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) 1. Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic 2. Seismological Laboratory, University of Patras, Greece 3. Swiss Seismological Service, ETH Zurich

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

The hybrid method

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 2 nd step hybrid binding

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

PEXT method (1 st step) (Perturbation LF and Extrapolation HF) 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 ( Hz) by a Gaussian noise, which is constrained by the envelope. Complete Green's functions computed by DW in 1D structure (Bouchon, 1981).

The fault size from the low and high frequency ranges are different LF fault-length estimation 15 km LF fault-length estimation 15 km HF fault-length estimation 7.5 km HF fault-length estimation 7.5 km supported by early aftershocks 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

EGF modeling of rupture propagation (K. Irikura) Grid search of the nucleation point Grid search of the nucleation point 3 stations enable unequal focal mechanism of the mainshock and aftershock 3 stations enable unequal focal mechanism of the mainshock and aftershock Matching waveforms Matching waveforms

Finite-source (composite) modeling DW synthetic (point-source) sub- events DW synthetic (point-source) sub- events “EGF-like” sub-event summation “EGF-like” sub-event summation Low-frequency enhancement included Low-frequency enhancement included Radial rupture propagation, Vr=const Radial rupture propagation, Vr=const Homogeneous slip, equal sub-event size Homogeneous slip, equal sub-event size  Need of a stochastic component

Athens 1999 earthquake Mw=5.9 Athens 1999 earthquake Mw=5.9

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

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

Fault geometry strike 112 o, dip 61 o, rake -84 o nucleation point assumed to be at N, E, depth 10 km this is the western bottom corner of the asperity the asperity top is at the depth of 8 km

The scenarios substitute variations of the asperity position green = entire fault green = entire fault yellow = asperity yellow = asperity blue star = hypocenter blue star = hypocenter red star = nucleation point of the asperity red star = nucleation point of the asperity

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

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.

Comparing records and synthetics

Fourier spectra

... another station

... and one more

Improving fit by the HF perturbation of the radiation pattern

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

Finite differences (2 nd step) 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

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

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

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

H/V ratio in Ano Liosia

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

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

1km A1

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

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

Geological vs. computational model

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.

.7.1

8181

The Ano Liosia area

Maxima of pseudoacceleration response (damp=.05)

Conclusions 1)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.

ACKNOWLEDGEMENTS: This research was supported by research project of Czech Republic MSM , Grant Agency of Czech Republic GACR 205/00/1047, GAUK grant 235/2003, EC projects EVG1-CT PRESAP and EVG1-CT SAFE (BBW Nr ); and by project: Study on the master model for strong ground motion prediction toward earthquake disaster mitigation-p.i. Kojiro Irikura, Kytoto University. References: Zahradnik, J., and Tselentis, G.-A., 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 Oprsal, I., Zahradnik, J. 3D Finite Difference Method and Hybrid Modeling of Earthquake Ground Motion, Journal of Geophysical Research, in press, (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, (see WWW for PDF) Lekkas, E., S.G. Lozios, G.D.Danamos, K.Soukis and E. Vasilakis, Microzonation Study of Ano Liosia (in Greek) Lekkas, E., S.G. Lozios, G.D.Danamos, K.Soukis and E. Vasilakis, Microzonation Study of Ano Liosia (in Greek) The animations, posters and referenced articles are available at The animations, posters and referenced articles are available at karel.troja.mff.cuni.cz -> people -> Ivo Oprsal andseismo.ethz.ch/~ivo

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