Seismological investigations in Western Anatolia, NW Turkey have identified linear patterns of earthquake epicentres outlining a wedge-shaped block in the area of the Marmara Sea. This block shows different seismic characteristics from the rest of Western Anatolia and appears to act as a separate tectonic unit. Earthquake fault-plane mechanisms show that the Marmara block is being rotated and sheared in order to accommodate the right-lateral motion of the north Anatolian Fault and the extensional tectonics of the southwestern Anatolia. Investigations of the behaviour of seismic shear-waves suggest that neotectonic stress aligns water-filled microcracks throughout the upper (brittle) 10–20 km of the crust and that in situ stress directions can be monitored by analysis of shear waves recorded on three-component instuments.

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