3D Hybrid simulation of source and site effects during the 1999 Athens earthquake

I. Oprsal (1,2), J. Zahradnik (2), A. Serpetsidaki (3), G-Akis Tselentis (3)

Proc.of 13th World Conference on Earthquake Engineering, Vancouver, B.C., Canada, August 1-6, 2004, Paper No. 3337.

(1)
Swiss Seismological Service,
ETH-Hoenggerberg/HPP, CH-8093 Zuerich, Switzerland.
FAX: +41-1-6331065,
Phone: +41-1-6332602,
e-mail:

(2)
Department of Geophysics, Charles University, Prague

(3)
Seismological Laboratory, University of Patras, Greece

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Summary:
Combined source-path-site effects due to the Mw=5.9 1999 Athens Earthquake are computed via efficient hybrid method. The method couples finite-extent source kinematic modeling, and 1D discrete wavenumber crustal propagation with 3D finite-difference method for the local site effects, thus saving considerably computer memory and time. Simulation up to 6Hz proves that the intensities of IX at the Ano Liosia suburban area was due to proximity and directivity of the source, as well as due to complex 3D site effects. The simulated ground acceleration locally exceeds 0.6 g, and the response spectra indicate major effects at 2-4 Hz.


Introduction:
The damaging 1999 Athens earthquake of Mw=5.9 occurred at about 20 km from the city center. The intensity distribution in the capital, ranging from V to IX, was quite irregular due to combination of the
source, path and site effects. The 30-stations temporary network, installed in Athens by the University of Patras, recorded and located more than 400 aftershocks. The horizontal-to-vertical spectral ratios from the 20 selected aftershock recordings provided site classification. The most significant anomaly (H / V exceeding 4 in the frequency range 1-4 Hz) was found at the Ano Liosia site, belonging to the most
heavily damaged zones with intensity IX. The site is situated in a shallow basin whose surface extent is about 4 km x 4 km, and the maximum depth is of about 150 m. The basin is filled with basically 3 layers.
The topmost layer includes alluvium and soft soil, the second one consists of stiff soil and alternations of conglomerates, clay and sand, while the third layer is represented by Neogene formations like marl, marly limestone and sandstone. The bedrock of the basin consists of Triassic limestone and schist. Borders of the basin, where the topmost layer directly overlies the bedrock, are locally quite steep. Based on
geophysical data (VP, VS, Q) measured at the site, the numerical modeling of the seismic site response was carried out. The 3D finite-difference technique was used, and significant effects were revealed. No
recording of the mainshock is available in Ano Liosia. Nevertheless, based on the finite-extent composite source model, validated in central Athens by the existing strong motion records of the mainshock, we
found that the bedrock motion in Ano Liosia had its PGA ranging from 0.2 to 0.3 g, resulting from the relatively small epicentral distance (~ 10 km) and the forward source directivity. The source and site effects were combined with each other by a hybrid technique, allowing fast full-wave 3D calculations up to ~6 Hz on a standard personal computer, and showing that the combined source and site effects in Ano Liosia have provided the PGA values locally exceeding 0.6 g.


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