1. Postseismic Deformation from the 1991 Racha, Georgia Earthquake May 16, 2006 Joel Podgorski Earth and Ocean Sciences University of British Columbia

2. Purpose of study To determine if and how postseismic deformation has occurred following 1991 earthquake in Racha, Georgia To determine if and how postseismic deformation has occurred following 1991 earthquake in Racha, Georgia This information will provide constraints on rheological properties of lithosphere in Arabia-Eurasia continental collision zone This information will provide constraints on rheological properties of lithosphere in Arabia-Eurasia continental collision zone

3. Outline Racha earthquake and similar earthquakes Racha earthquake and similar earthquakes Tectonic setting Tectonic setting GPS data GPS data Racha rupture area and GPS sitesRacha rupture area and GPS sites data acquisition and analysisdata acquisition and analysis velocity decay in datavelocity decay in data correcting data for secular displacementcorrecting data for secular displacement Earthquake afterslip Earthquake afterslip afterslip processafterslip process inverse modelinginverse modeling modeling resultsmodeling results Viscoelastic relaxation Viscoelastic relaxation viscoelasticity and modelingviscoelasticity and modeling modeling resultsmodeling results Summary Summary

4. Racha earthquake and similar earthquakes Racha earthquake, Georgia (April 29, 1991) Racha earthquake, Georgia (April 29, 1991) M w =6.9 (same as Loma Prieta)M w =6.9 (same as Loma Prieta) Dip slip/thrust mechanismDip slip/thrust mechanism 30˚ dip, hypocenter 6 km depth30˚ dip, hypocenter 6 km depth Similar thrust events: Similar thrust events: Chi-Chi, Taiwan (1999), M w =7.6, hypocenter depth 8 km, dip 30˚Chi-Chi, Taiwan (1999), M w =7.6, hypocenter depth 8 km, dip 30˚ Northridge, California (1994), M w =6.7, hypocenter depth 20 km, dip 45˚Northridge, California (1994), M w =6.7, hypocenter depth 20 km, dip 45˚ Loma Prieta, California (1989), M w =6.9, hypocenter depth 16 km, dip 70˚Loma Prieta, California (1989), M w =6.9, hypocenter depth 16 km, dip 70˚ UP DIP DOWN DIP

5. Setting Greater Caucasus: 1000 km long, ~5000 m high 1000 km long, ~5000 m high middle of Alpine Himalayan fold belt middle of Alpine Himalayan fold belt uplift began 3.5 Ma after collision of Arabia uplift began 3.5 Ma after collision of Arabia Greater Caucasus 25 MM/YR ARABIA

6. Racha rupture area and GPS sites Racha Epicenter Afterslip Plane GPS Sites

7. GPS data acquisition and analysis Data are from 8 sites for some or all of time points: 1991.56 (~3 months after earthquake), 1994.77, 1996.74, 1998.71, 2000.76 Data are from 8 sites for some or all of time points: 1991.56 (~3 months after earthquake), 1994.77, 1996.74, 1998.71, 2000.76 Field work by collaborators at MIT Field work by collaborators at MIT Data analyzed at MIT with GAMIT/GLOBK software: Data analyzed at MIT with GAMIT/GLOBK software: Estimate site coordinates, satellite orbital parameters, atmospheric delay corrections, and earth orientation parametersEstimate site coordinates, satellite orbital parameters, atmospheric delay corrections, and earth orientation parameters Combine parameter estimates and covariances and apply position and velocity constraints from global core sitesCombine parameter estimates and covariances and apply position and velocity constraints from global core sites 50% precision improvement 1991-1994 due to increased satellite coverage 50% precision improvement 1991-1994 due to increased satellite coverage

8. Logarithmic versus linear fit to data Logarithmic fit indicative of afterslip Logarithmic fit indicative of afterslip Residuals to linear fit and log fit compared Residuals to linear fit and log fit compared Sites showing better fit with logarithmic curve (misfit halved or better): Sites showing better fit with logarithmic curve (misfit halved or better): LESO (N,E,Up)LESO (N,E,Up) KHUR (N)KHUR (N) SACC (E, Up)SACC (E, Up)

