Using absolute gravity measurements to augment deformation studies in western Canada:

The heavier (+ “just-right”) and lighter side of long-term and transient deformation monitoring

Natural Resources Canada

Outline of Topics

(Brief) Introduction to AG Operators in Canada

Deformation Monitoring in Western Canada

Long-Term Interseismic Deformation (“heavier”)

[Mid-continental GIA Profile (“just-right”)]

Transient Deformation Monitoring (“lighter”)

Organizational “Homes” of AG in NRCan

Natural Resources Canada (NRCan) Earth Sciences Sector (ESS)

… Surveyor General Branch (SGB)

Canadian Geodetic Survey (CGS) … Geological Survey of Canada (AWCB)

Geological Survey of Canada – Pacific (GSC-P) …

Absolute and Continuous Gravimeters in Canada Absolute Gravimeters

FG5-236: NRCan/CGS (Field Operation) FG5-106: NRCan/GSC-P (Field Operations) A10-003: NRCan/CGS (Field Operations) FG5-105: NRC (Watt Balance) A10-024: Tecterra & University of Calgary

Superconducting Gravimeters Cantley (CAGS): CSGI university consortium & NRCan/CGS

[observing since November 1989 (no activities 1994-1996)]

iGrav-002: Tecterra & University of Calgary[observing since July 2012 near Victoria, BC]

Tidal Gravimeter ET-12: NRCan (GSC-P Vault)

Absolute Gravimetry in CGS and NRCan

Canadian Spatial Reference System objectives, including: Datum support for gravity surveys Support time evolution of vertical component of geometric

RF Maintenance of a new gravity-based/geoid height reference

system (e.g. direct measurement of g-dot/h-dot ratio to provide simplified connection for corresponding reference standards)

Scientific applications/priorities (with NRCan partners): Subduction/earthquake zone deformation studies Sea-level rise studies Hydrological/ground-water mass monitoring Post-glacial rebound studies Monitoring studies of carbon capture & storage

Absolute Gravimetry Field Operations by CGS

FG5-236

Survey Tent

Vertical gradient of

gravity

A10-003

Canadian Absolute Gravity Site (CAGS)

Fundamental Absolute Gravity Station in Canada

• Superconducting gravimeter • Canadian Active Control Station (GPS)• Multiple absolute gravity stations• Weather station• Two wells• Bedrock site (established 1987)• Possible North American Inter-Comparison site (4 inst. max.)

CAGS

TMGO

Superconducting Gravimeter at CAGS

National Absolute Gravity Array (CGS)

GPS Observed Vertical Rates

Long-Term Deformation Monitoring (Multiple year measurements) Comparison of absolute gravity (AG) and GPS results of

interseismic deformation along the northern Cascadia Subduction Zone (CSZ)

[Comparison of absolute gravity (AG) and GPS results of glacial isostatic adjustment in the mid-continental region]

Transient Deformation Monitoring (Measurements over weeks to months) Comparison of AG and GPS results of Episodic Tremor &

Slip (ETS) transient deformation along the forearc of the northern CSZ

Possible Linkage of Residual Gravity Change to Strain Change & Closing Comments

Outline of Western Canada AG Operations

Interseismic and Episodic Tremor & Slip Zones

[Image courtesy of Herb Dragert (NRCan)]

Western Canada AG & GPS Sites (Long-Term)

FG5-106 “Homebase”

▪ GPS Sites▪ Seismometers▪ Continuous Soil Moisture▪ PBO Borehole Strainmeters & Pore Pressure▪ Tide Gauge (DFO)▪ Met (EC & Others)▪ Cabled Ocean-Floor Sensors▪ Gravimeters (ET-12 & iGrav – UofC)

FG5-106 operates from the Pacific Geoscience Centre (PGC) near Victoria, BC. Ancillary data are available from:

Summary of AG Processing (Western Canada)

Summary of GPS Processing (Western Canada)

AG & GPS Comparison – Western Canada (I)

Preferred Solution (IGS05 Realization from Orbits & Clocks)

Regional Positive Gravity Bias (Vancouver Island Sites)

AG & GPS Comparison – Western Canada (II)

Alternate Solution (ITRF2008 through 7-Parameter Rate Transformation)

(Smaller) Regional Positive Gravity Bias (Vancouver Island Sites)

Residual Gravity Rates – Western Canada AG Sites

Residual gravity rates are the gravity rate offset from the predicted DGG (-0.2 µGal/mm).Maximum horizontal interseismic strain rates are predicted by an elastic dislocation model

at AG site.

We observe a tentative correlation between positive long-term gravity residual rates and interseismic strain rates.

‘Longer’ Term Gravity Differences (SVI)

[Modified from Wolynec (2004)]

Observations are projected onto a cross-island line from the deformation front to the Strait of Georgia (‘GS’): a profile similar to that from Ucluelet to Nanoose.

Gravity differences between surveys are not corrected for observed or modelled uplift.

The increase in gravity near the outer coast (‘Shelf’) is contrary to observed uplift and thus indicates a positive gravity residual rate.

AG & GPS Comparison – “Away from the West Coast”

Recent examinations [e.g., Mazzotti et al., 2011] of yearly AG and continuous vertical GPS time series at seven collocated (or nearby) sites in the mid-continental region of North America yielded a gravity-to-uplift ratio consistent with model predictions of postglacial rebound.

