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Earthquake Hazards Program

Tectonic Summary

Magnitude 4.3 GREECE
Tuesday, August 24, 2004 at 12:38:50 UTC

Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver

Northern Greece
Plate Boundaries

The earth's lithosphere beneath the eastern Mediterranean constitutes a broad boundary region between three major tectonic plates, the Eurasia, Africa, and Arabia plates. The motions of the major plates drive smaller plates, and it is the shapes and motions of these smaller plates that determine the locations and focal mechanisms of most earthquakes in the region.

Most shallow earthquakes in central and northern Greece (depths less than 50 km) result from interaction between the Eurasia plate and the small Aegean Sea plate, which is moving southwest with respect to the Eurasia plate with a velocity of about 30 mm/year. The boundary between the Aegean plate and the Eurasia plate in central and northern Greece is diffuse. Seismicity is concentrated in east-trending and northeast-trending zones of deformation. The east-trending zones are most prominent in mainland Greece, are characterized by predominantly normal faulting, and have produced earthquakes with magnitudes of about 7. The northeast-trending belts are characterized by predominately strike-slip fault earthquakes. A northeast-trending zone of predominantly strike-slip earthquakes occurs off the west coasts of Cephalonia and Lefkada, western Greece, and other northeast-trending zones occur beneath the Aegean Sea east of the Greek mainland. In the twentieth century, an earthquake of magnitude 7.2 occurred on a northeast-trending strike-slip fault beneath the northern Aegean Sea.

The Africa plate subducts beneath the Aegean Sea plate along the Hellenic arc, from the western Peloponnesus through Crete and Rhodes to western Turkey, at a rate of almost 40 mm/year. Intermediate-depth earthquakes (depths greater than 50 km) occur within the northward dipping Africa plate in the mantle below central Greece, beneath the overriding Aegean Sea plate. Intermediate-depth earthquakes typically cause less damage on the ground surface above their foci than is the case with similar magnitude shallow-focus earthquakes, but they are sometimes felt at greater distances from the epicenter.

A belt of shallow-focus seismicity along the western coast of Greece to the north of Lefkada, and extending north along the Adriatic coast of the Balkan Peninsula, is characterized by reverse fault earthquakes occurring in response to northeast-southwest crustal convergence. This zone produced an earthquake of magnitude 7.0 in 1979, centered beneath the coast of Montenegro.


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