The 8 January 2006 Earthquake (Mw 6.7) Offshore Kythira Island, Southern
Greece: Seismological, Strong-motion, and Macroseismic Observations
of an Intermediate-depth Event
Konstantinos I. Konstantinou, Ioannis S. Kalogeras,
Nikolaos S. Melis, Moissis C. Kourouzidis, and George N. Stavrakakis
See also the publication
in Seismological Research Letters, Vol 77, No 5 September/October
2006, 544-553
On 8 January 2006 at 11:34 GMT (13:34 local time),
a strong earthquake with a moment magnitude of 6.7 occurred in
southern Greece, off the eastern coast of the island of Kythira.
The epicentral coordinates as estimated by the European Mediterranean
Seismological Centre (EMSC-CSEM, http://emsc-csem.org were 36.31
deg;N, 23.24 deg;E, and the focal depth was 60 km. The shock was
felt in a spatially extended area that covered Greece, Italy,
Turkey, Egypt, Cyprus, Israel, Syria, Jordan, and Lebanon. Despite
the large magnitude of the earthquake, the reported damage was
not extensive mainly due to the intermediate focal depth. No casualties
were reported and the structural damage to buildings was limited
to the islands of Kythira and Antikythira and to the city of Chania,
western Crete. Furthermore, landslides and/or rockfalls were reported
only at the village of Mitata (Kythira island), where the main
square and the road were damaged due to a landslide.
In this article we take advantage of a multitude
of available observations to give a detailed report on this most
recent large intermediate-depth earthquake. First, we describe
the temporal and spatial distribution of the mainshock-aftershock
sequence and summarize all available moment tensor solutions reported
by various agencies. Then, we present preliminary analysis of
strong-motion recordings in an effort to check the relationship
between the shaking caused by such an event and the influence
of both attenuation and local geological conditions. Macroseismic
data collected from the whole of Greece also are included and
utilized toward understanding the regional intensity attenuation
pattern. Finally, we give an overview of the implications of such
an event in terms of regional seismotectonics and seismic hazard.
SEISMOLOGICAL OBSERVATIONS
Intermediate-depth earthquakes in the broader area
of south-western Greece are related to the southern Aegean subduction
zone, which is referred to as the Hellenic arc (figure).
The mainshock as well as several of its aftershocks
(8 January to 20 May 2006) were recorded by the Institute of Geodynamics'
seismological network.
The results of the recordings analysis (manually picking of P-
and S-phase arrival times and their inversion) are consistent
with the results reported by several institutes/agencies, which
show a predominently reverse slip component. In the map
the circle The circle size is proportional to the local magnitude
of each event, according to the scale shown at the right side
of the plot. In a similar way, the focal depth follows the color
scale shown at the right side. All available moment tensor solutions
for the mainshock and two of its aftershocks also are plotted.
The linked table shows
the moment tensor solutions included in the study.
STRONG MOTION OBSERVATIONS
The strong motion recordings included in the study
came from the accelerographs
operated by the Institute of Geodynamics, ITSAK (www.itsak.gr)
and PPC. The linked table
gives information about the sites and the PGA recorded.
Some examples of strong ground motion recorded during
the Kythira earthquake by a number of instruments in and around
the Athens metropolitan area are showed in the figure.
The star represents the epicenter of the mainshock and the different
symbols indicate PGA values registered at those locations.
The linked table
summarizes the PGA (g), PGV (cm/s) and PGD (cm) values calculated
from the NOAIG strong motion instruments. Follow the number-links
to view the respective graph.
Analysis of the accelerographs showed that
the corresponding response
spectra appear to be dominated by long period (up to 2.5s)
strong ground motion, which is in contrast to observations of
strong ground motions recorded during near-field events (dominant
periods usually shorter than 0.5s). A comparison
of near field (dashed line) and far field (continuous line) pseudoacceleration
response spectra for PGA of the same order (around 20-30mg) was
performed for 4 sites (PRSA, KORA, XLCA and ATHC) and showed the
existence of the long period waves recorded by the strong motion
instruments due to the Kythira earthquake in all situations, although
the spectral accelerations do not vary much for the four cases.
MACROSEISMIC OBSERVATIONS
According to first reports, the Kythira earthquake
was felt in most parts of Greece, so questionnaires were sent
to all the prefectures of the country. In total 666 questionnaires
were sent and 304 were completed and returned by the local authorities.
The reported answers clearly indicate that despite its large magnitude,
the earthquake did not cause serious damage owing to its large
focal depth. For example, at Potamos village (on Kythira island)
situated only 35 km from the epicenter, the reported intensity
was only V+. At the village of Mitata, where the most serious
damage occurred (intensity VII+), local site amplifications appear
to have been responsible for the relatively severe shaking, since
similar observations also were reported after another intermediate-depth
earthquake in 1903. A map
of isoseismals curves derived from the macroseismic
data following the methodology of Schenkova et al. (2006),
based on a kriging method. Moreover some simple
statistical diagrams concerning the macroseismic observations
are presented.
Γ.Ι.
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