Stress tensor inversion from focal mechanism solutions and earthquake probability analysis of Western Anatolia, Turkey

Shah, Syed Tanvir
Western Anatolia is one of the most important tectonic elements of Turkey, and constitutes the eastern margin of Aegean Extensional Province. The area is one of the most seismically active continental margins around the globe experiencing NS extension. Earthquake data in this study is used to analyze the active stress patterns and to estimate the earthquake probabilities for different sub-regions in western Anatolia. Various processing techniques to attain homogeneity are applied to the earthquake catalogue for the area, which is then used for investigation of spatial and temporal variations in frequency magnitude distribution. More frequently occurring smaller magnitude earthquakes represented by high b-values are associated with relatively low stress conditions. The prominent high b-value regions identified in the study area are along Bakırçay Graben and between İzmir and Manisa. Spatial distribution of high b-values correlates well with the distribution of hot springs and high heat flow anomalies. The temporal variations in b- values are associated with major earthquakes in the region. Before the earthquake, b-vales show a small scale decrease followed by an abrupt increase after the event and this variation disappears after some time. Focal mechanism solution data is used for determining the orientations and ratios of principal stress components in the area. Minimum principal stress (σ3) is sub horizontal in the region trending in almost NS (N15 E) direction and the region is characterized mainly by normal fault regime. The area is sub-divided into 10 sub-regions based on the variations in focal mechanism solutions and tectonic setting. The results for sub-regions showed that apart from the predominant extensional regime; Bakırçay and Gulf of Sığacık sub-regions are dominated by strike-slip, while Manisa and Soma sub-regions are dominated by extensional strike-slip regimes. These sub-regions roughly coincide with the İzmir Balıkesir Transfer Zone (IBTZ), which is characterized by mixture of normal and strike-slip faults. Computed principle stress directions shows that the area is mostly characterized by NS extension except Gökova region marked by NWSE extension and Dinar and Fethiye regions displaying more variable stress tensor solutions with more dominant NE-SW extension. The earthquake probabilities computed using Gutenberg-Richter relation and Gumbel extreme value method shows that the whole region has a return period of 3 and 7 years for a magnitude 6 earthquake. According to our results, Simav and Gulf of Gökova sub-regions have highest and Manisa has lowest earthquake probabilities. The fractal dimension (Dc) analysis illustrates that Gulf of Gökova and Fethiye sub-regions have highest and Dinar-Burdur, Büyük Menderes and Gulf of Sığacık have lowest levels of seismicity clustering. The comparison of results computed for sub-regions also showed that a-, b- values and stress variance are negatively correlated with Dc and stress ratio (R) while Dc correlates positively with R.
Citation Formats
S. T. Shah, “Stress tensor inversion from focal mechanism solutions and earthquake probability analysis of Western Anatolia, Turkey,” M.S. - Master of Science, Middle East Technical University, 2015.