Seismic Anisotropy in the East Mediterranean Lithosphere: The Mw6.3
Karpathos Intermediate Depth Earthquake of 22 January 2002 in
the Hellenic Arc
G.A Papadopoulos (1) , M. Ziazia (1) , A.
Plessa (1) , A. Ganas (1) , V.Karastathis (1), N. Melis (1)
, G. Stavrakakis (1)
(1) Institute of Geodynamics , National
Observatory of Athens, 11810 Athens , Greece, g . papad @ gein.noa.gr
1. Introduction
On 22 January 2002 , 04:53:54.3 UTC , a Mw6.3
(USGS) or M L 6.1(Geodynamic Institute , National Observatory
of Athens, NOAGI) earthquake of intermediate focal depth ( ?
100 km ) focusing near Karpathos (35.56 0N, 26.73 0E ), east
part of the South Aegean Sea (Fig. 1), shook a large part of
the East Mediterranean Sea . Felt areas were extended, by hundreds
of kilometers, into remote places like Egypt , Cyprus and Israel
. The asymmetric propagation of seismic energy radiated by intermediate
depth shocks in the Hellenic arc was described in the past from
both macroseismic observations and records of displacement and
velocity short-period seismometers (see a review in Papadopoulos
,1991) .
The Karpathos shock is the first strong intermediate depth event
in the Hellenic arc that can be studied on combined broadband
records and macroseismic intensity observations , thus providing
an excellent opportunity to reexamine the pattern of energy
propagation and its possible causes. In this preliminary report
we utilize broadband records and macroseismic observations collected
from stations and observation points located at hypocentral
distances up to about 900km , that is from the area of Greece
and the Eastern Mediterranean Sea .

Figure 1 . Epicenter of the Karpathos earthquake
of 22 January 2002 and its focal mechanism according to USGS
. Circles indicate macroseismic intensities .
2. Past Cases
The main, large- scale geodynamic feature that
controls the seismicity in the Hellenic arc and trench system
is the active subduction of the Mediterranean lithosphere beneath
the South Aegean Sea . The dynamic interaction between the Mediterranean
and the Aegean lithospheric plates results in the generation
of shallow earthquakes in the broad Aegean Sea area and of intermediate
focal depth earthquakes in the South Aegean Sea. Besides , intermediate
depth seismicity may be of particular interest for understanding
better volcanism in island arc structures since there is evidence
that the two phenomena are geodynamically related (e.g. Papadopoulos,
1986, 1987).
One of the most important properties of the intermediate depth
seismicity is the pattern of irregular geographic distribution
of seismic intensities which was described by a number of authors
(e.g. see in Ambraseys, 2001, for recent results and a short-review).
The pattern of seismic anisotropy incorporates a component of
high seismic conductivity towards the Mediterranean Sea, that
is towards the subducting lithosphere, and a component of low
seismic conductivity in the direction of the Central and North
Aegean Sea, that is in the back-arc domain. Seismic intensities
of degree III or even higher in 12-point intensity scales were
typically reported as far as South Italy , Malta , Cyrenaica
, Egypt ,Cyprus , Israel and Palestine, that is at epicentral
distances up to 800km and more. On the contrary, such intensity
values were not observed even at half distances towards the
Central and North Aegean Sea. In addition, Papazachos and Comninakis
(1971) and Delibasis (1982) presented instrumental evidence
indicating a similar pattern in the seismic energy propagation
due to the strong attenuation of mainly the shear waves towards
the back-arc area. This was attributed rather to the anisotropic
seismic energy propagation than to soil conditions that may
locally amplify seismic wave amplitudes. A plausible geophysical
explanation is that seismic waves traveling through the cold
and dense lithoshperic slab subducting from the Mediterranean
to the Aegean Sea do not attenuate significantly. However, the
hot asthenospheric wedge dominating the back-arc region of the
Aegean Sea area absorbs strongly the seismic energy(e.g. Hashida
et al., 1988).
In earlier times magnitudes up to about Ms8.0 were assigned
to intermediate earthquakes occurring in the instrumental era
. More recent examinations , however, showed that this was rather
an exaggeration possibly due to the extra large felt areas of
such earthquakes which were taken into account in the magnitude
calculation. In view of this one may argue that it is quite
realistic for magnitudes up to 7-7.5 to be observed in association
with the intermediate depth seismicity of South Aegean Sea.
3. Observational Material
3.1 Instrumental observations
Broadband records coming from nineteen stations
of the national seismograph network of NOAGI were processed
. Hypocentral distances range from 110 km to 690 km. The ratio
A = S a / P a was calculated from a number of records to determine
large-scale anisotropic seismic energy propagation, where S
a and P a is the maximum amplitude of the S and P waves , respectively.
