Presenter: D.Randolph Watts (University of Rhode Island)

Description:
Slow slip events (SSEs) release strain across faults on time scales that are long compared to earthquakes. Offshore shallow SSEs have been discovered near the trenches, where by shifting stress to neighboring locations may trigger earthquakes and tsunamis. Offshore SSEs are challenging to monitor, particularly to map their spatial structure. While seafloor pressure sensors can detect small vertical seafloor movements in SSEs, the measured bottom pressure includes "ocean noise" signals from pressure variations due to subtidal physical processes within the water column at comparable amplitude (~10 cm). Ocean models produce realistic pressure records that correlate highly with observed measurements, however the residual difference has standard deviation (STD) ~1.6- 3 cm, still unacceptably high. Ocean variability of large spatial scale (>10s of km) and long time scale (>4 days) is geostrophic, a balance between currents and lateral pressure gradients. Since currents are unaffected by the small bottom deflections in SSEs, improved estimates of ocean noise can be made by combining measurements of currents and pressures. A pilot study was conducted offshore Oregon, deploying current meters and pressure sensors at four sites from April to November 2017. Observational methods were compared to reduce ocean noise, producing lower residuals (STD ~0.4- 1cm). Because no SSE was observed during that study, for illustration purposes synthetic SSEs of 2 cm amplitude and 7-day duration were added and detected, one at a time at different spots. The dynamical constraint of geostrophy detected the synthetic SSEs with no false positives or negatives. Offshore New Zealand, where SSEs are prevalent, examination of historical bottom pressure records indicates that adding a modest 2D array of current observations could improve detectability and structural mapping of SSEs.