Three-dimensional structure of Vp in the upper crust of the Sakarya Basin, NW Turkey

Abstract

In this study, our aim is to obtain a 3-D velocity model of the Sakarya Basin and its surrounding. First of all, the various velocity models conducted previously are used as initial models and then a new model representing the region exactly is calculated. In selection of the earthquakes with particular criteria, the high-quality events are chosen to obtain reliable results. A total of 311 aftershocks located between Sakarya and Bolu provinces of the North Anatolian Fault Zone (NAFZ) are selected at the end of this process and a one-dimensional P-wave velocity model is calculated. The earthquakes have been selected among the 3000 aftershocks of the 1999 A degrees zmit earthquake recorded by the digital three-component seismometers that deployed in the section of the NAFZ remaining between Sakarya and Bolu provinces. The one-dimensional P-wave velocity model obtained in this study is used as a reference model in the tomographic inversion process to calculate the three-dimensional velocity structure of the region. Then, several three-dimensional cross sections have been generated. When those sections are examined, it is observed that the aftershocks are concentrated in the layers down to 16 km in depth. These events provide more accurate and reliable results. When 1-D velocity structure has been analyzed, it is observed that the deeper velocity structures of the initial model given the values remain constant. At this point, the P-wave velocity values in the layers starting from 4.21 to 7.23 km/s increase steadily and the velocity structure of the region is brought out as three dimensional, and its compatibility with its geology and tectonics is evaluated. By those results, while the different velocity anomalies are determined in the study areas, in particular presence of the low velocity zones are clearly seen in segments of the NAF. In this case, the region is said to be quite compatible with the geology and tectonics.