ALBERT

Notes: An interesting aspect of the nucleation of large Eastern Mediterranean earthquakes is the emergent nature of their source time functions. Several large and shallow, normal, strike-slip and thrusting events, which occurred in other parts of the world, and broke the surface for which the source time functions have been determined using the same inversion procedure, exhibit similar characteristics. These emergent source time functions may reflect a difficulty in breaking barriers, the localized region of the fault which remains unbroken at the completion of the dynamic fault process, at which ruptures nucleated, or some particular physical process (e.g. creep instability) that may commonly occur during the rupture of large earthquakes. Earthquakes in similar tectonic settings with similar source mechanisms reflect common patterns.

Notes: In this paper we examine the connection between the westward motion of Turkey relative to Europe and the extension in and around the Aegean Sea. The principal new data available since the last attempt to synthesize the tectonics of this region by McKenzie (1978) are much improved focal mechanisms of earthquakes, constrained by P and SH body wave modelling as well as by first motions. These mechanisms show that the faulting in the western part of the Aegean region is mostly extensional in nature, on normal faults with a NW to WNW strike and with slip vectors directed NNW to NNE. There is evidence from palaeomagnetism that this western region rotates clockwise relative to stable Europe. In the central and eastern Aegean, and in NW Turkey, distributed right-lateral strike–slip is more prevalent, on faults trending NE to ENE, and with slip vectors directed NE. Palaeomagnetic data in this eastern region is more ambiguous, but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations, relative to Europe, in the south. The strike–slip faulting that enters the central Aegean from the east appears to end abruptly in the SW against the NW-trending normal faults of Greece.The kinematics of the deformation is controlled by three factors: the westward motion of Turkey relative to Europe; the continental collision between NW Greece–Albania and the Apulia–Adriatic platform in the west; and the presence of the Hellenic subduction zone to the south. As the right-lateral slip on the North Anatolian Fault enters the Aegean region it splays out, becoming distributed on several parallel faults. The continental shortening in NW Greece and Albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed E–W right-lateral shear, and thus leads to E–W shortening in the northern Aegean, which is compensated by N–S extension as the southern Aegean margin can move easily over the Hellenic subduction zone. The dynamics of the system, once initiated, is self-sustaining, being driven by the high topography in eastern Turkey and by the roll-back of the subducted slab beneath the southern Aegean.The geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate. In spite of its extreme simplicity, a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the Aegean region, including: the slip vectors and nature of the faulting in the eastern and western parts; the senses and approximate rates of rotation; the overall extensional velocity across the Aegean; and the distribution of strain rates, as seen in the seismicity and topography or bathymetry, and using geodetic measurements.As part of this study, we re-examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 Gulf of Corinth earthquakes, and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting. An important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves.

Notes: The average S-P-wave traveltime residuals from 29 earthquakes in the Aegean and the Hellenic Trench near Crete imply that there are lateral variations in structure beneath these regions. Average S-P-wave residuals, measured with respect to the Jeffreys-Bullen (J-B) tables (1940), from 14 of 17 earthquakes in the Aegean are greater than +1 s. The largest positive values of residuals in the Aegean imply that the average velocities in the crust and the upper mantle of these parts of the Aegean are lower than those beneath Crete. The existence of volcanoes, heat flow observations, an assumption of relatively low seismic-wave velocities, and variations in S-P-wave residuals from earthquakes in the Aegean provide evidence of lateral variations in uppermost mantle structure beneath the region. Residuals reflect systematic lateral variations that do not correlate well with elevation or with simple aspects of regional geology.

Notes: Long period P and SH waveforms and first motion polarities of P waves were used to determine the source parameters of the Çubukdaǧ (in the eastern section of the Büyük Menderes graben) earthquake of 1986 October 11, the waveforms used in this event include the WWSSN, and the digitally recorded seismograms of the DWWSSN, SRO and ASRO networks. the minimum misfit solution obtained by the inversion of teleseismic body waveforms are better constrained that those of the previously published Harvard Centroid-Moment Tensor solution, and are in good agreement with field observations and the local geometry of the graben.

Notes: It is now widely accepted that the rapid extension observed in western Turkey is mainly accommodated by large active normal faults which control the geomorphology. The NE-SW trending Burdur, Acigöl and Baklan basins bounded by large faults form a system of half-graben whose orientation is evident in both the topography and the tilting of Neogene sediments adjacent to them.We used long-period P- and SH-waveforms to determine the source parameters of the two largest earthquakes of the 1971 May 12 sequence that occurred in the Burdur region. The main shock was followed by many aftershocks, which were distributed in a broad zone elongated NE-SW, parallel to the long axes of the basins in the region.Documented surface breaks of the 1971 event suggest that the northwest-dipping faults along the southern margin of the Burdur Basin are those which moved. Well-exposed fault planes are found dominantly to the south of Burdur lake; the surface dips of these faults are greater than that of the NW-dipping nodal plane obtained for the main shock. Combined seismological and geological observations suggest that these faults have listric geometries.

Notes: The East Anatolian Fault Zone accommodates most of the motion between the Arabian plate and the apparently little-deforming interior of central Turkey. The direction of overall slip across this zone is crucial to the determination of the slip rate on the North Anatolian Fault. We use long-period P- and SH-waveforms to determine the source parameters of the four largest earthquakes that occurred in, or near, the East Anatolian Fault Zone in the last 35 years. Only one of these actually involved left-lateral strike–slip motion on a NE–SW fault. But the other three, and the nearby 1975 Lice earthquake, all had steeply dipping nodal planes with a NNW strike: if these were the auxiliary planes then all the earthquakes had a slip vector direction within about 10° of 063°. If this direction represents the Arabia–Turkey motion, then the slip rate on the North Anatolian Fault must be in the range 31 to 48 mm yr−1, with a probable value of 38 mm yr−1, and the overall slip rate across the East Anatolian Fault Zone must be about 29 mm yr−1 with a range of 25–35 mm yr−1.