The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East

Abstract

This paper is concerned with the relationship between the overall motion across a zone of distributed continental deformation and the seismic moment tensors of earthquakes that occur within it. The overall deformation in the zone is described by the deformation gradient tensor L, which may be split into a symmetric part, S, and an antisymmetric part, A. S is the strain tensor, and can always be determined from the sum of the moment tensors, following the result of Kostrov (1974). A corresponds to a rigid body rotation, and is in principle unobservable seismically: the moment tensors contain no information about A, regardless of whether the ambiguity between fault and auxiliary planes is resolved. From S the integrated rates of motion normal and parallel to the zone boundary, as well as vertically, can be calculated. Of these, only the motion normal to the zone is specified by the motion across its boundaries. In general, S (and hence L) is not specified by the motion of the plates bounding the zone. Only if some a priori assumptions about L are made, can information about A be recovered from the seismic moment tensors. Otherwise A must be determined independently from paleomagnetic or geodetic measurements. These results are applied to the Mediterranean region to see whether the motion between the relatively rigid regions of central Iran, Turkey, Arabia, Africa, the Adriatic Sea and Eurasia is accommodated seismically within the upper crust of the wide deforming zones that bound these regions. In NE Iran, the North Anatolian Fault Zone and the Aegean Sea all or most of the deformation is probably taken up seismically. In the Zagros, Caucasus, Hellenic Trench and western Mediterranean probably 10 per cent or less of the upper-crustal deformation is seismic and the rest must be accommodated by creep. The Cyprus arc, the East Anatolian and Dead Sea Fault Zones have had insufficient seismicity this century for any conclusions to be drawn. The seismicity in central Italy and Yugoslavia accounts for velocities of about 2 mm yr−1 normal to the deforming zones, but there is no independent estimate of the velocities on the borders of the Adriatic Sea with which to compare these. The seismicity in the Aegean region indicates very high stretching velocities (c. 60 mm yr−1) and strain rates (c. 4 × 10−15 s−1). These in turn require correspondingly high subduction rates (c. 100 mm yr−1) in the Hellenic Trench. If these rates have been constant in time, it is unlikely that the tip of the sinking slab beneath the southern Aegean began to be subducted more than 5 Ma ago. These high-stretching rates in the Aegean, if extrapolated back to the Pliocene, are compatible with observed finite strains and paleomagnetic rotations. They are also likely to have raised to a shallower depth the isotherm corresponding to the seismic-aseismic transition in the crust, perhaps accounting for the relatively shallow focal depths of normal faulting earthquakes in the Aegean compared with those in other areas of continental extension where strain rates are lower. It is possible that the reason for the dominantly aseismic deformation in the Zagros and Hellenic Trench is related to the great thickness of sediments, partly decoupled from the basement by salt, in both places. This may lead to elevated basement temperatures and inhibit upward fault propagation, thus restricting the size of seismogenic fault planes (and hence seismic moment) and causing the sedimentary cover to deform independently from the basement, partly by folding. However, this explanation has serious drawbacks and is not easily applicable to other areas, notably the western Mediterranean.