Implications of interseismic deformaton in the western

1. Implications of interseismic deformaton in the western United States for the mechanics of strain localization Fred Pollitz, USGS Menlo Park Data sources: UNAVCO, IRIS DMC

2. Thatcher 2008 IGR

3. Western US Seismicity • Seismicity ~bounds Sierra Nevada microplate • SAFZ on west • Walker Lane FZ on east • Local seismicity bands: • west-central NV • Southern NV • SW UT - YS • YS - W central ID • YS - NW MT MT YS ID NV UT 1975-2000 Epicenters from CNSS Thatcher 2008 IGR

4. Wesnousky (2005 Tectonics) • Differences between SAF and Walker Lane/ECSZ due to • Differences in cumulative • geologic slip • Additional component of • extension / merging with • Basin & Range • Role of underlying mantle • asthenosphere in dragging • the crust Walker Lane / Eastern California Shear Zone San Andreas Fault system

5. Fault zone locations / strain localization are the product of: Coupling of mantle flow with the crust Pre-existing weaknesses Gravitational potential energy Laterally variable crust and mantle viscosity structure 5) Other crust and mantle thermal / compositional heterogeneities

6. Laterally variable crust and mantle viscosity structure • Crustal rheology • -- felsic vs. mafic lower crust • Mantle rheology • -- dry vs. wet olivine • -- low vs. high temperature • Lateral rheological discontinuities • -- sharp discontinuities • -- broad weak zones surrounded by strong zones • -- variations in effective elastic plate thickness

7. Thatcher and Pollitz (2008)

8. Seismic shear-wave velocity GPS Crustal velocity field

9. Geodetic inference of rheology

10. How do the lower crust and upper mantle deform after an earthquake? Upper crust Upper crust Lower crust Lower crust Upper mantle Upper mantle Afterslip Relaxation Figures courtesy of Liz Hearn

11. crust mantle (asthenosphere)

12. crust mantle (asthenosphere)

13. crust mantle (asthenosphere)

14. crust mantle (asthenosphere)

15. crust mantle (asthenosphere)

16. 1999 M7.1 Hector Mine earthquake

17. Freed et al. (2007): • Time-dependent • GPS displacements • after 1999 Hector Mine • earthquake best explained • with mantle relaxation • Acceptable rheologies • have `strong’ lower crust • and weak upper mantle

18. Pollitz and Thatcher (2010)

19. Post-earthquake (M7.5 1959 Hebgen Lake, Idaho, earthquake) Nishimura and Thatcher (2003)

20. Geologic inference of rheology

21. Figures courtesy of Bruce Bills

22. Paleolake Lahontan, Nevada High-viscosity crust Low-viscosity mantle Bills et al (2007)

23. Thatcher and Pollitz (2008) Western US rheology based on laterally homogeneous models

24. Modified from Dixon et al. (2004)

25. Strike-slip faults often localize along sharp structural boundaries  Influence of laterally heterogeneous rheology structure Molnar and Dayem (2010)

26. Postseismic relaxation following 2001 M7.8 Kokoxili earthquake Kunlun fault Ryder et al. (2011)

27. Postseismic relaxation following 2010 M7.2 El Mayor-Cucapah earthquake Pollitz et al. (2012)

28. Postseismic relaxation following 2010 M7.2 El Mayor-Cucapah earthquake Pollitz et al. (2012)

29. Postseismic relaxation following 2010 M7.2 El Mayor-Cucapah earthquake Pollitz et al. (2012)

30. Seismic structure around 2010 M7.2 El Mayor-Cucapah earthquake 50 km depth Pollitz et al. (2012)

31. Pollitz et al. (2012)

32. Interseismic velocity field # velocity vectors PBO: 1595 Various: 2414 Payne et al.: 672 Total: 4681

33. Viscoelastic-cycle (`blockless’) model • Time-dependent viscoelastic relaxation of the lower crust • and mantle from earthquakes occurring on a few major faults, • dominated by faults close to the major plate boundaries • (SAF system; Pacific-Juan de Fuca transform faults and • spreading centers; Cascadia megathrust) • Time-independent relaxation from numerous minor faults • Viscoelastic relaxation from broadly distributed dislocation sources over a ~106 km2 area within the plate interior • Lateral variations in effective (vertically-averaged) rigidity • Steady slip on creeping faults

35. Data • Inverted parameters: • Fault slip rates • Lateral variations in effective rigidity • Slip distribution of past large quakes, e.g., 1700 Cascadia eq

36. Components of Model Velocity Field Total

38. Mantle Seismic Shear-Wave Velocity (40 km depth) Lateral Crustal Rigidity Variations Pollitz and Snoke (2010)

39. Mantle Seismic Shear-Wave Velocity (40 km depth) Elastic plate thickness Lowry et al., 2000 Pollitz and Snoke (2010)

40. Correlation of low-vp/vs with actively deforming areas  Long-term weakening role of high-silica crust Lowry and Perez- Gussinye (2011)

41. CONCLUSIONS • Both sharp lateral discontinuities and • localized `weak’ zones can concentrate strain • in the crust. • Crustal strain accumulation is largest within the major strike-slip fault zones (SAF, ECSZ) and eastern boundary of the Basin & Range (ISB). • Zones of low depth-averaged rigidity help concentrate crustal strain around the SAF, ECSZ, and ISB. There is likely upper mantle control on these zones of strain accumulation.