The previous version of FMC has a small bug in the code that produces a bad computation of the seismic moment tensor components from the focal mechanism nodal planes. This error only appears if you are working with the Aki & Richards format as input and you are obtaining the Global CMT components as output. This bug has been corrected and now it works properly. Additionally the column number is now included in the header of the output file if you select the “ALL” parameters as output.
The link to the previous version of the program has been updated.
I would like to share with you a new contribution of our group on the active tectonics of Central America that has been recently published in the journal Tectonics. This time the responsible of the research is Jorge Alonso. As part of his PhD project he did a research stay at Berna, under the supervision of Guido Schreurs, to apply the analog modeling (those funny sandbox experiments) to El Salvador. He was interested on the development of the present day main active structures in the Volcanic Arc of El Salvador, and in order to constrain it he performed a series of sandbox experiments and studied its evolution through time using a XRCT Scan. This work links with his previous work on the morphotectonics of the area and with the works on the structure of El Salvador Fault Zone by Carolina Canora.
The El Salvador Fault Zone (ESFZ) is an active, approximately 150 km long and 20 km wide, segmented, dextral strike-slip fault zone within the Central American Volcanic Arc striking N100°E. Although several studies have investigated the surface expression of the ESFZ, little is known about its structure at depth and its kinematic evolution. Structural field data and mapping suggest a phase of extension, at some stage during the evolution of the ESFZ. This phase would explain dip-slip movements on structures that are currently associated with the active, dominantly strike slip and that do not fit with the current tectonic regime. Field observations suggest trenchward migration of the arc. Such an extension and trenchward migration of the volcanic arc could be related to slab rollback of the Cocos plate beneath the Chortis Block during the Miocene/Pliocene. We carried out 4-D analog model experiments to test whether an early phase of extension is required to form the present-day fault pattern in the ESFZ. Our experiments suggest that a two-phase tectonic evolution best explains the ESFZ: an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures. This extensional phase is followed by a strike-slip dominated regime, which results in intersegment areas with local transtension and segments with almost pure strike-slip motion. The results of our experiments combined with field data along the Central American Volcanic Arc indicate that the slab rollback intensity beneath the Chortis Block is greater in Nicaragua and decreases westward to Guatemala.
The work I show you today is a result of our field work in El Salvador, and particularly of the excellent Ph.D. thesis of my colleague and friend Carolina Canora.
During her research on paleoseismicity in El Salvador Carol realized that an earthquake that was present on our paleoseismological trenches on the Volcanic Arc faults, has the same date of an earthquake described in the literature as a subduction earthquake. The difference is key, because the Volcanic Arc earthquakes are shallower than the subduction earthquakes and the epicenters are located directly beneath the populations. The implications are bold for the seismic hazard assessment because Carol show that earthquakes greater than M 7.0 could be generated on the volcanic arc faults, and some earthquakes classified as subduction earthquakes in the literature could be in fact more dangerous continental crust shallow earthquakes. She analyzed also the macroseismic isoseismal maps in Modified Mercally Intensisty for the last big El Salvador earthquakes (January and February 2001), and compared it to the published 1719 isoseismal map and with a new isoseismal map proposed for the same event in the light of the new evidences.
The work has been recently published in the journal “Seismological Research Letters” [link].
Today I’m sharing with you a new episode on our research on the active tectonics of Central America, and a part of the Jorge Alonso Henar Ph.D. thesis. Jorge is the first author of this publication, which deals with the geomorphology associated to the activity of some main strike-slip faults in El Salvador. In this paper we show how the hypsometric analysis can be useful even in strike-slip faults with small vertical displacement component. We used the hypsometry and the morphometric analysis in a new way to active faults that allow us to estimate the rate of motion of the faults in both components (dip-slip and strike-slip). In addition we can make some interesting deductions about the recent tectonics of Central America. The work has been published in Tectonophysics (link).
We have used hypsometric analysis to improve our understanding of the current tectonic deformation and structure of El Salvador Fault Zone; a N90°E oriented strike-slip fault zone that extends 150 km through El Salvador, Central America. Our results indicate an important amount of transtensive strain along this fault zone, providing new data to understand the tectonic evolution of the Salvadorian volcanic arc. We have defined kilometric scale tectonic blocks and its relative vertical movements, length of segments with homogeneous vertical motions and lateral relay of active structures. We have identified and quantified slip-rate variations along-strike of the El Triunfo fault within El Salvador Fault Zone, ranging from 4.6 mm/year in its central parts to 1 mm/year towards the tips of the fault. This study supports the hypothesis of a recent rotation in the maximum shortening direction, and the accommodation of the current deformation through the reactivation of pre-existing structures inherited from a previous tectonic regime.
Tomorrow I am taking a plane to Vienna to enjoy, one more year, the awesome European Geosciences Union general assembly. In case you are not going to be there (or you could not be there) I am posting here my two poster contributions as first author so you can take a look at them.