9. Data correction for secular displacement: two options Subtract 1996-2000 velocities Subtract 1996-2000 velocities Assumption: postseismic deformation finished by 1996 – likely true if no viscoelastic deformationAssumption: postseismic deformation finished by 1996 – likely true if no viscoelastic deformation Advantage: values based on actual measurementsAdvantage: values based on actual measurements Subtract predicted MIT block model velocities Subtract predicted MIT block model velocities Based on 1988-2005 GPS measurements to fit large-scale block model for eastern Mediterranean regionBased on 1988-2005 GPS measurements to fit large-scale block model for eastern Mediterranean region Advantage: data from entire earthquake cycleAdvantage: data from entire earthquake cycle Disadvantage: rough fit, not meant for smaller scaleDisadvantage: rough fit, not meant for smaller scale

10. Earthquake afterslip Motion on a fault after an earthquake due to stresses induced by the earthquake Motion on a fault after an earthquake due to stresses induced by the earthquake Happens on coseismic fault; sometimes also on adjacent faults Happens on coseismic fault; sometimes also on adjacent faults Usually occurs where coseismic slip was at a minimum Usually occurs where coseismic slip was at a minimum Occurs at shallow depths in zone of velocity strengthening Occurs at shallow depths in zone of velocity strengthening Begins immediately after earthquake and can last for several years – modeled as logarithmic decay Begins immediately after earthquake and can last for several years – modeled as logarithmic decay Examples of afterslip Examples of afterslip Chi-Chi: 16% of coseismic moment in 15 monthsChi-Chi: 16% of coseismic moment in 15 months Northridge: 22% of coseismic moment in 2 yearsNorthridge: 22% of coseismic moment in 2 years Loma Prieta: 10% of coseismic moment in 5 yearsLoma Prieta: 10% of coseismic moment in 5 years

11. Afterslip inverse modeling Kinematic model inverting for fault displacement in an elastic half-space using displacements on Earth's surface Kinematic model inverting for fault displacement in an elastic half-space using displacements on Earth's surface Green‘s functions, G, are calculated on each fault tile to relate slip to each GPS displacement Green‘s functions, G, are calculated on each fault tile to relate slip to each GPS displacement Smoothing parameter, ß, facilitates trade-off between best fit to data and a smoothly varying solution by minimizing: Smoothing parameter, ß, facilitates trade-off between best fit to data and a smoothly varying solution by minimizing: ||W(Gs-d)|| 2 + ß 2 ||Ls|| 2 ||W(Gs-d)|| 2 + ß 2 ||Ls|| 2 (misfit) (roughness) (misfit) (roughness) s i is slip on each fault tile d j is displacement at each GPS site L is the Laplacian operator W T W = ∑ -1 (data covariance matrix)

12. Fault plane resolution tests  Forward modeled checkerboard slip distribution to GPS sites  Inverted for slip using forward modeled displacement vectors Modeled Afterslip Plane SLIP INPUT INVERSION OUTPUT Racha Hypocenter SURFACE DEPTH 40 KM 120 KM

13. Afterslip 1991-1994 Subtracting 1996-2000 Subtracting 1996-2000 90% error reduction90% error reduction 35% coseismic moment35% coseismic moment max slip: 35 cm Subtracting block model Subtracting block model 66% error reduction66% error reduction 28% coseismic moment28% coseismic moment max slip: 45 cm SURFACE DOWN DIP

14. 1991-1994 afterslip with coseimic slip distributions Using data subtracting 1996-2000 velocities Using data subtracting block model Coseismic model DOWN DIP SURFACE

15. Forward-modeled afterslip versus data Red: data with 1-σ errors (67% confidence interval) Blue: model Subtracting 1996-2000 velocities Subtracting 1996-2000 velocities Subtracting block model Subtracting block model

16. Afterslip with seismicity Afterslip plotted with first 2 months of aftershocks Afterslip plotted with first 2 months of aftershocks  Afterslip is from 3 months to 3 years after earthquake “shallow” aftershocks beneath 1500m high Racha Ridge

17. Afterslip 1994-1996 Subtract 1996-2000 Subtract 1996-2000 22% error reduction max slip: 18 cm Afterslip was likely not occurring after 1994 Subtract block model Subtract block model 27% error reduction

18. Viscoelasticity Viscoelasticity: elastic on short time scale, viscous on long time scale (e.g. Earth's mantle) Viscoelasticity: elastic on short time scale, viscous on long time scale (e.g. Earth's mantle) Indication in lower lithosphere: seismic attenuation and high heat flow Indication in lower lithosphere: seismic attenuation and high heat flow Maxwell viscoelastic material explained by: Maxwell viscoelastic material explained by: dε/dt = σ/2η + dσ/Edt from which: σ = σ 0 exp(-Et/2η) (stress decay at 0 strain rate) where: σ-stress, ε-strain, η-viscosity, E-Young's modulus

