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Preliminary
report for the strong motion data of the 8 June 2008 Earthquake
(Mw 6.4)
in Achaia - Elia, Western Greece
Kalogeras I., Loukatos D. and
G. Stavrakakis
On 8 June 2008 at 12:25 GMT (15:25 local
time), a strong earthquake with a moment magnitude of 6.4 occurred
in western Greece.The epicentral
coordinates as estimated by the Institute of Geodynamics
were 37.98°N, 21.51°E, and the focal depth was 25
km while the moment tensor solution gave a strike-slip focal mechanism
with a NE-SW direction. Information can be found in:
(http://bbnet.gein.noa.gr/alerts_manual/evman080608122527_info.html),
(http://bbnet.gein.noa.gr/MT.htm).
The broader area hosted significant earthquakes
during the last 20 years: Vartholomio 16/10/1988 M=6.0, Pyrgos
26/03/1993 M=5.5 and M=5.8, Patras 14/07/1993 M=5.6, Vartholomio
2/12/2002 M=5.8 The shock was felt in almost all over Greece and
caused 2 deaths, about a hundred injuries and about 10000 houses
with small to severe damage according to the reports. Serious
problems occurred within the municipalities of Olenia, Larissos,
Dymi, Vrachneika and Fares (Newspaper Eleftherotypia, 9/6/2008,
www.enet.gr). Old masonry houses
suffered the most damage. Benching of the rails occurred in Kato
Achaia, while liquefactions were observed at many sites. The Patras
hospital evacuated due to serious damage.The results of the preliminary
analysis of strong-motion recordings are presented here, obtained
from the stations of the Institute of Geodynamics' accelerographic
network either by telemetry or by visiting the stations. The
collection of the data from other stations continues and will
be presented together with seismological and macroseismic data
shortly. The linked map shows
the sites of the instrument trigerred (red triangles), plus the
sites of some instruments not triggered (yellow triangles) by
the main shock (green star).
Table summarizes
the PGA (g), PGV (cm/s) and PGD (cm) values calculated from the
NOAIG strong motion instruments. Click on the peak ground values
to view the respective graph.Analysis of the accelerographs showed
a considerable long duration of the strong motion (Patras 80 s,
Xylokastro 78 s, Zakynthos 75 s) which is compatible with the reports
of the people in the meizoseismal area. The situation in Athens
area is also of great interest, since the Acropolis' instrument
(free-field) was triggered by the earthquake, while the neighbouring
Thissio instrument was not triggered, although it is located at
about the same geological conditions (cretaceous limestone), but
perhaps the topography played a significant role. Since the source
and the path is generally the same for the case of Athens, local
conditions in Athens probably specify the triggering threshold for
each site.For indicative reasons the response spectra for Amaliada
(AMAA0801), Patras (PATB0807)
and Pyrgos (PYRA0801) are
also shown for the three components and for 5% damping factor.
Pyrgos shows two maxima at about 2Hz and 3 Hz, Patras shows high
spectral values between 0.8 and 2.5 Hz, while Amaliada shows maximum
spectra values at about 4Hz.Patras instrument (PATB) recorded also
the strongest aftershocks (M 3.5 - 4.5) occurred during the first
two days after the main shock with uncorrected accelerations not
exceeding the 15cm/s/s.
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