Crustal seismicity in and around Turkey

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 dat a for the period 1964-1998. Fits of various magnitude scales to the observa tions have been used to construct a homogeneous catalogue. Depth distributi on of the parameters derived from the Gutenberg-Richter relationship reveal s 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 d ominant in the total seismicity reported, and give rise to substantial chan ges to the vertical distribution of these parameters. It is also observed t hat the parameters of either a or b alone may not be used as a measure of r egional seismicity of the study area in which high and low seismic activity observed over short distances. In contrast, distribution of modal (a/b) va lues provides detailed images of the local areas presented by high and low seismic zones. This study presents evidence that low b-values are associate d with major strike-slip faults, particularly exhibiting high apparent stre sses.