19. Viscoelastic modeling Basis: P and S attenuation in lower crust of Caucasus Basis: P and S attenuation in lower crust of Caucasus Used code to forward model response of coseismic slip using different viscosities and layer thicknesses Used code to forward model response of coseismic slip using different viscosities and layer thicknesses Best model: Best model: 4e+17 Pa s for bottom4e+17 Pa s for bottom 20 km of crust Only 21% error reductionOnly 21% error reduction Viscoelastic relaxation not responsible for early postseismic deformation Viscoelastic relaxation not responsible for early postseismic deformation

20. Summary Eight GPS sites produced sparse time series of nine-year postseismic period of 1991 Racha earthquake Eight GPS sites produced sparse time series of nine-year postseismic period of 1991 Racha earthquake Logarithmic decay in position measurements from three sites indicate postseismic deformation Logarithmic decay in position measurements from three sites indicate postseismic deformation Two methods of correcting data for secular motions produced similar afterslip inversions over 1991-1994: Two methods of correcting data for secular motions produced similar afterslip inversions over 1991-1994: shallow aseismic afterslipshallow aseismic afterslip 65-90% error reduction65-90% error reduction ~30% of coseismic moment~30% of coseismic moment No evidence for afterslip post-1994 No evidence for afterslip post-1994 No evidence for viscoelastic relaxation in 1991-1994 No evidence for viscoelastic relaxation in 1991-1994 Afterslip dominated Racha deformation as anticipated by studies of similar earthquakes Afterslip dominated Racha deformation as anticipated by studies of similar earthquakes A viscoelastic layer may be in the lower lithosphere, but Racha event not strong enough to activate it A viscoelastic layer may be in the lower lithosphere, but Racha event not strong enough to activate it

21. Thank You

23. Velocity calculation from raw data Displacements found by differencing measurements relative to station SACC located near fault on footwall Displacements found by differencing measurements relative to station SACC located near fault on footwall Secular displacements corrected by subtracting velocities from the 1996- 2000 time period Secular displacements corrected by subtracting velocities from the 1996- 2000 time period Errors found by summing squares of errors used in differencing measurements Errors found by summing squares of errors used in differencing measurements

24. Fault afterslip Inverse modeling in an elastic half-space with Poisson‘s ratio of 0.25 Inverse modeling in an elastic half-space with Poisson‘s ratio of 0.25 Green‘s functions, G, are calculated on each fault slip tile Green‘s functions, G, are calculated on each fault slip tile Roughness, L, of inversion result is minimized by applying smoothing factor, ß, and minimizing weighted residual sum of squares (WRSS): Roughness, L, of inversion result is minimized by applying smoothing factor, ß, and minimizing weighted residual sum of squares (WRSS): ||W(Gs-d)|| 2 + ß 2 ||Ls|| 2 ||W(Gs-d)|| 2 + ß 2 ||Ls|| 2where: s i is slip on each fault tile d j is displacement at each GPS site W T W = ∑ -1 (data covariance matrix)

25. Determine smoothing Choose smoothing where curvature of misfit vs. roughness graph is greatest Choose smoothing where curvature of misfit vs. roughness graph is greatest

26. Afterslip fault planes & aftershocks

27. Afterslip inversion result Smoothing = 5.8 EQslip Smoothing = 10.0

28. Comparison of inversion and data (BLUE: model, RED: data) All stations All stations Close up near fault Close up near fault

29. Viscoelastic relaxation code Code forward models response of earth to earthquake stresses Code forward models response of earth to earthquake stresses Uses spherical layers with variable density, bulk modulus, shear modulus, and viscosity Uses spherical layers with variable density, bulk modulus, shear modulus, and viscosity Calculates spherical harmonic expansion of spheroidal and toroidal motion components Calculates spherical harmonic expansion of spheroidal and toroidal motion components

30. Viscoelastic relaxation vs. fault afterslip Viscosity = 10 17 Pa s (below 16km)

31. Things to do… Confirm that investigating only 1991- 1994 is adequate Confirm that investigating only 1991- 1994 is adequate Do more modeling of viscoelastic relaxation to gain clearer picture of what viscosity is needed to fit data Do more modeling of viscoelastic relaxation to gain clearer picture of what viscosity is needed to fit data Perhaps try viscoelastic modeling of 1994-1996 deformation Perhaps try viscoelastic modeling of 1994-1996 deformation