The relationship between AG and GPS uplift rates is “just-right” using the same analysis methodologies as performed for the west coast.

Mid-continent AG & GPS Sites

AG & GPS Comparison - Mid-continent Region

Observations of Episodic Tremor & Slip (ETS)

[Image courtesy of Herb Dragert (NRCan)]

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10 E

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Regional Observations of Episodic Tremor & Slip

[Image courtesy of Herb Dragert (NRCan)]

Tremor Activity - 2010 Regional ETS Event

Image Source:http://pnsn.org/tremor/

Port Renfrew

Continuous GPS Monitoring - PTRF

GPS Horizontal Observations at Port Renfrew (2010)

GPS Vertical Observations at Port Renfrew (2010)

ETS window defined by regional tremor, horizontal GPS and borehole strain-meter observations.

AG Observations at Port Renfrew (2010)

Near-surface mass from precipitation may (slightly) mask the magnitude of the full transient-related gravity decrease during the 2010 ETS event (i.e., ‘tectonic’ Δg may tend towards being larger).

Seasonal variability in water mass storage at Port Renfrew is expected to be small (although more gravity data is required to quantify the effect).

Predicted (Thrust Fault): Δg/Δh = -0.20 μGal/mm

[e.g., Rundle, 1978; Walsh & Rice, 1979]

Observed (2010 ETS): Δg/Δh = -0.30 μGal/mm[mean free-air effect: -0.3086 µGal/mm]

Although the uncertainty on this preliminary DGG is on the order of 50%, it initially appears to be more consistent with a free-air effect alone. This may imply height change without any associated net change in mass (from thickening and/or density changes) for the region.

OR, mass may be regionally redistributed during the ETS event(s).

Expected ‘deformation’ gravity change (for Δh = 6.87 mm):

ΔgDEF = -1.37 µGal

The residual gravity (ΔgOBS - ΔgDEF): ΔgRES = -0.68 µGal[equivalent loss of -1.62 cmWATER (i.e., thickness of an infinite sheet of water)]

Deformation Gravity Gradient (DGG) - Implications

Residual Gravity (Block Density)

ΔgRES = -0.68 µGal → Δρ = -3.62 x 10-6 g/cm

(density change: -1.4 ppm)

Elastic Model:Toda et al. (2011)

Single Thrust Fault:Tapered dip slip

Young Modulus:800000 bar

Poisson’s Ratio:0.25

Frictional Coefficient:

0.4

Origin at Port Renfrew;Adjusted to GPS

Observations at PTRF

Mass Redistribution – Elastic Model of Density Change

Model Profile and Depth

Using uppermost 10 km for modelling

Shallower zone has higher ‘weighting’

Rheology, crustal seismicity (etc) arguments

Predicted Gravity Change from Elastic Model

Gravity effect modelled by prisms (2 km x 2 km) of infinite strike length (e.g., Telford et al., 1976).

Modelled strain-related gravity decrease (but rather small).

Port Renfrew is located in a region where relatively subtle changes in the fault geometry and kinematics may significantly change the estimate of net gravity change.

Location above updip limit of fault Different tapers of slip distribution 3-D geometry not incorporated (yet)

Fluid mobility (and/or poroelastic effects) not considered.

Model Notes - Gravity Change (2010 at PTRF)

Borehole Strainmeter (PBO/UNAVCO)

(B004 is across the straight of Juan de Fuca on the Olympic Peninsula.)Continuous measurements of gravity in an area of high-strain would be beneficial (but logistically

difficult)!

Earlier ETS Monitoring at Ucluelet with AG

Possible large gravity decreases at a site that experiences little vertical (or hydrologic) change during ETS events.

However, this region is may exhibit more complexity with respect to ETS (i.e., close to ‘edges’ of two ‘zones’).

“Continuous” AG Observations at PGC (2008)

2008 – Inconclusive (Likely No Significant Change)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.

2011 – No Significant Change ObservedThe 2011 northern Cascadia ETS event arrived “earlier than expected”; 2011 monitoring efforts had been scheduled for Port Renfrew.

2012 – Efforts Enhanced by Superconducting GravimeterThe University of Calgary (JW Kim) has deployed an iGrav SG to the Vancouver Island (study in progress). The 2011 northern Cascadia ETS event also arrived “earlier than expected” (conflicting with the other field operations by FG5-106; 2012 monitoring efforts had also been scheduled for Port Renfrew).

AG Notes - ETS Monitoring at PGC (Sidney)

Closing Comments (“The Three Bears”)

— “HEAVIER” DEFORMATION — We have observed a tentative correlation between positive long-

term gravity residual rates and interseismic strain rates, possibly related to deformation of the forearc crust (density increase from the closing of cracks?) due to the interseismic loading of the subduction fault.

— “LIGHTER” DEFORMATION — For ETS we will further focus on studies of AG transient records

in areas that experience large strains (and/or large strain gradients). In the near-shore portion of northern Cascadia, the peak near-surface transient strains are more likely to be dilatational – and the residual gravity changes may then be negative.

— “JUST RIGHT” DEFORMATION — For GIA in the mid-contintent, GPS uplift, AG trands, and model

predictions are consistent.