News, January-June 2009: see pages FranceTaiwan (International Laboratory update),  India (seismotectonic regimes and plate collision), Taiwan (seismic hazard in western Taiwan, also earthquake cycle in eastern Taiwan)

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   From left to right:

(1) Stereoplot of fault slips and tension veins (DIAGRA) and stress from right dihedra method (DIEDRE)  

 (2) Lithospheric structure of the Taiwan collision zone (see also page FranceTaiwan)

(3) Example of stereoplot issued from stress tensor inversion (TENSOR)

(4) Regional earthquake hazard levels inferred from historical seismicity, western Taiwan (see page Taiwan)

 

 

Origin and main features of the Tector software system 

 

Since 1974, the Tector system (Tector means Tectonic orientation) is mainly devoted

to the analysis of brittle tectonic data (faults, non-faults, tension and pressure structures)

and focal mechanisms of earthquakes. It provides stereoplots of various types, stress

tensor determinations, earthquake mapping, etc.

         

        The first related papers were published from 1975 to 1980, most of them in France (more recent papers are referred to later in these pages): ANGELIER J. (1975). - Sur l'analyse des déplacements dus au jeu d'une population de failles. Exemples en Crète (Grèce). C. R. Acad. Sci. Paris, (D), 280, p. 1657-1660 ; ANGELIER J. (1975). - Sur l'analyse de mesures recueillies dans des sites faillés: l'utilité d'une confrontation entre les méthodes dynamiques et cinématiques. C. R. Acad. Sci. Paris, (D), 281, p. 1805-1808 (Erratum: Ibid., (D), 283, 1976, p. 466); ANGELIER J. (1975). - Sur l'analyse des phases superposées de la tectonique cassante: la néotectonique et les failles du massif de l'Ida (Crète, Grèce). Ann. Soc. géol. Nord, 95, 3, p. 183-200 ; ANGELIER J. (1975). - Sur un apport de l'informatique à l'analyse structurale: exemple de la tectonique cassante. Rev. Géol. dyn. Géogr. phys., (2), XVII, fasc.2, p. 137-146 ; ANGELIER J. (1977). - La reconstitution dynamique et géométrique de la tectonique de failles à partir de mesures locales (plans de faille, stries, sens de jeu, rejets): quelques précisions. C. R. Acad. Sci. Paris, (D), 285, p. 637-640 ; ANGELIER J. (1979). - Determination of the mean principal directions of stresses for a given fault population. Tectonophysics, 56, p. T17-T26 ; ANGELIER J. et GOGUEL J. (1979). - Sur une méthode simple de détermination des axes principaux des contraintes pour une population de failles. C. R. Acad. Sci. Paris, (D), 288, p. 307-310 ; ANGELIER J. et MANOUSSIS S. (1980). - Classification automatique et distinction de phases superposées en tectonique cassante. C. R. Acad. Sc. Paris, (D), 290, p. 651-654.

 

The last century sofware package, Tector 1994, may be obtained for academic, non-profit use following request to author. Users are requested to (1) let author know about their work, (2) provide acknowledgement and (3) refer to author's related papers. The Tector XXI softwares, including dedicated analyses of focal mechanisms, window-based screen graphics and Postcript output, are not exportable but their use can be involved in co-operation scientific projects. 

International papers partly describe the Tector system regarding the stress inversion of fault slip data and earthquake focal mechanism solutions (as done by Tector softwares such as TENSOR and PHASES). These papers are referred to in the following Sections. Other papers with application to various regions of the world are mentioned in several pages of this site.

 

For the beginner who needs to learn about brittle tectonic analysis and inversion

of fault slip data, a general introduction is given in ANGELIER J. (1994). - Palaeostress

analysis of small-scale brittle structures. Chapter 4 in: 'Continental Deformation',

edited by P. Hancock, Pergamon Press, 421 p. (p. 53-100).

 

Data input and basic operations

 

         After downloading executable Tector 1994 programs, the user can enter his own data using MESURE, draw stereoplots using DIAGRA, quantify stress compatibility domains with DIEDRE and determine stress tensors with TENSOR. One can draw a variety of steroplots and rose diagrams, perform data selection, rotate data sets, etc. Two examples are given below, using programs ROSACE and ROTILT. Help to user exists (request in program or "?" key in Tector 1994, menu bar in Tector XXI). 

 

  

 Example of basic geometrical analysis: orientation distribution illustrated with program ROSACE. Set of 33,878 earthquake focal mechanisms from the South Iceland Seismic Zone. Source: ANGELIER J., BERGERAT F., STEFANSSON R. and BELLOU M. (2008). - Seismotectonics of a newly formed transform zone near a hotspot: earthquake mechanisms and regional stress in the South Iceland Seismic Zone, Tectonophysics, 447, p. 95-116 [doi:10.1016/j.tecto.2006.07.016]. Note the consistency of P-axes (b) and T-axes (c) despite the large number (nearly 40,000) and azimuthal dispersion (a) of mechanisms. In Tector 2000 series, program ROSACE also provides various statistics and histograms, such as for depth or magnitude distributions of earthquakes (see page Iceland).

