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Absolute Gravity Monitoring Efforts on Vancouver Island

Absolute Gravity Monitoring Efforts on Vancouver Island. J. Henton 1 , N. Courtier 2 , A. Lambert 2 , H. Dragert 2 , M. Schmidt 2 1 Geodetic Survey Division; 2 Geological Survey of Canada. Cascadia 2007 and Beyond Workshop Seattle, WA – March 2-3, 2009. AG Sites in Southwestern BC.

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Absolute Gravity Monitoring Efforts on Vancouver Island

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  1. Absolute Gravity Monitoring Efforts on Vancouver Island J. Henton 1, N. Courtier 2, A. Lambert 2, H. Dragert 2, M. Schmidt 2 1 Geodetic Survey Division; 2 Geological Survey of Canada Cascadia 2007 and Beyond Workshop Seattle, WA – March 2-3, 2009

  2. AG Sites in Southwestern BC (All/Various Sites)

  3. AG Monitoring on Vancouver Island (Regularly Visited – Including New Sites) Ucluelet PGC

  4. AG Variations at Ucluelet (1/2) (Long-term Monitoring) Lambert et al. – J. of Geodynamics (2006)

  5. AG Variations at Ucluelet (2/2) (Including 2004, 2005 & 2007 Episodes)

  6. AG Notes - ETS Monitoring at UCLU (2004, 2005 & 2007 Episodes) • The AG trend associated with the early-2007 event is not at present clear. Furthermore it cannot be directly compared to GPS results due to instrumental difficulties with the co-located GPS site. • AG monitoring of the 2005 ETS episode was suspended due to a scheduling conflict for FG5-106. • The AG signal coincident with the mid-2004 ETS event demonstrates a large negative gravity transient. The co-located GPS time series shows little if any vertical or horizontal displacement. However, the GPS time series from nearby Bamfield displays a moderate (~3mm) horizontal displacement associated with this ETS event (resulting in a local gradient in the displacement field). ● For the mid-2004 ETS event the AG decrease may imply that the strain field has a significant effect on the observed gravity signal.

  7. AG at PGC-V – 2008 ETS Episode (FG5-106 at PGC-Vault)

  8. “Continuous” AG Measurements (2008 Episode at PGC-V)

  9. Corrected AG Measurements (2008 Episode at PGC-V)

  10. AG Notes - ETS Monitoring at PGC-V (2008 Episode) The transient AG signal coincident with the mid-2008 ETS event at PGC is largely due to (or masked by) the drying of the near-surface soils. (Note that the AG at the PGC-Vault is measured below ground-level. Any decrease in water mass above the instrument will result in a observed increase in the gravity value).

  11. Future Gravity Efforts (ETS at Southern Vancouver Island) • We plan to target an area that has typically experienced large strains during past ETS episodes. Such areas often have a large vertical displacement measured by GPS (as well targeting a site/area with a smaller soil moisture signal). • The chosen location will require a co-located GPS site to determine a local Δg/Δh ratio. This ratio will greatly assist the understanding the fundamental physical processes involved within the observed gravity signal. • Additionally, it is hoped that gravity monitoring efforts can be augmented by employing precise relative (e.g., earth-tide) gravimeters.

  12. Future AG Efforts (ETS Monitoring) (Target High Strain Area on Southern Vancouver Island) ▪Courtesy of Kelin Wang (GSC-Pacific)

  13. Seasonal Soil Moisture Corrections (Estimated from Southern Vancouver Island AG Sites) Lambert et al. – J. of Geodynamics (2006)

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