TR method: a practical tool to analyze focal mechanisms and identify the ‘real’ seismogenic fault of an extensional or compressional shallow earthquake sequence

Abstract

The recently proposed TR method that separates heterogeneous fault-slip data in extensional and compressional stress regimes is applied on focal mechanisms of a number of earthquake sequences aiming to identify the main seismogenic fault plane. The method only requires the earthquake sequence to be classified as related to an extensional or compressional stress regime. In particular, the nodal planes of the compressional or extensional focal mechanisms have been subdivided into two opposite dipping groups taking into account the average trend of their horizontal P or T axis. In the TR diagrams where we plot the slip deviation (from the dip-slip activation) of the nodal plane versus the angle between the dip direction of the nodal plane and the average trend of the P or T axis, the comparison of the geometric, kinematic, dynamic and mechanical compatibility of the two opposite nodal plane groups is possible. Moreover, a quality number (TRn) is devised in order to assess the two opposite facing groups quantitatively. Five earthquake sequences related with an extensional stress regime, and two related with a compressional stress regime, are processed in order to demonstrate and validate the potential and efficiency of the method. These cases indicate that the TR method can be fairly used as a practical tool to assess which nodal planes are more likely to identify the main seismogenic fault plane. The main advantages of the method are (a) the assessment of the nodal plane groups without the necessity to determine all the parameters of the driving stress tensor, i.e., stress axes and stress ratio, (b) the consideration of the slip tendency.