Two groups of stations were considered : the first includes
stations lying in the front -arc domain while the second includes
the stations lying in the back-arc domain.
3.2 Macroseismic observations
The macroseismic data used were compiled from
about 120 NOAGI standard questionnairies completed on the responsibility
of local authorities in a respective number of observation points.
Many of them were verified through a procedure of rapid telephone
or e-mailing interviews with local earth scientists and / or
civil protection officers . Press reports and macroseismic information
sent by seismological institutes were taken into account for
some observation points located in Cyprus , Israel and Egypt.
Macroseismic intensity in each observation point was evaluated
in both modified Mercalli scale (MM) and EMS- 92 scale . Identical
intensity degrees were found for nearly all the observation
points. Only intensity values equal to or larger than II were
considered.
4. Results
Plots of A as a function of hypocentral distance
, D , reveals quite different patterns in the two arc domains
as far as the vertical components of the records are concerned.
In the front- arc domain, the ratio A gradually increases by
a factor of ~2.5 while in the back-arc domain A gradually decreases
by a factor of ~3 up to at least D = 750km (Fig. 2) . This means
a much higher S-wave amplitude decrease in the back-arc region
with respect the front-arc region. This pattern is compatible
with the macroseismic intensity distribution which indicates
systematically higher intensities in the front-arc domain with
respect to the back-arc domain. In fact , intensities observed
in the front -arc, like in Crete island, are systematically
larger by about one degree as an average from the intensities
observed in the back-arc at equal hypocentral distances (Figures
3a, b , c).
Figure
2. Variation of the ratio A (vertical axis) calculated from
the vertical component records with distance D (in km, horizontal
axis) in the front -arc domain (top) and the back-arc domain
(bottom) .
Figure
3a . Intensity attenuation in the front-arc region.
Figure
3b . Intensity attenuation in the back-arc region.
Figure
3c . Comparison of intensity attenuations in the front-arc and
back-arc regions.
5. Conclusion
Our preliminary results verify the anisotropic
propagation of seismic energy radiated by intermediate depth
shocks in the Hellenic arc . The high attenuation of S waves
passing through the low-velocity and low -Q material of the
asthenospheric wedge lying beneath the back-arc region , along
with the relatively low attenuation particularly of the S waves
traveling through the dense and cold subducting lithosphere
, explains well the large-scale anisotropy observed .
6 . References
Ambraseys , N.N., 2001. Far-field effects of Eastern
Mediterranean earthquakes in Lower Egypt . J.
Seismology , 5 , 263 -268.
Delibasis, N., 1982 . Seismic wave attenuation in the upper
mantle beneath the Aegean . Pure Appl.
Geophys. , 120 , 820 - 839.
Hashida, T. , Stavrakakis , G. and Shimazaki, K. , 1988. Three-dimensional
seismic attenuation
structure beneath the Aegean region and its tectonic implication.
Tectonophysics , 145, 43-54.
Papadopoulos , G.A. , 1986. Large intermediate depth shocks
and volcanic eruptions in the Hellenic arc
during 1800-1985. Phys. Earth Planet. Inter., 43 , 47 -55.
Papadopoulos , G.A. , 1987. Large deep-focus shocks and significant
volcanic eruptions in convergent
plate boundaries during 1900-1980. Tectonophysics, 138, 223-233.
Papadopoulos , G.A., 1991. Seismicity and related phenomena
in the Hellenic arc : Their influence on
the Eastern Mediterranean region. In: M.J. Salem et al. (Eds.
) The Geology of Libya , v. 4,
Eslevier, 2441- 2449.
Papazachos, B.C. and Comninakis , P.E. , 1971. Geophysical and
tectonic features of the Aegean
arc. J. Geophys. Res . , 76 , 8517 - 8533.
Addendum
According to the determinations of NOAGI
, Athens , on 21 May 2002 at 20: 53: 30.8 and 6 June 2002 at
22: 35: 43.8 (UTC) another two strong earthquakes of magnitudes
M L 5.1 and M L 5.2 , focal depths of 97km and 93km and epicentral
coordinates 36.57 0 N , 24.31 0 E and 35.65 0 N , 26.22 0 E
, occurred in the central and east parts of the South Aegean
Sea Wadati-Benioff zone , respectively, exactly as it was expected
from a seismic activity migration pattern described by Papadopoulos
(1989). Such a space - time clustering of strong, intermediate
depth earthquakes was also observed in the past in the South
Aegean. Although the processing of instrumental records is in
progress , a preliminary examination of some macroseismic data
already available verify the characteristic, anisotropic propagation
of seismic energy radiated by intermediate depth shocks in the
Hellenic arc.