33D HYBRID SIMULATION OF THE SOURCE AND D HYBRID SIMULATION OF THE SOURCE AND SITE EFFECTS DURING THE 1999 (Mw=5.9)SITE EFFECTS DURING THE 1999 (Mw=5.9)

ATHENS EARTHQUAKE ATHENS EARTHQUAKE

byby

Ivo OprIvo Opršal šal (1,3)(1,3), , JiJiříří Z Zahradníkahradník (1) (1), , Anna SAnna Serpetsidakierpetsidaki (2) (2), , G-Akis TG-Akis Tselentisselentis (2) (2)

1.1. Faculty of Mathematics and Physics, Charles University in Prague, Czech Faculty of Mathematics and Physics, Charles University in Prague, Czech RepublicRepublic

2.2. Seismological Laboratory, University of Patras, GreeceSeismological Laboratory, University of Patras, Greece

3.3. Swiss Seismological Service, ETH ZurichSwiss Seismological Service, ETH Zurich

There are 4 million There are 4 million inhabitants in Athens inhabitants in Athens

now !now !

Dedicated to those who lost their Dedicated to those who lost their relatives and still live in relatives and still live in

temporary houses...temporary houses...

The hybrid methodThe hybrid method

The hybrid methodThe hybrid method

The hybrid methodThe 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 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 (1PEXT method (1stst step) step)(Perturbation LF and Extrapolation HF)(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 The fault size from the low and high frequency ranges are differentfrequency ranges are different

LF fault-length LF fault-length estimation 15 kmestimation 15 km

HF fault-length HF fault-length estimation 7.5 kmestimation 7.5 km

supported by early supported by early aftershocksaftershocks

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

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

deterministic < 1 Hzdeterministic < 1 Hzextrapolation < 5 Hzextrapolation < 5 Hz

a strong-directivity model

Strong-motion Strong-motion modelingmodeling

Slip velocity (independent Slip velocity (independent on N)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

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

Out attempt was to fit the velocigrams. Out attempt was to fit the velocigrams.

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

Comparing records and Comparing records and syntheticssynthetics

Fourier spectraFourier spectra

... ... another station another station

... ... and and one moreone more

Modeling the Modeling the velocityvelocity waveformswaveforms

(data of NOA and ITSAK)(data of NOA and ITSAK)

dominant frequencies of dominant frequencies of about 0.6 Hzabout 0.6 Hz

(in the deterministic range) (in the deterministic range)

Finite differences (2Finite differences (2ndnd step) 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 Stations used for the H/V ratios measurements (University of Patras, seismological laboratory) (University of Patras, seismological laboratory)

--the ambient noise the ambient noise vibrationsvibrations

--the aftershocks of the 1999 the aftershocks of the 1999 Athens earthquakeAthens earthquake

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

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

H/V ratio in Ano LiosiaH/V ratio in Ano Liosia

-pronaounce H/V ratio indicates a -pronaounce H/V ratio indicates a 'singular' site'singular' site

-Intensity as high as IX during 1999 -Intensity as high as IX during 1999 EQEQ

Hence the main motivation to model Hence the main motivation to model LOCALLY BY HYBRID FD LOCALLY BY HYBRID FD the ground the ground motions in this highly populated motions in this highly populated areaarea

Ano LiosiaAno Liosia

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

situationsituation

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 Geological vs. computational modelmodel

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

8

1

The Ano Liosia area The Ano Liosia area

Maxima of Maxima of pseudoacceleration pseudoacceleration

response (damp=.05)response (damp=.05)

ConclusionsConclusions1) 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 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.

References: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 Lekkas, E., S.G. Lozios, G.D.Danamos, K.Soukis and E. Vasilakis, 2000. Microzonation Study of Ano Liosia (in Greek)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 Oprsalkarel.troja.mff.cuni.cz -> people -> Ivo Oprsalandandseismo.ethz.ch/~ivoseismo.ethz.ch/~ivo

The animations, posters and referenced articles: The animations, posters and referenced articles:

karel.troja.mff.cuni.cz -> people -> Ivo Oprsalkarel.troja.mff.cuni.cz -> people -> Ivo Oprsal

andand

seismo.ethz.ch/~ivoseismo.ethz.ch/~ivo

All codes are available free on requestAll codes are available free on request

ivo@seismo.ethz.chivo@seismo.ethz.ch

jz@karel.troja.mff.cuni.czjz@karel.troja.mff.cuni.cz