 

 

Example of basic geometrical analysis: backtilting of a data set performed with program ROTILT. Normal faults green, reverse faults purple, poles as open dots. Bedding (thin grey dashed lines) used as guide to backtilting. The set of fault data observed in the field includes reverse and flat faults (diagram on left). Backtilting restores the original fault pattern (diagram on right). Two intermediate stages of backtilting are added to illustrate changes in fault geometry during folding. These data illustrate normal faults of the Mesozoic Arabian margin tilted during the late Cenozoic folding of the Zagros. Because folds trend parallel to paleo-margin, the tilt axis trend and the paleo-fault strikes coincide in this case. For more detail about these events, see page Iran.

 

Determining stress regimes from tectonic data or focal mechanisms

 

         The rigth dihedra (or P&T dihedra) method is described in ANGELIER J. et MECHLER P. (1977). - Sur une méthode graphique de recherche des contraintes principales également utilisable en tectonique et en séismologie: la méthode des dièdres droits. Bull. Soc. géol. France, (7), XIX, 6, p. 1309-1318. 

 

 

Example of application of the right dihedra method. Program DIEDRE, version 2004. Distributions of pressure and tension on the sphere for contrasting magnitude classes. Set of 33,878 earthquake focal mechanisms from the South Iceland Seismic Zone. The map and depth distribution of these earthquakes are shown on page Iceland. Upper row, primary subset. Lower row, secondary subset. Stereoplots: equal-area projection, lower hemisphere. From left to right: negative earthquake magnitudes (a and b), magnitudes higher than 2 (c and d), magnitudes higher than 3 (e and f), magnitudes higher than 4 (g and h). Note the smoothed aspect and symmetrical pattern of pression and tension on stereoplots to the left, decreasing with the number of earthquakes towards the right. More important, note that in each subset the stress axes resulting from inversion are the same for different magnitudes ranges, suggesting that even the focal mechanisms of very low magnitude earthquakes, as determined by the Iceland Meteorological Institute, are reliable. Source: ANGELIER J., BERGERAT F., STEFANSSON R. and BELLOU M. (2008). - Seismotectonics of a newly formed transform zone near a hotspot: earthquake mechanisms and regional stress in the South Iceland Seismic Zone, Tectonophysics, 447, p. 95-116 [doi:10.1016/j.tecto.2006.07.016]. 

 

        Several methods for stress tensor determinations are involved in programs TENSOR and PHASES. The oldest ones are described in ANGELIER J. (1975). - Sur l'analyse de mesures recueillies dans des sites faillés: l'utilité d'une confrontation entre les méthodes dynamiques et cinématiques. C. R. Acad. Sci. Paris, (D), 281, p. 1805-1808 (Erratum: Ibid., (D), 283, 1976, p. 466), and  ANGELIER J. (1979). - Determination of the mean principal directions of stresses for a given fault population. Tectonophysics, 56, p. T17-T26. A heavier approach, not used now, is described in ANGELIER J., TARANTOLA A., MANOUSSIS S. and VALETTE B. (1982). - Inversion of field data in fault tectonics to obtain the regional stress. I: single phase fault populations: a new method of computing the stress tensor. Geophys. Journ. roy. astr. Soc., 69, p. 607-621.

        The simplest stress determination method, present in programs TENSOR and PHASES, is the direct inversion method fully described in ANGELIER J. (1990). - Inversion of field data in fault tectonics to obtain the regional stress. III. A new rapid direct inversion method by analytical means. Geophys. J. Int., 103, p. 363-376. This paper also provides practical examples of fault slip data inversion, including display of data sets (see Annexes), which can be used for Tector training.

         Application of these stress tensor inversion methods can be found in ANGELIER J. (1984). - Tectonic analysis of fault slip data sets. Journ. geophys. Res., 89, n°B7, p. 5835-5848 and ANGELIER J. (1989). - From orientation to magnitudes in paleostress determinations using fault slip data. Journ. struct. Geol., 11, 1/2, p. 37-50.

         A new method of seismotectonic stress inversion especially designed for sets of focal mechanisms of earthquakes was described in ANGELIER J.  (2002) - Inversion of earthquake focal mechanisms to obtain the seismotectonic stress IV- a new method free of choice among nodal planes. Geophys. J. International, 150, 3, p. 588-609.

