1. SUSHI II Long-term stress modelling: Implications for large earthquake forecasting Suleyman S. Nalbant, John McCloskey, Shane Murphy, Nuno Simao and Tony Lindsay Environmental Sciences Research Institute University of Ulster, Coleraine, Co. Derry, N. Ireland

2. Background There were two gigantic earthquakes on the Sunda megathrust in 2004 and 2005 with 3 months separation. Stress transfer from these earthquakes sparkled a worry that Mentawai section would rupture soon

3. Background There were historical evidences that supported this worry. A potential earthquake on the Mentawai section with a magnitude in the range of M~8.7-8.9 would have disastrous consequences. A tsunami as a result of this potential earthquake would invade population centres along the west coast of Sumatra, specifically the city of Padang with 800,000 population.

4. Padang McCloskey et al EPSL 2008 Knowing slip distribution is important

5. Motivation Can we forecast a future earthquake with its possible rupture area and slip distribution on the Sunda megathrust? To have a reliable answer to this question –stress/strain accumulation over at least a seismic cycle should be known; Initial stress level before the last seismic cycle Stress perturbation due to the all earthquakes in the last seismic cycle Stress accumulation over inter-seismic periods.

6. Strain accumulation It is defined as the ratio of interseismic slip rate to plate convergence rate Interseismic slip rates are inverted from geodetic and palaeogeodetic measurements. strong coupling is a necessary but not sufficient condition for high slip in a single event Chlieh et al. (2008) JGR Natawidjaja et al. (2006) JGR

7. Rupture planes of all earthquakes M>7.0 occurred between 1797 and 2005 (22 earthquakes)

8. Slip Function for earlier historical earthquakes Seismic moment is kept the same

9. 1797-1833 We are calculating stress changes assuming a ‘zero’ stress level in 1797.

10. 1797-1861

11. 1797-1931

12. 1797-2004

13. 1797-2005 The rupture limits may have been defined by the pre‐existing stress state shaped by previous earthquakes

14. Rupture extend and slip distribution of the 1797 EQ Natawidjaja et al. 2006 (JGR)

15. Rupture extend and slip distribution of the 1833 EQ Natawidjaja et al. 2006 (JGR) One thing we know for sure that slip distributions and rupture extend weren’t like this.

16. What do we know? Newcomb and McCann JGR 1987 Increasing Information

17. Class 4 February 16 1861 Eq Using Monte-Carlo method, we can produce many scenario earthquakes satisfying the observations

18. Location (lat/long) -0.06/98.21 Magnitude:7.7 Class 3 December 28 1935 Eq

19. Coral IDLatitudeLongitude Vertical Displacement(m) 22 1-0.53998.4640.20.05 2-1.25499.0600.680.45 3-2.13299.5360.690.56 4-2.37099.7430.70.1 6-2.75199.9950.750.2 7-2.599100.1060.05 8-2.826100.2830.370.09 9-3.039100.4630.490.07 10-3.128100.3110.90.2 11-3.285100.44600.05 12-3.163100.5050.05 14-3.211100.3320.030.05 Class 2 1797 and 1833 Eqs. Sumatra

20. 17971833 Maximum Magnitude 8.79.2 Minimum Magnitude 8.68.9 Maximum Slip 4.315.3 Fault Coupling 1833 1797

21. Year Month Day Long Lat Depth Mag. #Scenario Eq 16811211000050 1756113000050 177011000050 17972100008.610 1818318000050 181851000050 183311240008.9 8 184315000050 18521111000050 18612160008.550 190963-2.00101.0007.750 19351228-0.0698.2107.750 194368-2.82102.0907.350 194369-0.94100.9107.650 194658-0.5199.3107.150 19766203.4096.33337.050 198411170.2098.30337.250 200211295.992.65237.250 2004122694.263.09299.01 200532897.071.67268.61 2007912100.99-3.78248.51 200791399.39-2.31177.01 200822095.982.69157.31 200822599.95-2.66147.21 20104696.742.07187.81 20105995.783.36377.21 2010102599.32-3.71127.81 All events Class 1 Modern Eqs

22. Slip deficit scenarios; Which History is most likely?

23. A preliminary result The well-resolved red bulge under Siberut would produce an M8.7 earthquake if it all failed simultaneously.

24. What’s next? Genetic algorithm for improved inversion for corals Searching for further coral displacements to constrain other historical earthquakes – particularly 1861 and 1935 Use Bayesian techniques to assess the probability of each model given the data and uncertainty Look at the stress field before recent damaging earthquakes Communicate risk mitigation by engaging key stakeholders, so that science-based hazard maps can be presented in a readily-understood local context

25. 1797-2010

26. 1797-2011

27. From Loading and Earthquakes to Absolute Stress? Slip Deficit Initial Slip Deficit Seismic Slip Interseismic Loading + - = Interseismic Loading = Plate Velocity Coupling  Seismic Slip = Sum of Slip distributions

28. 1797-2005