Relocation of the December 20, 2007 Bala earthquake (Mw 5.4) and it's aftershocks by using a new one-dimensional seismic velocity model for the Central Anatolia

The use of highly accurate ground structure models in seismological studies has importance for the reliability of research findings and results. A seismic velocity structure of the region with high accuracy was required for the determination of the locations of Mw 5.4 Bala, Ankara earthquake (December 20th, 2007) and its aftershocks. In order to provide this requirement, one-dimensional seismic velocity structure was determined by using data of 13 seismograph stations deployed in Central Anatolia. 2955 P- and S-wave phases read from 652 earthquakes down to 30 km depth were used to determine the model. Two accuracy tests have been applied to check the accuracy of the new model. One of them is the numerous shifting tests which are performed by shifting the earthquake’s latitude, longitude and depth in different directions and quantities. The other one is the comparison of the travel time residuals before and after the inversion for the model determination. The new model, assessed by accuracy tests, was used to relocate the Bala earthquakes. Earthquake locations are also calculated simultaneously in the iterative inversion process in which the new model is identified. The preliminary location parameters of the earthquakes calculated by using the previous velocity models introduced by other researchers were accepted as the initial parameters for the inversion. The double difference method which is based on the minimization of the residuals between the differences of the theoretical and observed travel times for earthquake pairs was performed to relocate the earthquakes by using the new one-dimensional model. The average location error is observed to be lower in the new model when compared to other seismic velocity models