Both posters have been designed with Inkscape as usual, and most of the figures where done with GMT.
FMC: a one-liner Python program to manage, classify and plot focal mechanisms
This poster deals with a program I made in Python to work with earthquake focal mechanisms. I tried to keep the poster simple but attractive. I used the typical terminal colors over a gray background and a “hand-made” program design to explain its concept. You can download the abstract here.
Tsunamigenic seismic sources characterization in the Zagros fold and thrust belt. Implications for tsunami threat in the Persian Gulf
This is a classical “scientific” poster. It is divided in three columns; the first for the seismicity, the second for the tectonics and the third for the tsunami model. The background with a sand color tries to remind the study area landscape. You can download the abstract here.
Download the program here; it is released under the GNU GPL license. The manual is distributed under the Creative Commons License: Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0).
About the program
The analysis of earthquake focal mechanisms (or Seismic Moment Tensor, SMT) is a key tool on seismotectonics research. Each focal mechanism is characterized by several location parameters of the earthquake hypocenter, the earthquake size (magnitude and scalar moment tensor) and some geometrical characteristics of the rupture (nodal planes orientations, SMT components and/or SMT main axes orientations). The aim of FMC is to provide a simple but powerful tool to manage focal mechanism data. The data should be input to the program formatted as one of two of the focal mechanisms formatting options of the GMT (Generic Mapping Tools) package: the Harvard CMT convention and the single nodal plane Aki and Richards (1980) convention. The former is a SMT format that can be downloaded directly from the Global CMT site (http://www.globalcmt.org/), while the later is the simplest way to describe earthquake rupture data. FMC is programmed in Python language, which is distributed as Open Source GPL-compatible, and therefore can be used to develop Free Software. Python runs on almost any machine, and has a wide support and presence in any operative system. The program has been conceived with the modularity and versatility of the classical UNIX-like tools. Is called from the command line and can be easily integrated into shell scripts (*NIX systems) or batch files (DOS/Windows systems). The program input and outputs can be done by means of ASCII files or using standard input (or redirection “<“), standard output (screen or redirection “>”) and pipes (“|”). By default FMC will read the input and write the output as a Harvard CMT (psmeca formatted) ASCII file, although other formats can be used. Optionally FMC will produce a classification diagram representing the rupture type of the focal mechanisms processed. In order to count with a detailed classification of the focal mechanisms I decided to classify the focal mechanism in a series of fields that include the oblique slip regimes. This approximation is similar to the Johnston et al. (1994) classification; with 7 classes of earthquakes: 1) Normal; 2) Normal – Strike-slip; 3) Strike-slip – Normal; 4) Strike-slip; 5) Strike-slip – Reverse; 6) Reverse – strike-slip and 7) Reverse. FMC uses by default this classification in the resulting diagram, based on the Kaverina et al. (1996) projection, which improves the Frohlich and Apperson (1992) ternary diagram.
Why this program?
I have been using different versions of this program during the last decade. Initially I reworked some of the Gasperini and Vannucci (2003) FORTRAN subroutines on Matlab in order to obtain all the focal mechanism parameters from the Harvard CMT psmeca formatted catalog. During my research on seismotectonics I started to use the Frohlich and Apperson (1992) diagram, but after Kagan (2005) I decided to try the Kaverina et al. (1996) one. From the original Matlab program I jumped to the Free Software world adapting it to Octave. The program only produced the x and y positions of the events and all the plotting where done by means of GMT (Wessel et al., 2013).
Some colleagues wanted to use the diagram for their work, but they were not familiar with GMT, so I decided to make a big improvement in the program to make it easy to use, distributable and with plotting support. I choose to program it in Python with the following basic ideas:
It should be called from the terminal or the command line in order to be incorporated into shell scripts
It should behave like any other shell unix tool, compatible with redirection, piping and ASCII format
It should be compatible with the GMT psmeca formats to allow the mapping of the focal mechanisms
It should has the option to produce a Kaverina et al. (1996) type classification diagram
Part of the programming where done during my happy days as Ph.D. candidate at the UCM; so I have to acknowledge the UCM scholarship that allow me to start my scientific career.
I would like to thank the beta testers Jorge L. Giner-Robles and Alberto Jiménez-Díaz for their comments and suggestions.
If you use FMC, and consider it appropriate, you can acknowledge it by citing the following reference:
Álvarez-Gómez, J.A. (2014) FMC: a one-liner Python program to manage, classify and plot focal mechanisms. Geophysical Research Abstracts, Vol. 16, EGU2014-10887.
Durante este curso un grupo de profesores de la Facultad de Geología de la UCM estamos realizando una serie de vídeos para la enseñanza de la utilización de la plantilla estereográfica en Geología Estructural e Ingeniería Geológica. Por el momento hemos subido el bloque básico de iniciación a la utilización de esta plantilla, pero seguiremos incorporando más vídeos para completar los problemas que se pueden resolver con el uso de la plantilla estereográfica.