Abstract

The main goal of studying double couple sources in seismology is to obtain a complete description of the physics of the source. In other words, it is to describe the force system equivalent to physical mechanism in earthquake focus. Seismic moment tensor is a mathematical quantity defining the earthquake source. It characterizes all the information about the source. With this purpose, it describes the equivalent forces of a seismic source. Then, seismic moment tensor is a necessary and sufficient condition for the description of the physics of seismic sources. Theoretically, a seismic source may be composed of different force systems. Each force system describes an elementary source model. A double couple source is just one of them. Nevertheless, the double couple system of equivalent forces is the most appropriate model for most earthquakes. However, an earthquake may also be a non-double couple. In practice, there is some evidence that supports the theory of non-double couple earthquake source mechanism. Therefore, the following four source models are considered: i) General moment tensor, ii) Pure-deviatoric moment tensor, iii) A moment tensor characterizing the pull-apart rupture, iv) A moment tensor characterizing the listric fault model. Any moment tensor can be represented by a linear combination of different elementary sources. Consequently, each source model above is a complex source and defined by a moment tensor. These are considered as time-independent moment tensors. The first source is generated by an isotropic component, two strike-slip faults, two vertical dip slip faults, and a reverse fault. The second source is like the first source, but there is no isotropic part in this complex source. The third source is a complex source contributed with respect to specific proportion of two strike-slip faults and a normal fault. The analysis of complex moment tensors calculated for these three source models shows that each source has a non-double couple mechanism. Finally, the fourth source model is a linear combination of more than one normal fault, the dips of which are gradually decreasing with depth. Nevertheless, this complex source is a double couple. However, this evaluation is a result of a calculation only due to fault geometry. Thus, to assume double-couple for the earthquake source is an incomplete approximation since the complex source assumption also contains the deviation from double couple.

Key words: Earthquake source, equivalent body forces, moment tensor, complex source, fault plane solution, pull-a-part structure, listric normal fault, Turkish earthquakes.