Computer derived focal mechanisms for selected earthquakes off the south and west coasts of Turkey

Abstract

Focal mechanisms of earthquakes, two of them in the Gulf of Antalya and one in the eastern Aegean Sea, were determined by using both P-wave polarities and S-wave polarization angles. Additionally, a focal mechanism solution for a third Gulf of Antalya earthquake was determined from P-wave polarities only. The polarity data were read from long period vertical (LPZ) seismograms of World Wide Standard Seismograph Network (WWSSN) stations. Additional polarity data were obtained from published earthquake listings. S-wave polarization angles were determined from particle motion diagrams for both the linear, and non-linear distance ranges. To determine polarization angles for the non-linear range stations, a correction technique by Nuttli (1964) was applied. A revised version of the computer program written by Guinn (1977) was employed to obtain. the mechanism solutions. The focal mechanisms for the Gulf of Antalya events indicated strike-slip normal (28 May 1979 and 26 April 1981 events) and strike-slip thrust faulting (1 June 1977 event). All of the earthquakes occurred in this region are not far more than 50 kilometers from each other with focal depths ranging from 66 to 98 kilometers. The occurrence of opposite type of faults in the Gulf of Antalya indicate a complex tectonics in that region. Additionally, the focal mechanism of the Aegean Sea earthquake showed east-west striking normal faulting. The solution is in agreement with the focal mechanisms of the previous regional events (McKenzie, 1972 and Alptekin, 1973), indicating the continuation of the east-west oriented graben systems of western Turkey in the Aegean Sea.