1. Probabilistic Ground Motions for Scoggins Dam, Oregon Chris Wood Seismotectonics & Geophysics Group Technical Service Center July 2012

2. Scoggins Dam 45 km east of locked part of CSZ interface (red) –M 9.0 megathrust earthquakes 60 km above slab (blue, green) –M 7.5 deep earthquakes

3. Reclamation Dam Safety Guidelines Use risk-based approach for decision making that evaluates the following risks: –annual probability of failure –annualized loss of life Consider a wide range of loading conditions to determine risk Probabilistic seismic loadings are needed for input into engineering analyses of how dams respond to seismic loadings –Specifically, we need scenario ground motion time histories for a wide range of return periods

4. How probabilistic ground motions were developed for Scoggins Dam Perform a Probabilistic Seismic Hazard Analysis (PSHA) to get hazard curves, uniform hazard spectra (UHS), and disaggregation of hazard Determine response spectra for scenario earthquakes using the Conditional Mean Spectrum (CMS) method Select initial time histories that are representative of the main contributors to the hazard –from historical records (e.g., M 9.0 Tohoku, Japan) –from Empirical Green’s Function (EGF) models Spectrally match the selected initial time histories to the scenario earthquake response spectra

5. Seismic sources Subduction zone interface Intra-slab (deep) earthquakes Crustal: mapped faults and shallow background seismicity Source: USGS

6. PSHA Probabilistic seismic hazard analyses (PSHA) conducted by R. LaForge, Fugro Wm. Lettis & Assoc. (FWLA) for Reclamation, 2006, 2009, 2011 Considered commonly recognized earthquake sources (CSZ interface, intraslab, and crustal)

7. Cascadia (Interface), Intraslab & Crustal seismic sources Earthquake Sources used for PSHA CZS Interface Puget intraslab Portland intraslab Crustal faults

8. PHA Hazard 10,000-yr PHA = 1.4 g Mean hazard contributors (for return periods of interest): 1)Cascadia SZ Interface 2)Portland Intraslab 3)Crustal sources (Gales Creek fault)

9. Contributions by source – PHA

10. 0.75-sec. SA Hazard – All Sources 10,000-yr SA = 1.7 g Mean hazard contributors (for return periods of interest): 1)Cascadia SZ Interface 2)Crustal sources (Gales Creek fault) 3) Portland Intraslab

11. Contributions by source – 0.75-sec. SA

12. Computed hazard strongly depends on subjective GMPE weights

13. Ground Motion Prediction Equations (GMPEs) and Weighting Cascadia Subduction Zone Interface –Atkinson & Boore, 2003 (worldwide): 0.1 (predicts lowest ground motions for R < 150 km) –Atkinson & Macias, 2009: 0.1 –Youngs et al, 1997: 0.3 –Zhao et al, 2006: 0.5 (predicts highest ground motions for R < 150 km) Cascadia Subduction Zone Intraslab –Atkinson & Boore, 2003 (worldwide): 0.17 –Youngs et al, 1997: 0.0 –Zhao et al, 2006: 0.83 Shallow Crustal Sources –Abrahamson & Silva, 2008 (NGA): 0.25 –Boore & Atkinson, 2008 (NGA): 0.25 –Campbell & Bozorgnia, 2008 (NGA): 0.25 –Chiou & Youngs, 2008 (NGA): 0.25 Note: Same GMPEs and weights used both for PSHA and for development of scenario response spectra

14. Median CSZ interface deterministic spectra ZEA06 gives much larger estimated accelerations

15. Comparison of M 9.0 Tohoku, Japan EQ observatio ns with GMPE predictions

16. Scenario Response Spectra Development Scenario spectra were developed using the Conditional Mean Spectrum (CMS) method developed by Baker & Cornell, 2006 Requires specification of one or more critical spectral response periods If the critical periods of a structure are poorly known, may require target spectra from numerous trial periods to fully envelope total hazard defined by UHS Provides a more realistic response spectrum for a single earthquake than the UHS For Scoggins, simplified to long- and short-period scenario spectra –Results in larger ground motions, but fewer time histories to analyze Scenario spectra constraints –Scenario spectra for a specified return period envelope the UHS –No scenario spectrum is allowed to exceed the UHS

17. Uniform Hazard Spectra (UHS)

18. Development of CMS Spectra

19. CMS – CSZ Interface, 10k, T c =0.75 sec.

20. CMS – Crustal sources, 10k, T c =0.75 sec.

21. Time History Development Obtain initial time histories for scenario earthquakes from historical or synthetic records with magnitudes, distances and characteristics representative of the Cascadia interface, crustal, and intraslab sources Use a wavelet-based spectral matching method to modify the initial time histories so that the response spectrum of the final time history matches that of the scenario spectrum Match spectra for 500, 1,000, 5,000, 10,000, and 50,000 year return periods

22. CSZ interface source characteristics (M, R Rupt,ε) for PHA, 10k, YEA97 Largest contribution from M 9.0, and R Rupt 45-50 km Similar results for other return periods, response spectral periods, and GMPEs

23. Cascadia interface initial time histories Historical records from 3/11/2011 M 9.0 Tohoku, Japan earthquake Synthetic M 8 to 9 Cascadia interface earthquake records based on Empirical Green’s function (EGF) method.

24. HKD070 EGF R = 104 km M 6.4 aftershock of 2003 M 8.0 Tokachi-oki, Japan earthquake Soil site HKD070 EGF

25. Simulation using EGF with R = 100 km Rupture length is 600 km Hypocenter in center HKD070 (soil) Duration ~ 130 sec

26. Spectrally-matched 5k time history for CSZ interface earthquake source Initial time history: M 9.0 EGF simulation using HKD070 EGF Target spectrum: 5k interface, CMS PHA = 0.56 g Duration = 180 sec I A = 30.2 m/s Spectral Match

27. Comparison of target and matched response spectra Target and matched response spectra, 5k CSZ interface from M 9.0 EGF simulation using HKD070 EGF

28. Non-interface Time Histories Initial time histories representing crustal earthquake source (Gales Creek) Initial time histories representing the Portland intraslab Spectrally matched to CMS non-interface scenario spectra Initial time histories selected from historical strong-motion records based on PSHA disaggregation results

29. Spectrally-matched 5k time history for crustal earthquake source Initial time history: M 6.5, 1992, Big Bear, CA, Big Bear Lake, R=9 km Target spectrum: 5k non-interface, CMS PHA = 0.96 g Duration = 10 sec I A = 11.2 m/s Initial Spectral Match

30. Scenario Earthquake Time History Summary Seven sets of 3-component time histories for each scenario spectra. –Time histories for CSZ interface, CSZ intraslab, and crustal fault scenarios –Five return periods: 500, 1,000, 5,000, 10,000 and 50,000 years –Baseline-corrected acceleration, velocity and displacement Free-surface acceleration time histories Velocity time histories for use as compliant base input to FLAC model