at different burial depths as the profile extends from the middle to the edge of the Zodiac fan. Velocities are higher and porosity is lower (5-10%) where fan sediments are thick. Offshore Sumatra, sediment thickness is very thin and the Indo-Australian plate is known to be deforming. Seismic velocities in the upper crust are overall slightly lower than the upper crust in Alaska with porosities ranging from 11-14% at the top basement, 4-5% at 550m depth below the top basement and is less than 2% within the sheeted dikes. For Sumatra, doubly converted shear wave arrivals allow us to perform S-wave tomography and highlight high Poissons ratio (>0.32) in the uppermost 500 m. All these results show that variability exist in upper oceanic crust over short distances and some of this variability could contribute to the along-strike variations seen in subduction zone processes.