Challenges in Achieving Height Modernization in Alaska Crustal
- Slides: 11
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 geoidellipsoid separation is by far the largest source of error – (Relative) geoid heights used, error may approach 20 cm 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. • Strong gradient near Anchorage (100 cm mid. Turnagain Arm, 30 cm at Port of Anchorage tide gauge) could include a component of geoid error.
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(B 4), doi: 10. 1029/2001 JB 001685, 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 NAVD 88 • 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 earthquakedriven 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