Riedel, Michael and all Cruise Participants (2022) CLOCKS: Northern Cascadia: Extent of locked zone, prism deformation, slip-to-toe, and the edge of subduction, and CASCADIA CO2: Seismic multi-parameter study at a possible site for CO2 storage in basalt in the Cascadia Basin utilizing shear wave events (secondary user), Cruise No. SO294, 13.09. 2022 - 27.10. 2022, Vancouver (Canada) - Port Hueneme (USA), CLOCKS & CCO2. . SONNE-Berichte, SO294 . UNSPECIFIED, Bonn, 150 pp. DOI 10.48433/cr_so294.

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Abstract

Expedition SO294 served two scientific projects. The main program was designated towards the
project CLOCKS: “Northern Cascadia: Extent of locked zone, prism deformation, slip-to-toe, and
the edge of subduction”. The Cascadia subduction zone extending from northern California to the
northern tip of Vancouver Island, remains unbroken by a megathrust earthquake since January 26,
1700, which is known from Tsunami records in Japan. The megathrust fault on the subducting
Juan de Fuca (JdF) Plate is believed to be fully locked based on previous studies. We address
several open questions on the state of locking of the megathrust by long-term monitoring
experiments: nine short-period ocean bottom seismometers (OBS) were deployed combined with
six ocean bottom pressure (OBP) gauges and six broad-band seismometers (deployed in a small
sub-grid) allowing analyses of very low-frequency earthquakes and tremors. A set of eleven short-
period OBS were deployed north of the Nootka Fault zone separating the JdF and Explorer Plates.
The deeper structure of the subducting JdF Plate and the overlying North American plate was the
target of an amphibious Magnetotelluric (MT) experiment. Along a 2D profile from the
deformation front to the shelf off Vancouver Island, marine MT data were recorded for a month.
A landward extension of the profile across Vancouver Island was recorded by the University of
Alberta in the summer of 2022. These data are being jointly analyzed to identify and quantify
hydration and dehydration processes, which play a major role in controlling seismicity. In order to
image faulting and identify potential slip-to-toe events where the megathrust may have ruptured
all the way to the deformation front, we acquired seismic data across the fragmented deformation
front at the central northern Cascadia margin. Seismic data were also collected in the Winona
Basin (Explorer Plate) to address the extent of subduction off northern Vancouver Island. These
experiments were complemented by acquisition of heat-probe data measuring heat-flux. This will
improve the thermal model used to define the limits of the locked zone by estimating the 125°C
(upper limit) and 350°C (lower limit) isotherms. To study the recurrence rate of megathrust
earthquakes on the Explorer Plate, we collected twelve cores at four slope failures.
A secondary user project was added to SO294 to utilize the opportunity provided by the vessel
operating off the West Coast of Canada. This project is a collaborative study between GEOMAR
and Ocean Networks Canada (ONC) on potential CO2 storage in marine basalt complexes called
CASCADIA CO2 (CCO2). ONC leads a project that aims at CO2 capture from the atmosphere
and injection into oceanic crust in the Cascadia Basin, where boreholes from previous scientific
drilling campaigns provided prior information on the physical properties of the basalt formations
and overlying sediments. CCO2 studies the lateral variations of the basalt and its physical
properties by the acquisition of seismic data on OBS optimized for the measurement of S-waves,
a dataset yet missing in the assessments of the CO2 storage potential of basalt. In total, 22 OBS
were deployed along two perpendicular profiles receiving seismic signals from the G-Gun array.
Projects CLOCKS and CCO2 are accompanied by a mitigation program to protect marine
mammals and other endangered species. Eight observers conducted continuous visual observations
during daylight hours and 24h passive acoustic monitoring (PAM) during all seismic acquisition.
An Infra-Red camera system was tested as identification tool for the presence of marine mammals.

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