The strain pattern in the western Hellenic arc deduced from a microearthquake survey

Abstract

SUMMARY than loo0 earthquakes recorded during 7 weeks in 1986 in the Peloponnese and surrounding areas show shallow seismicity spread over a wide area bounded by the Hellenic trench. The highest level in the energy release is for clusters located along the Hellenic trench, where changes in the morphology are seen. A few clusters are also observed, as at the intersection between the Gulf of Corinth and the Gulf of Patras, near the Lake Trikhonis, or between Kythira and Crete. In the Peloponnese, the shallow earthquakes do not define single faults, but are diffusely distributed. A higher concentration and a deepening of the foci (within the whole crust) towards the west seem to reflect a higher strain rate there. Fault plane solutions exhibit a scattered pattern for earthquakes shallower than 11 km, but for earthquakes deeper than 11 km, they show some consistency in the orientation of P-, T-, and B-axes. Gently dipping nodal planes are seen all over the Peloponnese, with the conjugate vertical plane striking in various directions, and with no consistency in the sense of motion. These earthquakes are located between 8 and 18 km deep and could reflect a decoupling between the lower and the upper crust. Reverse faulting is seen in the Gulf of Kefallinia and the western Peloponnese. The P-axes trend NE-SW to E-W. Normal faulting is seen in the Gulf of Corinth and in central Peloponnese with the T-axes trending N-S, and in the southern Peloponnese, where the T-axes trend NW-SE. The deformation over the western Hellenic arc, revealed by fault plane solutions of earthquakes with depth between 11 and 40 km, appears to be the superposition of two phenomena. First, throughout the Aegean, crustal extension with orientation roughly parallel to the trench dominates, with crustal shortening and subduction along the Aegean arc. Superimposed on this is a more local effect of the collision between the Aegea and Apulia which seems to induce horizontal compression roughly perpendicular to the Aegean arc west of the Peloponnese and Epirus. The compression due to this decays eastward, at a distance comparable with the width of the collision zone. The deepening of the brittleductile transition is likely to be due to an increase of the strain field towards the Ionian islands.