3 D HYBRID SIMULATION OF THE SOURCE AND SITE EFFECTS DURING THE 1999 (Mw=5.9) ATHENS EARTHQUAKE

1. 3D HYBRID SIMULATION OF THE SOURCE AND SITE EFFECTS DURING THE 1999 (Mw=5.9)ATHENS EARTHQUAKE by Ivo Opršal (1,3), Jiří Zahradník (1), Anna Serpetsidaki (2), G-Akis Tselentis (2) • Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic • Seismological Laboratory, University of Patras, Greece • Swiss Seismological Service, ETH Zurich

2. There are 4 million inhabitants in Athens now ! Dedicated to those who lost their relatives and still live in temporary houses...

3. The hybrid method

4. The hybrid method

5. The hybrid method

6. Some of implications of the hybrid method: • the excitation box represents all sources (thus including the finite-extent sources) and source-path details from the 1st step model. • the hybrid technique of the excitation in the 2nd step keeps the excitation box fully permeable for all the scattered waves created in the 2nd step model • the frequency content of the computation may be relatively high even for cases of a distant source because the FD area is performed on a fraction of the original large all-in-one model • performing the 2nd step computation on (unchanged) 1st step model (so called 'REPLICATION TEST') should give the same wavefield as in the 1st step inside the box, while it should give 'zero‘ (or neglectably small) wavefileld outside the box • thus the REPLICATION TEST is a measure of the consistency between the 1st ant the 2nd step hybrid binding

7. A recipe for the hybrid method 1. Calculate 3D wavefield due to source and crustal part - Composite PEXT finite-extent source 2. Solve 3D site model by FD, thus you get combined source - path -site effect

8. Composite source modeling up to 2.8Hz yields deterministic envelope of accelerograms. Radial rupture propagation: rupture velocity varies up to 10% around mean constant value. The acceleration spectral plateau is extrapolated up to 6Hz from the deterministic part (2.0-2.8Hz) by a Gaussian noise, which is constrained by the envelope. Complete Green's functions computed by DW in 1D structure (Bouchon, 1981). PEXT method (1st step)(Perturbation LF and Extrapolation HF)

9. The fault size from the low and high frequency ranges are different • LF fault-length estimation 15 km • HF fault-length estimation 7.5 km • supported by early aftershocks  The source model currently used for our modeling: L=10, W=8 km (corresponds to empirical relation of Somerville et al. 1999) The asperity size: 4.5x4.5 km The asperity slip contrast=2

10. Strong-motion modeling one of previous attempts with a homogeneous-slip 7.5x6 km (J. Zahradnik)deterministic < 1 Hzextrapolation < 5 Hz a strong-directivity model

11. Slip velocity (independent on N) average slip velocity = subevent slip / subevent duration =0.41 m/s (same for any Mw due to self-similarity) maximum slip velocity depends on wavelet e.g., for Brune’s wavelet: = average slip velocity * 2.3 = 0.9 m/s

12. PEXT (1st step):Stations where the source model was verified against the real strong-motion data, and the example of such a comparison (0-6Hz)

13. Out attempt was to fit the velocigrams. This may be a bit more difficult then fitting only the accelerations because one-asperity model may not be always sufficient, and asperities with lower contrast have to be added.

14. Comparing records and synthetics

15. Fourier spectra

16. ... another station

17. ... and one more

18. Modeling the velocity waveforms(data of NOA and ITSAK) dominant frequencies of about 0.6 Hz (in the deterministic range)

