Computers & Geosciences 30 (2004) 259–265 SORTAN: a Unix program for calculation and graphical presentation of fault slip as induced by stresses $ Christophe Pascal* Laboratoire de Tectonophysique, UPMC, bte 129, 4, pl. Jussieu 75252 Paris c ! edex 05, France Received 27 January 2003; received in revised form 17 October 2003; accepted 30 October 2003 Abstract Stress inversion programs are nowadays frequently used in tectonic analysis. The purpose of this family of programs is to reconstruct the stress tensor characteristics from fault slip data acquired in the field or derived from earthquake focal mechanisms (i.e. inverse methods). Until now, little attention has been paid to direct methods (i.e. to determine fault slip directions from an inferred stress tensor). During the 1990s, the fast increase in resolution in 3D seismic reflection techniques made it possible to determine the geometry of subsurface faults with a satisfactory accuracy but not to determine precisely their kinematics. This recent improvement allows the use of direct methods. A computer program, namely SORTAN, is introduced. The program is highly portable on Unix platforms, straightforward to install and user-friendly. The computation is based on classical stress-fault slip relationships and allows for fast treatment of a set of faults and graphical presentation of the results (i.e. slip directions). In addition, the SORTAN program permits one to test the sensitivity of the results to input uncertainties. It is a complementary tool to classical stress inversion methods and can be used to check the mechanical consistency and the limits of structural interpretations based upon 3D seismic reflection surveys. r 2004 Elsevier Ltd. All rights reserved. Keywords: Tectonic stress,Fault slip directions,Subsurface faults,Uncertainties,Graphical output 1. Introduction The methods of stress inversion are nowadays commonly used in tectonic analysis (e.g. Barrier et al., 2002) and are applied to both fault slip data acquired in the field and nodal planes associated to earthquake focal mechanisms. These methods are based upon the Wallace–Bott hypothesis (Wallace, 1951; Bott, 1959) that assumes that the resolved shear stress vector applied to a fault plane and the fault slip vector are parallel. Since the 1970s numerous computer programs (e.g. Carey and Brunier, 1974; Angelier, 1979, 1990; Gephart and Forsyth, 1984; Michael, 1984; Reches, 1987; Lisle, 1988; Sperner et al., 1993) were developed to handle stress inversion. Although some differences in the treatment of the numerical problem exist, all the stress inversion programs are based on similar algorithms, that aim to minimise the angular misfit between a collection of actual fault slip directions and the corresponding collection of computed shear stress vectors. As a result the collection of optimal stress tensors, characterised by similar orientations for the principal stress axes and similar shapes for the stress ellipsoids, is determined (e.g. Angelier, 1994; Ramsay and Lisle, 2000). In contrast, and despite the pioneering work of Wallace (1951), little attention has been paid to direct methods (i.e. reconstruction of potential slip directions given the characteristics of the applied stress tensor). The increase in resolution of reflection seismic data during the 1990s opened the way to the use of direct methods in tectonic analyses. The geometry of ARTICLE IN PRESS $ Code available from server at http://www.iamg.org/ CGEditor/index.htm. *Present address: Faculteit der Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, Amsterdam 1081 HV, Nether- lands. Fax: +31-20-646-2457. E-mail address: pasc@geo.vu.nl (C. Pascal). 0098-3004/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cageo.2003.10.008