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

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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.