Monitoring the 2021 M_w 8.2 Alaska Earthquake by an Offshore Seismic and Fluid Pressure Observation Network and Implications for Ocean‐Crust Dynamic Coupling

Abstract

Ground shaking caused by earthquakes is accompanied by seafloor and sub‐seafloor formation fluid pressure variations in offshore areas, but there have been few collocated observations of these signals. In this work, we report seismic and high‐sampling‐rate fluid pressure records of the 2021 Mw 8.2 Alaska earthquake by the Ocean Networks Canada (ONC) NEPTUNE observatory at an epicentral distance of ∼2,200 km in the northeast Pacific Ocean. The system comprises observatory nodes in various tectonic environments, with each node including buried broadband seismometers, seafloor pressure sensors, and, at two nodes, borehole pressure sensors. Seismic and tsunami waveforms of the Mw 8.2 earthquake were documented in detail. Seismic seafloor pressure variations (Psf) were dominated by Rayleigh waves of periods between 5 and 50 s, with peak amplitudes of 3–4 kPa at most sites. Waveform similarity and the linear scaling between Psf and vertical ground acceleration indicate forced acceleration of the water column being dominant in governing Psf during long‐period surface‐wave arrivals, with an additional component of elastic oscillation occurring at higher frequencies (>0.1 Hz) causing extra pressure signals. Analysis of formation pressure variations due to various types of ocean loading of distinctly different frequencies (e.g., tides, tsunami, and infragravity waves) shows stable one‐dimensional vertical loading efficiencies that depend on lithology at each borehole site, with loading response being strongly influenced by the presence of free gas at shallow depths within the Cascadia accretionary prism. Inter‐site comparisons of seismic and seafloor pressure waveforms demonstrate a key role of sediment thickness in the amplification of surface wave amplitudes.

Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking

“…Sun et al. (2021) and Sun and Davis (2022) have examined the relationship between pressure variations and strain imposed by Raleigh waves (with dominant energy in period of 20–50 s) from distant earthquakes and inferred a matrix compressibility that is consistent with what is derived from the tidal loading response.…”

“…The absence of complicating factors is also supported by observations for higher-frequency loading, such as ocean infra-gravity waves and seismic ground motion. Sun et al (2021) and Sun and Davis (2022) have examined the relationship between pressure variations and strain imposed by Raleigh waves (with dominant energy in period of 20-50 s) from distant earthquakes and inferred a matrix compressibility that is consistent with what is derived from the tidal loading response.…”

“…Relationships among collocated ground motion, seafloor pressure, and formation fluid pressure measurements have proven useful for studying dynamic ocean-crust coupling (Sun & Davis, 2022) and for characterizing the mechanical properties of the oceanic crust and the outer accretionary prism and their response to seismic and oceanographic loading (e.g., Davis & Farrugia, 2021;Sun et al, 2021). In an examination of the Hole U1364A 11-month borehole seismometer, tilt meter, and fluid pressure data, McGuire et al (2018) found no indications of deformation during that monitoring period.…”

“…In each row, the left panel shows seismic signals of formation fluid pressure P fm (blue) and radial ground velocity V R (red) recorded by the collocated (CQS64) or nearby seismometer (NC89, ∼3.5 km away), with numbers indicating peak signal amplitudes. A time shift of 7 s was applied to the P fm record, to account for the pressure diffusion time (associated with local lithology and observatory configuration) to optimize the phase agreement between the two signals (Sun & Davis, 2022); the right panel shows pressure anomalies relative to hydrostatic, with tidal and other ocean loading signals and noise removed using methods explained in Section 3, over a longer 2-month period spanning the earthquake origin time (marked by the orange line). No transients or offsets were observed after any of these earthquakes.…”