A shallow sub-seafloor seismic model that includes well-determined seismic velocities and clarifies sediment-crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska-Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean-bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub-seafloor S-wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the accretionary wedge. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep-sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S-wave velocities, contrasting with the clay-rich sediments in the forearc and accretionary wedge. A difference in S-wave velocities is observed for upper oceanic crust formed at fast-rate (Shumagin) and intermediate-rate (Semidi) spreading centers. The reduced S-wave velocities in the Semidi crust may be caused by increased faulting and a less mafic composition, related to a previous period of intermediate-rate spreading.

The EarthCube BALTO broker (Brokered Alignment of Long-Tail Observations) provides streamlined access to both long-tail and big data using Web Services through several distinct mechanisms. First, we updated the OPeNDAP framework Hyrax, software that serves big data from USGS, NASA, and other sources, with a BALTO extension that tags dataset landing pages with JSON-LD encoding automatically. Therefore, the big data made available through Hyrax are now searchable via EarthCube GeoCODES (formerly P418) and Google Dataset Search. The BALTO broker extension to Hyrax makes thousands of datasets easily searchable and accessible. Second, we focused our efforts on a geodynamics use-case aimed at advancing our understanding of continental rifting processes through the use of an NSF mantle convection code called ASPECT. By addressing this use-case, we implemented a web services brokering capability in ASPECT that allows for remotely accessing datasets via a URL defined in an ASPECT parameter file. Third, through another use-case in ASPECT aimed at testing hypotheses involving global mantle flow, we developed a brokering mechanism for a “plug-in” that accesses NetCDF seismic tomography data from the NSF seismology facility IRIS, then transforms it into the format needed by ASPECT to run global mantle flow models constrained by seismic tomography. Fourth, we demonstrate methods to allow any scientist or citizen scientist to make their in-situ IoT based sensor data collection efforts available to the world. Finally, we are developing a Jupyter Notebook with a GUI that allows for users to search Hyrax servers for big datasets and long-tail data. These cyberinfrastructure developments comprise the entire EarthCube BALTO brokering capabilities.

In the northern Hikurangi margin, aseismic slip events known as slow slip events (SSEs) occur approximately every 18-24 months with a long duration (weeks to months), and have been shown to cause 1 to 3 cm of horizontal surface displacement onshore and 1.5 to 5.4 cm of vertical displacement (uplift) offshore. The discovery of SSEs has expanded our scientific understanding of slip behavior at subduction zones to encompass behaviors beyond earthquake-producing (fast) slip events. Using converted phase seismic signals from local earthquakes in addition to direct P and S arrivals allows us to further analyze the properties of the plate interface along the subduction zone, including its composition, ­topography and heterogeneity. It has been hypothesized that these properties play a role in the process of slow slip, therefore our investigation of converted waves focuses on the area offshore the east coast of the North Island of New Zealand, where dozens of SSEs have been observed in the past twenty years. We examine an earthquake catalog comprised of local slab earthquakes occurring recorded on ocean-bottom seismometers (OBSs) from the one-year-long Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) experiment as well as New Zealand’s permanent GeoNet on-land seismic network. Preliminary findings indicate strong secondary arrivals on the vertical component of the land stations which are likely Sp conversions from the plate interface. We aim to identify conversions from the plate interface on the OBSs from the HOBITSS experiment, as these data will help us resolve a part of the plate interface that was not investigated in previous converted wave studies. We will conduct a systematic search for phase-converted arrivals from the plate interface, initially focusing primarily on Sp conversions by examining the vertical component seismograms.