Challenges in Achieving Height Modernization in AlaskaCrustal 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 Non-linear sea level trends in transition and subduction zones Linear sea level trends along strike-slip boundary 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 Yakutat Icefields: Peak Uplift Rate 3.5 cm/year Glacier Bay: Peak Uplift Rate 2.5 cm/year 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.
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/t)) (viscosity) • Afterslip log(1 + t/t) (frictional parameters) • Poroelastic relaxation maybe erf(t/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