# Presentation: 180492

## Challenges in Achieving Height Modernization in AlaskaCrustal Deformation Has Invalidated Much of the Historical Data 

**Jeff Freymueller**
- Geophysical Institute, University of Alaska Fairbanks

## Height Accuracy Reaches New Low

- Terrestrial height network (leveling) is significantly compromised
- Last systematic leveling in southern Alaska in 1964-1965, immediately after 1964 Prince William Sound earthquake
- There has been up to 1.25 METERS of vertical crustal motion since then.
- Substantial areas of >20-30 cm deformation since last survey
- Geoid height errors in Alaska can be very large, as is well-known at NGS
- GRACE geoid compared to EGM-96 showed up to 1 meter geoid height errors
- CONCLUSION: Alaska needs an essentially new vertical reference network – just like starting over

## Main Sources of Vertical Motion

- Glacial-Isostatic Adjustment (“post-glacial rebound”) up to 3.5 cm per year
- 1 meter change every 30-50 years
- But signal is strongest in parts of Alaska that had minimal height information aside from tide level
- Postseismic Deformation Following the 1964 earthquake up to 1.25 meters since 1964
- Other faulting-related deformation much smaller (several mm/yr)
- Deformation assoicated with 2002 Denali fault earthquake substantial but localized
- Rarely more than 10 cm vertical change away from fault
- Postseismic changes continue, may reach 10 cm level

## Post-1964 Postseismic Uplift

- 1964 to present
- Units: cm

## Post-1964 Postseismic Uplift

- Estimate based on GPS surveys of leveling BMs
- Correction for geoid-ellipsoid separation is by far the largest source of error
- (Relative) geoid heights used, error may approach 20 cm
- Strong gradient near Anchorage (100 cm mid-Turnagain Arm, 30 cm at Port of Anchorage tide gauge) could include a component of geoid error.

- Data: Cohen, S. C., and J. T. Freymueller, Crustal Deformation in Southcentral Alaska: The 1964 Prince William Sound Earthquake Subduction Zone, *Advances in Geophysics*, 47, 1-63, 2004.

## Tide Gauges

**Linear** sea level trends along strike-slip boundary

**Non-linear** sea level trends in transition and subduction zones

- Tectonic influence on long term uplift records is strongest following the 1964 earthquake

- Larsen, C. F., R. Motyka, J. Freymueller, and K. Echelmeyer, Tide gauge records of uplift along the northern Pacific-North American plate boundary, 1937 to 2001, *J. Geophys. Res., 108*(B4), doi:10.1029/2001JB001685, 2003.

## Uplift Rates

- 2 mm/yr contour interval

- Larsen, C. F., R. J. Motyka, J. T. Freymueller, K. A. Echelmeyer and E. R. Ivins, Rapid uplift of southern Alaska caused by recent ice loss, *Geophys. J. Intl.*, 158, 1118-1133, 2004.

- Yakutat Icefields:
- Peak Uplift Rate 3.5 cm/year

- Glacier Bay:
- Peak Uplift Rate 2.5 cm/year

## What Needs to be Done

- Existing vertical datum is in error by >30 cm in much of southern Alaska, as much as 1 m in places.
- Because of crustal movement since surveys that defined it.
- May be difficult to even maintain a consistent definition with past
- Repeating all the leveling effectively impossible
- All southern coastal tide gauges have moved >30 cm since surveys used to define NAVD88
- Need new definition of vertical datum based on GPS
- CORS plus some number of monuments in the ground
- Need new geoid model
- If Arctic Ocean can have a 5-10 cm geoid, why should (populated) Alaska have to settle for meter-level errors?
- It’s almost like starting from scratch.
- Might be better to start over with a new system based on ITRF. The Lower 48 will have to go that route some day.

## What is Postseismic Deformation?

- Transient deformation triggered by an earthquake
- Afterslip on the fault zone
- Viscoelastic relaxation of the mantle or lower crust
- Poroelastic deformation associated with earthquake-driven fluid flow (changes elastic constants)

## Characteristic Relaxation

- Postseismic deformation rate decays with time
- Viscoelastic relaxation (1-exp(-t/)) (*viscosity*)
- Afterslip log(1 + t/) (*frictional parameters*)
- Poroelastic relaxation maybe erf(t/) (*hydraulic diffusivity*)
**In general, all of these will be active and multiple timescales should be expected**

## Postseismic Uplift Rate Was Very Fast Right After Quake

- Uplift rate declined with time but still peaks at 1 cm/year