           Example of result of stress inversions performed with the INVGLI program (Tector XXI version), based on the method mentioned above (Angelier, 2002), for the primary subset containing 73 % of the total data set of 33,878 earthquakes analysed in the South iceland Seismic Zone (see also page Iceland). Earthquake mapping done with program CARTAX. (Tector XXI). Each stereoplot refers to the inversion of a magnitude-based subset: (a) total subset, (b) negative magnitudes, (c) magnitudes below 1, (d) magnitudes 1 and above, (e) magnitudes 2 and above, (f) magnitudes 3 and above). All stress tensors are similar (confidence eliipses however differ), indicating that even very low magnitude earthquakes recorded by the SIL network monitored by the Icelandic Meteorological Institute provide significant focal mechanisms. All stress tensors shown for a 40% threshold value of ω (explanation: Angelier 2002), with stress axes σ1, σ2 and σ3 as 5, 4 and 3-branch stars respectively, confidence ellipses (60, 75, 90 %), pairs of arrows indicating trends of compression and extension, azimuth uncertainties, 0-1 scale (0=base, 1=top) for Ф value added in the upper-right corner. Map of epicentres of the primary subset added in (a). Source: ANGELIER J., BERGERAT F., STEFANSSON R. and BELLOU M. (2008). - Seismotectonics of a newly formed transform zone near a hotspot: earthquake mechanisms and regional stress in the South Iceland Seismic Zone, Tectonophysics, 447, p. 95-116 [doi:10.1016/j.tecto.2006.07.016].            

 

Advanced: separation of multiple stress regimes

 

     The determination and separation of multiple stress tensors can be done with heretogeneous data sets, using a dynamic clustering approach. This is done by the program PHASES described first by ANGELIER J. et MANOUSSIS S. (1980). Classification automatique et distinction de phases superposées en tectonique cassante. C. R. Acad. Sc. Paris, (D), 290, p. 651-654 and, in more detail, by ANGELIER J. (1984). Tectonic analysis of fault slip data sets. Journ. geophys. Res., 89, n°B7, p. 5835-5848. However, for users who are not extremely familiar with stress inversion, using the program PHASES is not recommended . 

 

 Example of automatic separation of data using the program PHASES (Tector XXI version). Within a total data set of 110,261 focal mechanisms of earthquakes from South Iceland (220,522 nodal planes) three main seismotectonic regimes of variable importance are distinguished. The largest subset fairly illustrates the main stress regime of the South Iceland Seismic Zone. For details, see ANGELIER J., SLUNGA R. F., BERGERAT F., STEFANSSON R. and HOMBERG C. (2004) Perturbation of stress and oceanic rift extension across transform faults shown by earthquake focal mechanisms in Iceland. Earth Planet. Sci. Letters, 219, p. 271-284 and ANGELIER J., BERGERAT F., STEFANSSON R. and BELLOU M. (2007). Seismotectonics of a newly formed transform zone near a hotspot: earthquake mechanisms and regional stress in the South Iceland Seismic Zone, Tectonophysics, 447, p. 95-116 [doi:10.1016/j.tecto.2006.07.016].

 

Technical aspects

 

 

        In addition to stress determination tools, geometrical applications are present in the 1994 Tector package, such as ROTILT (rotation of data sets and tensors), ROSACE (statistics and rose diagrams), etc. Graphics output needs specific processing in this MS-DOS-based package built before adoption of Postscript graphics. For user's convenience, some programs make graphic output easier on screen or paper, and to translate the graphic files (Calcomp code) into exportable HP code. In any case, as the graphic files are written in ASCII, the user can easily write his own interfaces with his favourite graphic softwares. Examples of graphic output obtained with Tector softwares are displayed in the other pages of this site, especially Softwares and Structures.  

        Most applications concerning focal mechanisms of earthquakes, including inversion, mapping and cross-sections, appeared later than Tector 1994. Inversion of focal mechanism solutions can however be done with Tector 1994 versions if nodal planes are selected and treated as fault slips. The right dihedra method, DIEDRE, is applicable to focal mechanisms without any choice between nodal planes regardless of Tector version (1994 or XXI).

        The Tector 1994 executable programs can be obtained for non-profit, academic purposes and used on almost any MS-DOS compatible PC (including XP and Vista systems). 

        The Tector XXI executable programs designed for NT, XP and Vista systems require special installation, are not exportable and only used under specific research agreements.

 

Application to regional tectonic and seismotectonic analyses

      

        Applications of these methods have been done in Greece, Taiwan, BasinRange, Iceland, Iran, India and other countries. They are mentioned in the following pages. More information about the Tector softwares, structures and files is given in pages Softwares and Structures.

        Page Structures contains simple examples of small, very simple data sets that the beginner can process by himself using Tector 1994 or other available softwares to become familiar with basic aspects of computer-based brittle tectonic analysis and stress inversion. The outputs and solutions obtained by the user can be checked using the illustrations of page Structures.

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Mots-clés: inversion, contraintes, paléocontraintes, déformation, cisaillement, pression, tension, déplacement, cinématique, dynamique, tenseur, glissement, faille, filon, dyke, fracture, débit, pression-dissolution, diaclase, double couple, mécanisme au foyer, séisme, tectonique, sismotectonique, tectonique de plaques, lithosphère, programme Fortran, logiciel, fichier Postscript, projection stéréographique, rotation, cartographie, coupe, structures cassantes, travail de terrain, géologie, sismologie, géodynamique, base de données, polyphasage, analyse de groupes, épuration, statistique, données d'orientation, dièdres droits, extension, compression, décrochement, coopération internationale