11:00 〜 11:15
[SSS04-07] Using Ocean Networks Canada’s Seismic and Fluid Pressure Monitoring to Study Dynamic Seafloor Loading and Formation Mechanical Properties
★Invited Papers
キーワード：cabled observatory, Ocean Networks Canada, fluid pressure monitoring, 2021 Mw 8.2 Chignik Alaska earthquake, formation mechanical properties, dynamic ocean-crust coupling
Recent development of offshore geophysical observations, including via cabled networks that allow high-sampling-rate and real-time monitoring, has provided rich opportunities to study sub-marine seismotectonic and ocean loading processes, as well as early warning of geohazards. In this study, we report several new applications of the Ocean Network’s Canada (ONC) NEPTUNE observatory established offshore of Vancouver Island, spanning the northern Juan de Fuca Ridge, the ridge flank, and the Cascadia subduction zone. The observatory comprises several observation nodes in different geological settings, ranging from sediment-free mid-ocean ridge to normal oceanic plate covered by thick sediment, to subduction zone upper-plate accretionary prism. At each node, multiple seafloor pressure sensors form a local array, accompanied by collocated buried broadband seismometers. At two nodes, sub-seafloor formation pressure signals are monitored at sealed IODP borehole observatories. These multi-dataset observations provide insights into 1) the ocean-seafloor coupling during dynamic seismic and oceanic loading and 2) the in-situ characterization of sub-seafloor formation structure and properties.
Regarding the ocean-seafloor coupling, measurements associated with the seismic and tsunami wave arrivals of the 2021 Mw 8.2, Chignik, Alaska earthquake demonstrate a wide spectrum of seafloor pressure (Psf) variations across the hydrostatic and elasto-dynamic regimes. Long-period (>1 hr) tsunami arrivals caused Psf variations with a peak amplitude of ~0.3-0.4 kPa – reflecting a hydrostatic sea-surface elevation of ~3-4 cm – at our sensor locations at epicentral distances of ~2,200 km. For shorter-period (~5-50 s) surface wave arrivals, waveform agreement between Psf and vertical ground acceleration (AZ) suggests the dominance of forced acceleration of the ocean column in causing dynamic Psf variations; further spectral analysis of the Psf and AZ records reveals an additional role of elastic (hydroacoustic) oscillation of the water column in causing extra Psf signals at high frequencies (>0.1 Hz). The site-dependent magnitude of the dynamic Psf variations is governed by not only the local water depth, but also the sub-seafloor lithological conditions. Specifically, inter-site comparison of Psf records from the multiple observation nodes suggests the primary role of sediment layer thickness in governing surface wave amplification.
Regarding the in-situ characterization of sub-seafloor formation properties, formation pressure signals (Pfm) in response to the tsunami of the Chignik earthquake and various other types of ocean loading (e.g., infra-gravity waves during strong storms) suggest stable one-dimensional vertical loading efficiencies over a wide range of loading frequencies (10-5–10-2 Hz). Site-specific loading efficiency agrees with previous estimates based on tidal signals only, and reflects lithology-dependent formation matrix compressibility, porosity, and at the shallower monitoring levels of the Cascadia subduction prism, the existence of free methane gas. An expanded understanding of the oceanic crust’s mechanical properties has been achieved by studying Pfm anomalies due to volumetric strain perturbations caused by passing Rayleigh waves from many distant large earthquakes at various back azimuths. For each earthquake, companion Pfm and seismic records can be used to determine a “horizontal pressurization efficiency” that reflects formation matrix compressibility in the corresponding radial direction. Our analysis reveals that the shallow igneous oceanic crust is most compliant in the ridge-normal (i.e., plate-spreading) direction, most likely the consequence of preferentially aligned crustal fabrics. This in-situ determined mechanical anisotropy is equivalent to a seismic P wave velocity anisotropy of >50%, reflecting a previously unresolved degree of extensive fracturing.
Regarding the ocean-seafloor coupling, measurements associated with the seismic and tsunami wave arrivals of the 2021 Mw 8.2, Chignik, Alaska earthquake demonstrate a wide spectrum of seafloor pressure (Psf) variations across the hydrostatic and elasto-dynamic regimes. Long-period (>1 hr) tsunami arrivals caused Psf variations with a peak amplitude of ~0.3-0.4 kPa – reflecting a hydrostatic sea-surface elevation of ~3-4 cm – at our sensor locations at epicentral distances of ~2,200 km. For shorter-period (~5-50 s) surface wave arrivals, waveform agreement between Psf and vertical ground acceleration (AZ) suggests the dominance of forced acceleration of the ocean column in causing dynamic Psf variations; further spectral analysis of the Psf and AZ records reveals an additional role of elastic (hydroacoustic) oscillation of the water column in causing extra Psf signals at high frequencies (>0.1 Hz). The site-dependent magnitude of the dynamic Psf variations is governed by not only the local water depth, but also the sub-seafloor lithological conditions. Specifically, inter-site comparison of Psf records from the multiple observation nodes suggests the primary role of sediment layer thickness in governing surface wave amplification.
Regarding the in-situ characterization of sub-seafloor formation properties, formation pressure signals (Pfm) in response to the tsunami of the Chignik earthquake and various other types of ocean loading (e.g., infra-gravity waves during strong storms) suggest stable one-dimensional vertical loading efficiencies over a wide range of loading frequencies (10-5–10-2 Hz). Site-specific loading efficiency agrees with previous estimates based on tidal signals only, and reflects lithology-dependent formation matrix compressibility, porosity, and at the shallower monitoring levels of the Cascadia subduction prism, the existence of free methane gas. An expanded understanding of the oceanic crust’s mechanical properties has been achieved by studying Pfm anomalies due to volumetric strain perturbations caused by passing Rayleigh waves from many distant large earthquakes at various back azimuths. For each earthquake, companion Pfm and seismic records can be used to determine a “horizontal pressurization efficiency” that reflects formation matrix compressibility in the corresponding radial direction. Our analysis reveals that the shallow igneous oceanic crust is most compliant in the ridge-normal (i.e., plate-spreading) direction, most likely the consequence of preferentially aligned crustal fabrics. This in-situ determined mechanical anisotropy is equivalent to a seismic P wave velocity anisotropy of >50%, reflecting a previously unresolved degree of extensive fracturing.