Insight into the crustal structure of the eastern Marmara region, NW Turkey

In order to investigate crustal structure beneath the eastern Marmara region, a seismic refraction survey was conducted across the North Anatolian Fault (NAF) zone in north west Turkey. Two reversed profiles across two strands of the NAF zone were recorded in the Armutlu Highland where a tectonically active region was formed by different continents. We used land explosions in boreholes and quarry blasts as seismic sources. A reliable crustal velocity and depth model is obtained from the inversion of first arrival travel times. The velocity-depth model will improve the positioning of the earthquake activities in this active portion of the NAF. A high velocity anomaly (5.6-5.8 km s(-1)) in the central highland of Armutlu block and the low velocity (4.90 km s(-1)) pattern north of Iznik Lake are the two dominant features. The crustal thickness is about 26 +/- 2 km in the north and increases to about 32 +/- 2 km beneath the central Armutlu block in the south. P-wave velocities are about 3.95 km s(-1) to 4.70 km s(-1) for the depth range between about 1 km and 5 km in the upper crust. The eastern Marmara region has different units of upper crust with velocities varying with depth to almost 8 km. The high upper crust velocities are associated with Armutlu metamorphic rocks, while the low velocity anomalies are due to unconsolidated sedimentary sequences. The western side of Armutlu block has complex tectonics and is well known for geothermal sources. If these sources are continuous throughout the portions of the crust, it may be associated with a granitic intrusion and deformation along the NAF zone. That is, the geothermal sources associated with the low velocity may be due to the occurrence of widespread shear heating, even shear melting. The presence of shear melting may indicate the presence of crustal fluid imposed by two blocks of the NAF system.