|Title||Report on Cruise 2010007PGC, C.C.G. Vessel John P. Tully, 30 June - 10 July 2010, SeaJade-I Seafloor Earthquake Array - Japan Canada Cascadia Experiment, Ocean bottom seismometer recovery, methane
gas-plume acoustic imaging, and CTD-water sampling program|
|Author||Riedel, M; Côté, M M; Neelands, P J; Obana, K; Wania, R; Price, A; Taylor, S|
|Source||Geological Survey of Canada, Open File 7557, 2014, 85 pages, https://doi.org/10.4095/295545 (Open Access)|
|Publisher||Natural Resources Canada|
|Related||This publication is related to the following
|Province||Western offshore region|
|Area||Pacific Ocean; Vancouver Island|
|Lat/Long WENS||-128.5000 -126.0000 49.6667 44.9167|
|Subjects||geophysics; geophysical surveys; earthquakes; earthquake studies; earthquake magnitudes; epicentres; aftershocks; seismic interpretations; seismicity; seismographs; seismological network; seismology;
seismic waves; gas; gas bubbles; acoustic surveys; acoustic surveys, marine; water analyses|
|Illustrations||location maps; tables; photographs; profiles; logs|
|Program||Public Safety Geoscience,
Western Canada Geohazards Project|
|Released||2014 12 04|
This expedition 2010007 is the second cruise of a major international collaboration of NRCan ("Public Safety Geoscience" (PSG) Program) with Japan (JAMSTEC) and
the Woods Hole Oceanographic Institution (WHOI). The main objective of this collaboration is to acquire passive seismic data using Ocean Bottom Seismometers (OBS) for earthquake studies. The OBS monitoring will provide direct information on seismic
activity beneath the continental slope and further offshore which cannot be provided by land-based seismic networks and therefore will fill a critical knowledge gap. The filling of this knowledge gap is essential to the assessment of earthquake
hazard due to the Cascadia subduction fault, an important output of the "Targeted Hazard Assessment in Western Canada" project under the PSG program.
In total, 33 OBSs were deployed during the cruise 2010005-PGC (SeaJade-I) from June 30 to July
10, 2010. The objective of cruise 2010007 was to collect back these OBS.
Secondary objectives of the expedition included acoustic imaging of known methane vent sites (plumes). This imaging was combined with conducting casts using a
Conductivity-Temperature-Depth (CTD) measurement-unit attached to a 24-Niskin-bottle rosette for water-sampling. The water-sampling was conducted to measure the concentration of dissolved methane in the water. The analyses of the water samples are
carried out in collaboration with Prof. Dr. Michael Whiticar from the School of Earth and Ocean Sciences, University of Victoria.
1.2 Accomplishments and Results
Despite extreme weather conditions due to a storm at the beginning of the cruise
forcing a major shutdown of all operations for 60 hours (2.5 days) from Friday evening, September 24th to Monday morning, September 27th the OBS were all, except one, recovered. OBS Station 11 did not respond to the acoustic release commands, and the
station was abandoned after 4 hours of trials to recover the system.
All 32 OBS were physically (from the outside) in good shape, all transponders and light-strobes worked making the identification of floating instrument and recovery relatively
simple, despite, in part, heavy seas. All OBS were brought back to the Pacific Geoscience Centre (PGC), where the glass spheres were opened and all data recovered. Copies of the data set were distributed to the different scientific parties involved.
Due to the weather interruption, only 3.5 days remained in the program to complete the acoustic imaging and water-sampling program. In total, 84 lines were acquired over seven (7) plume locations (2 grids at Bullseye Vent, 2 grids at Spinnaker
Vent, 2 grids at Amnesiac Vent, and 2 cross-lines at Barkley Canyon). Out of these lines seven (7) locations for CTD casts and water-sampling were chosen. Onboard analyses of some samples showed overall very small amounts of dissolved methane (not
exceeding 20 n-moles/l). However, sufficient volumes of gas were collected allowing for post-cruise analyses including isotope measurements at all seven vent locations.
|Summary||(Plain Language Summary, not published)|
This Geological Survey of Canada Open File summarizes the activities undertaken during the retrieval phase of the SeaJade expedition (Seafloor Earthquake
Array Japan Cascadia Experiment). A summary of the deployment phase can be found in Open File 7558. The main objective of this expedition was to retrieve the 35 Ocean Bottom Seismometers (OBS), which were deployed 3 months earlier to monitor
earthquake activity offshore Vancouver Is. The OBS monitoring provides direct information on seismic activity beneath the continental slope and further offshore which cannot be provided by land-based seismic networks. The filling of this knowledge
gap is essential to the assessment of earthquake hazard due to the Cascadia subduction fault. Secondary objectives of the expedition include acoustic imaging of known methane vent sites and conducting water-sampling. The water-sampling was conducted
to measure the concentration of dissolved methane in the water above the vent sites.