Abstract
The Izmir Gulf and surrounding areas are located within an important geothermal area of Turkey in the Aegean region (western Anatolia). The region represents a natural laboratory where multi-disciplinary approaches in Earth sciences find a venue for the study of the dynamics of the coupled lithosphere-fault system. Seismological studies are being carried out to investigate the origin and the nature of the Earth structure. In the present study, we perform a first attempt to define the crust and upper mantle seismic properties using local earthquake tomography. The major tectonic features within the 170x90 km² rectangular area include the Aliaga, Balcova, Menemen, and Seferihisar geothermal systems and the Urla hot spring. 783 well-located earthquakes, each one with at least 11 travel time readings, are selected to assemble a bulk of 7676 P-phase and 6431 S-phase picks. The 3D distribution of P-wave velocity (V_p), S-wave velocity (V_s), and the P-wave/S-wave velocity ratio (V_p/V_s) allows a resolution of the Earth crust down to 30 km of depth. The thickest and broadest low-V_p velocity anomalies are found beneath the Outer Gulf and north of Izmir Bay. A high V_p/V_s ratio corresponding to a low-V_s model is detected beneath the Menemen geothermal system. From local tomography, we identify four main seismic layers at variable depths from the surface to the lower crust. The lower crust/mantle discontinuity is found at approximately 27 km of depth. This transition is characterized by an undulated shape, and intrusive bodies are imaged beneath the main geothermal systems.

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