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.