Mathilde B. S ørensen (GFZ Potsdam, Germany) Nelson Pulido (NIED, Tsukuba, Japan)

1. Simulation of ground motion based on earthquake scenarios - uncertainties related to defining the scenario earthquake source Mathilde B. Sørensen (GFZ Potsdam, Germany) Nelson Pulido (NIED, Tsukuba, Japan) Kuvvet Atakan (University of Bergen, Norway)

2. Motivation • Deterministic seismic hazard assessment in cases with • Seismic hazard dominated by a single or few structures • High level of knowledge about rupture properties • Segmentation • Asperities • Source parameters • Hybrid method for simulating ground motions due to finite-extent earthquake source

3. Outline • Methodology • Application – Marmara Sea region, NW Turkey • Sensitivity to uncertain source and attenuation parameters • Future perspectives • Conclusions

4. Hybrid ground motion simulation methodology

5. Hybrid ground motion simulation methodology • Low frequency (0.1-1Hz): • Deterministic wave propagation from an asperity model in a flat layered velocity structure (Discrete Wave Number Method, Bouchon 1981) • High frequency (1-10Hz): • Semi-Stochastic simulation based on an asperity model . The model combines the stochastic methodology of Boore (1983) with the empirical Green’s function method of Irikura (1986), and a high frequency radiation pattern model (Pulido et. al 2002)

6. Scenario earthquake • Location of the rupturing faults • Focal mechanism • Area of rupturing faults (length and width) • Starting point of the earthquake rupture (hypocenter) • Location of asperities in the fault plane • Asperity parameters • Crustal Velocity structure (Vs, Vp and Q) • High frequency attenuation of S-waves

7. Application – seismic hazard in Istanbul Armijo et al., 2005

8. Westward migration of earthquakes along NAF Barka et. al. (2002)

9. Approach for assessing the seismic hazard • Perform simulations for a reference scenario • Based on results of Pulido et al., 2004 • Conservative scenario for hazard in Istanbul • Run additional sets of simulations changing the source and attenuation parameters one by one

10. Reference scenario – geometry Okay et. al 2000

11. Reference scenario – asperities and seismicity After Gurbuz et. al. (2000), Tectonic fault model from Okay (2000)

12. Reference scenario – fault segmentation The depth of the seismogenic zone is 20 km The hypocenter is located at a depth of 15 km

13. Reference scenario – fault parameters

14. Velocity structure Baris et al. (2003)

15. Reference scenario – simulation result

16. Test scenarios

17. Example of result – rupture velocity

18. Results – summary Most significant parameters in terms of ground motion level are: Rise time Rupture velocity Rupture initiation Stress drop Largest variability near asperities and in forward directivity direction High frequencies: Stress drop Attenuation (Q) Low frequencies: Rupture velocity Rise time

19. PEN SIT ATA KUM HIS BUS Response spectra Peaks at both low and high frequencies

20. Variation of response spectra Standard deviation in three frequency bands

21. Summary - Istanbul • Istanbul is under a significant seismic hazard with a potential M=7.5 earthquake causing significant ground shaking over a large area • Simulated ground motion is sensitive to source and attenuation parameters, mainly rise time, rupture velocity, rupture initiation point and stress drop • High frequency ground motion is mainly controlled by the stress drop and Q. These parameters have a strong effect on PGA and PGA attenuation. • Rupture velocity and rise time have a strong effect on the PGV values controlled by the coherent low frequency ground motion. • Variability ofresponse spectra is highly frequency dependent. Velocity response spectra are relativey stable to varying source parameters

22. Future perspectives Implementation to the IEEWRRS Simulation of rupture of individual segments Inclusion of local site effects Simulation of future earthquakes near Izmir Combination of hazard due to several structures

23. Conclusions • Ground motion simulations provide a strong tool in seismic hazard assessment in regions where the hazard is dominated by few structures and when sufficient information is available • Simulation result can provide us information on past earthquakes when recordings are lacking and can also be used to estimate hazard due to future earthquakes • When used for hazard assessment, the uncertainties associated with the input parameters must be dealt with, for example by running scenarios with different levels of conservatism • When applied carefully, the ground motion simulations provide important information about the seismic hazard directly applicable to engineering problems

25. Figure 4

26. Figure 5

27. Figure 6

28. Figure 7

29. Figure 8

30. IEEWRRS Birgören et al., 2004

31. Peak Ground Accelerations (PGA) Atakan and Sørensen, 2006

32. Spectral displacements (<1 Hz) Atakan and Sørensen, 2006

33. Spectral displacements (1 to 5 Hz) Atakan and Sørensen, 2006

34. Spectral displacements (>5 Hz) Atakan and Sørensen, 2006

35. Department of Earth Science University of Bergen Estimated building damage Atakan and Sørensen, 2006

36. Fault linkage between the two segments of NAF in the Marmara Sea is investigated through modeling Oglesby et al., 2005

37. Earthquake rupture at fault segments B and C are not likely to propagate to neighbouring segments Oglesby et al., 2005

38. Super-shear rupture scenario Reference (Vr=3km/s) Super-shear (Vr=5km/s)