Vertical and horizontal variations of the frequency of crustal earthquakes (h less than or equal to 35 km) that have occurred in and around Turkey are analyzed, using data from the Global Hypocenter Data Base and the IRIS data for the period 1964-1998. Fits of various magnitude scales to the observations have been used to construct a homogeneous catalogue. Depth distribution of the parameters derived from the Gutenberg-Richter relationship reveals that there is a depth dependence in the a- and b-values of the frequency distribution. It is observed that unknown focal depths (0, 10, 33 km) are dominant in the total seismicity reported, and give rise to substantial changes to the vertical distribution of these parameters. It is also observed that the parameters of either a or b alone may not be used as a measure of regional seismicity of the study area in which high and low seismic activity observed over short distances. In contrast, distribution of modal (a/b) values provides detailed images of the local areas presented by high and low seismic zones. This study presents evidence that low b-values are associated with major strike-slip faults, particularly exhibiting high apparent stresses.