Possible Shallow Tectonic Tremor Signals Near the Deformation Front in Central Cascadia

To better constrain the locking state of the shallow Cascadia megathrust, we investigate whether shallow tectonic tremor occurs near the deformation front at ~44.5°N during 2015-2024. We focus on two cabled buried ocean bottom seismometers (OBSs) on the portion of Cascadia that has evidence of partial locking offshore: one at Slope Base on the incoming plate ~5 km from the deformation front, and another ~20 km east on the overriding plate at Southern Hydrate Ridge. We first use in situ measured bottom currents to show that shallow burial successfully prevents current-generated noise on OBSs. We then develop a single-station approach to isolate tectonic tremor-like signals based on waveform and spectral characteristics. This technique allows the use of isolated stations and small networks and accounts for emergent signals specific to the marine environment, namely T-phases and ship noise. Application of this approach to the buried OBSs in central Cascadia detects tectonic tremor-like signals at the Slope Base site only that cannot easily be attributed to instrumental or environmental noise. Additional observations are required to verify the origin of these signals, but possible sources include localized slow slip on the décollement, protothrusts, faults on the incoming plate, nearby strike-slip faults, or deformation within the outermost accretionary wedge.