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 (All/Various Sites)

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

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

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

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.

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

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

Corrected AG Measurements (2008 Episode at PGC-V)

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).

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.

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

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