19. Formulation of the problem elastodynamic partial differential equation in time domain displacement formulation Hook’s isotropic generally inhomogeneous medium with discontinuities free-surface topography point source double couple, plane wave, arbitrary (hybrid) excitation a simplified employment of a variable Qp=Qs=Q(x,y,z)=c*f Numerical aspects numerical solution of 2nd order hyperbolic PDE of motion explicit finite-difference formulation, 2nd order of accuracy in space and time one FD approximation everywhere (easy to implement) interface conditions implicitly satisfied through treatment of elastic parameters (heterogeneous approach), including the free surface (vacuum formalism) ‘transparent’ boundaries and damping tapers at the edges of the model source and path effects coupled with local effect at the so called ‘excitation box’ (!) stable at high (vp/vs) contrasts (!) stable at high (vp/vs) ratio contrasts Finite differences (2nd step)

20. Stations used for the H/V ratios measurements (University of Patras, seismological laboratory) -the ambient noise vibrations -the aftershocks of the 1999 Athens earthquake

21. Map of the H/V MAXIMA-the ambient noise vibrations

22. Map of the H/V MAXIMA-the aftershocks of the 1999 Athens earthquake

23. H/V ratio in Ano Liosia

24. Ano Liosia -pronaounce H/V ratio indicates a 'singular' site-Intensity as high as IX during 1999 EQHence the main motivation to model LOCALLY BY HYBRID FD the ground motions in this highly populated area

25. The most damaged part of the Athens and the Ano Liosia situation

26. 1km A1

27. The largest observed damage (only in the densely inhabited area) 1km

28. geological model – profile G1-G2 (Lekkas 2000) computational model slice close to G1G2 profile

29. Geological vs. computational model

30. The present geological information is too sparse to be simply interpolated. In the future: A (carefully) smoothed model (in E-W direction) will be used for the next computations. The smoothing will keep the velocities’ limits.

32. .7 .1

33. 8 1

34. The Ano Liosia area

35. Maxima of pseudoacceleration response (damp=.05)

36. Conclusions • HYBRID approach allows joint treatment of finite-extent source, path and site effect (here up to 6 Hz) • 2) The 3D input (bedrock) motion calculated by PEXT method validated by comparison of synthetic and observed strong motion records in the other sites in Athens • 3) Ano Liosia, strong damage and intensity IX - proved to be combined effect of proximity and directivity of source, and complex 3D site effects.

37. References: • Zahradnik, J., and Tselentis, G.-A., 2002. Modeling strong-motion accelerograms by PEXT method, application to the Athens 1999 earthquake. Proc. of XXVIII Gen. Ass. of Europ. Seismol. Comm, 1-6 Sep. 2002, Genoa (CD-ROM), or http://seis30.karlov.mff.cuni.cz/ • Oprsal, I., Zahradnik, J. 3D Finite Difference Method and Hybrid Modeling of Earthquake Ground Motion, Journal of Geophysical Research, in press, 2002. (see WWW for PDF) • Oprsal I., Brokesova J., Faeh D., Giardini D., 3D Hybrid ray-FD and DWN-FD Seismic Modeling for Simple Models Containing Complex Local Structures, Stud. geophys. geod., in press, 2002. (see WWW for PDF) • Lekkas, E., S.G. Lozios, G.D.Danamos, K.Soukis and E. Vasilakis, 2000. Microzonation Study of Ano Liosia (in Greek) • The animations, posters and referenced articles are available at karel.troja.mff.cuni.cz -> people -> Ivo Oprsal and seismo.ethz.ch/~ivo ACKNOWLEDGEMENTS: This research was supported by research project of Czech Republic MSM 113200004, Grant Agency of Czech Republic GACR 205/00/1047, GAUK grant 235/2003, EC projects EVG1-CT-1999-00001 PRESAP and EVG1-CT-2000-00023 SAFE (BBW Nr. 00.0336); and by project: Study on the master model for strong ground motion prediction toward earthquake disaster mitigation-p.i. Kojiro Irikura, Kytoto University.

38. The animations, posters and referenced articles: karel.troja.mff.cuni.cz -> people -> Ivo Oprsal and seismo.ethz.ch/~ivo • All codes are available free on request ivo@seismo.ethz.ch jz@karel.troja.mff.cuni.cz