3D HYBRID SIMULATION OF THE SOURCE AND SITE EFFECTS DURING THE 1999 (Mw=5.9)ATHENS EARTHQUAKE byIvo Opral (1,3), Ji Zahradnk (1), Anna Serpetsidaki (2), G-Akis Tselentis (2)

Faculty of Mathematics and Physics, Charles University in Prague, Czech RepublicSeismological Laboratory, University of Patras, GreeceSwiss Seismological Service, ETH Zurich

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

The hybrid method

The hybrid method

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 modelthe 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 boxthus the REPLICATION TEST is a measure of the consistency between the 1st ant the 2nd 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 (1st 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 (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).

The fault size from the low and high frequency ranges are differentLF fault-length estimation 15 km

HF fault-length estimation 7.5 km

supported by early aftershocksThe 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 point3 stations enable unequal focal mechanism of the mainshock and aftershockMatching waveforms

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

Athens 1999 earthquake Mw=5.9

mainshock length x width (km)

7.5 x 6 *)

mainshock moment (Nm)

7.8e17

mainshock stress drop (MPa)

6.3

mainshock average slip (m)

0.55

number of sub-events

5 x 5

sub-event moment (Nm)

4.0e15

sub-event size (km)

1.5 x 1.2

sub-event corner frequency (Hz)

1.9

sub-event duration (sec)

0.54

sub-event slip (m)

0.07

*) Depth of fault top and bottom: 7.1 and 12.0 km. The rupture starts at the western bottom corner (38.08o N, 23.58o E, depth 12 km), and spreads radially at 2.8 km/s.

one of previous attempts with a homogeneous-slip 7.5x6 km (J. Zahradnik)

deterministic < 1 Hzextrapolation < 5 Hz

a strong-directivity modelStrong-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 Brunes wavelet: = average slip velocity * 2.3 = 0.9 m/s

Fault geometrystrike 112o, dip 61o, rake -84o

nucleation point assumed to be at38.08 N, 23.58 E, depth 10 kmthis 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 positiongreen = entire faultyellow = asperityblue star = hypocenterred star = nucleation point of the asperity

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)

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 (2nd step)Formulation of the problemelastodynamic partial differential equation in time domain displacement formulationHooks isotropic generally inhomogeneous medium with discontinuities free-surface topography point source double couple, plane wave, arbitrary (hybrid) excitationa simplified employment of a variable Qp=Qs=Q(x,y,z)=c*fNumerical aspectsnumerical solution of 2nd order hyperbolic PDE of motionexplicit finite-difference formulation, 2nd order of accuracy in space and timeone 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 modelsource 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

1kmA1

1km

computational model slice close to G1G2 profilegeological 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.

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.1

8

1

The Ano Liosia area

Maxima of pseudoacceleration response (damp=.05)

ConclusionsHYBRID 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.

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 Oprsalandseismo.ethz.ch/~ivoACKNOWLEDGEMENTS: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.

The animations, posters and referenced articles:

karel.troja.mff.cuni.cz -> people -> Ivo Oprsalandseismo.ethz.ch/~ivo

All codes are available free on requestivo@seismo.ethz.chjz@karel.troja.mff.cuni.cz