MoPaD--Moment Tensor Plotting and Decomposition: A Tool for Graphical and Numerical Analysis of Seismic Moment Tensors

Abstract

Online material: Software source code; examples; sample plots. The graphical representation of a focal mechanism as a plot of the radiation pattern on the focal sphere (sometimes referred to as “beachball diagram”) is well established. It consists of a spherical projection on a unit sphere centered at the source point of a seismic event. The sign of the P -wave radiation pattern is shown as colored areas on the sphere (as described, e.g. , by Udias 1999; Stein and Wysession 2003; and Crotin 2004). For pure shear motion on a plane, the focal sphere may be fully described by the angles of strike, dip, and slip-rake of this fault plane ( cf. , Shearer 1999; Aki and Richards 2002). In this case the construction of the spherical projection into the R 2 is straightforward ( e.g. , Stein and Wysession 2003, chapter 5). Several tools are available to do this work. They exist either as a more or less configurable, independent software package (Snoke 2003; Srivastava et al. 2006; Luis 2007; Haeussler and Labay 2007), as supplements for GMT, MATLAB (Creager 1997; Center for Earthquake Research and Information 2008) or Mathematica (Scherbaum et al. 2010), or as an online applet (Helffrich 2007). Almost all of the existing software consider only these pure shear cracks. The visualisation of a more complex source mechanism, e.g. , an arbitrary seismic moment tensor with six independent entries, is often not possible. Only a few tools exist at all that can handle this general case. The most commonly used one is the psmeca -tool in GMT (Wessel and Smith 1999). However, projections for arbitrary viewpoints are not possible with psmeca . We present the free platform-independent command line tool MoPaD , written in the Python programming language that allows one …