Absolute Gravity Monitoring Efforts on Vancouver Island

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

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AG at PGC-V 2008 ETS Episode
(FG5-106 at PGC-Vault)
8
Continuous AG Measurements
(2008 Episode at PGC-V)
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Corrected AG Measurements
(2008 Episode at PGC-V)
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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).
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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 ?g/?h 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)