1. Geodetic Deformation, Seismicity and Fault Friction Ge277- 2007 Sensitivity of seismicity to stress perturbations, implications for earthquakes nucleation

2. Key Observation : Seismicity rate and Geodetic strain rate are generally proportional, and both decays as t -1 following an earthquake Aftershocks and postseismic relaxation following the : –Mw7.6, Chichi earthquake (1999) –Mw 7.2 Landers Earthquake (1992) –Mw 8.1, Peru earthquake (2001) –Mw 8.7, Nias earthquake (2005) Background seismicity in the Nepal Himalaya

3. (Svarc and Savage, 1997) CPA analysis show that all GPS stations follow about the same time evolution f(t) Postseismic Displacements following the Mw 7.2 1992 Landers Earthquake

4. Comparing aftershocks and afterslip following Landers EQ

5. (Perfettini and Avouac, 2004a) Seismicity and Postseismic displacements follow the same relaxation curve consistent with the Omori Law.

6. Velocities relative to India (Bettinelli et al, 2006) Geodetic deformation across the Nepal Himalaya

7. Creeping Zone  17-18mm/yr Locked Fault Zone, width  110km

8. Seismicity and Coulomb stress change due to interseismic stress accumulation (Bollinger et al, JGR, 2004) Seismicity coincides with the area where Coulomb stress increases by 4-6 kpa/a

9. Conceptual Model

10. Seismicity and Coulomb stress change due to interseismic stress accumulation (Bollinger et al, JGR, 2004) Seismicity coincides with the area where Coulomb stress increases by 4-6 kpa/a

11. Winter seismicity rate is twice as large as summer seismicity rate

13. Detrend GPS time-series Seasonal variation of Horizontal Displacements

14. Water level in Ganges Basin determined from TOPEX-POSEIDON and GRACE GUMBA-SIMRA

16. TOPEX-POSEIDON GRACE

17. Displacements induced by surface water level variations in the Ganges basin Summer Winter

18. Finite Element Modeling

20. Seasonal variations of seismicity and water level in the Ganges Basin

21. Compression Extension

22. Strain induced by surface water level variations in the Ganges basin Summer: Extension Winter: Compression

23. Seasonal Coulomb stress variations Coulomb stress variations are estimated to about 500 pa (<earth tides, 2-3kPa for T=12h and 14 days)

24. Coulomb stress rate Seasonal variations of geodetic displacements reflect deformation due to water level variations in the Ganges basin Interseismic Coulomb stress increase by 4-5 kPa/yr is modulated by seasonal variations of 500pa (corresponding to stress rate variation of 2-3kPa/yr) modulating the seismicity rate by a factor of 2. Seasonal variation of Coulomb stress rate 6 kPa/yr 8 kPa/yr With secular term added 10 kPa/yr

26. In these examples: –Seismicity rate and stress rate are approximately proportional –The characteristic time associated with the stress variations is of the order of 1 yr.

27. Standard Coulomb failure - seismicity rate obeys : - so, for periodic loading : Assuming,

28. (Lockner and Beeler, 1999; Heki, 2003)

29. Standard Coulomb failure For periodic loading : Seismicity rate should be much more sensitive to earth tides. This is not observed, probably because rupture is a time- dependent process, as suggested by rock mechanics experiments.