Since the last entry on FMC there is a lot of water under the bridge. The last version (1.3) includes new plot options, a better output data filter and management and the possibility to perform clustering analysis using hierarchical clustering algorithms (other algorithms could be added in the future). Originally FMC was programmed in Python 2.7 and now is also compatible with Python 3.
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.
A new entry of FMC with updates can be found 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.
Alberto Jiménez-Díaz (link to his page), a Ph.D. candidate of our research group, has published an excellent work with the title: “Spatial variations of effective elastic thickness of the lithosphere in Central America and surrounding regions” on Earth and Planetary Science Letters (link). He has developed this work under the supervision of J. Ruíz and in close collaboration with M.Pérez-Gussinyé (Royal Holloway, University of London) and John F. Kirby (Curtin University, Perth). I collaborated basically on the tectonic and geodynamic contextualization and discussion of the results.
As a proxy for long-term lithospheric strength, the effective elastic thickness (Te) can be used to understand the relationship between lithospheric rheology and geodynamic evolution of complex tectonic settings. Here we present, for the first time, high-resolution maps of spatial variations of Te in Central America and surrounding regions from the analysis of the coherence between topography and Bouguer gravity anomaly using multitaper and wavelet methods. Regardless of the technical differences between the two methods, there is a good overall agreement in the spatial variations of Te recovered from both methods. Although absolute Te values can vary in both maps, the qualitative Te structure and location of the main Te gradients are very similar. The pattern of the Te variations in Central America and surrounding regions agrees well with the tectonic provinces in the region, and it is closely related to major tectonic boundaries, where the Middle American and Lesser Antilles subduction zones are characterized by a band of high Te on the downgoing slab seaward of the trenches. These high Te values are related to internal loads (and in the case of the southernmost tip of the Lesser Antilles subduction zone also associated with a large amount of sediments) and should be interpreted with caution. Finally, there is a relatively good correlation, despite some uncertainties, between surface heat flow and our Te results for the study area. These results suggest that although this area is geologically complex, the thermal state of the lithosphere has profound influence on its strength, such that Te is strongly governed by thermal structure.
Following with the results of our research on tsunami hazard and risk at El Salvador, I want to share with you our last publication of 2013. This paper has been published in Natural Hazards and Earth System Sciences (link) with the self-explanatory title: “Tsunami evacuation modelling as a tool for risk reduction: application to the coastal area of El Salvador”. The leader of the work is Pino González-Riancho; researcher and Ph.D. candidate at the “IH Cantabria” Insitute of Environmental Hydraulics.
Advances in the understanding and prediction of tsunami impacts allow the development of risk reduction strategies for tsunami-prone areas. This paper presents an integral framework for the formulation of tsunami evacuation plans based on tsunami vulnerability assessment and evacuation modelling. This framework considers (i) the hazard aspects (tsunami flooding characteristics and arrival time), (ii) the characteristics of the exposed area (people, shelters and road network), (iii) the current tsunami warning procedures and timing, (iv) the time needed to evacuate the population, and (v) the identification of measures to improve the evacuation process. The proposed methodological framework aims to bridge between risk assessment and risk management in terms of tsunami evacuation, as it allows for an estimation of the degree of evacuation success of specific management options, as well as for the classification and prioritization of the gathered information, in order to formulate an optimal evacuation plan. The framework has been applied to the El Salvador case study, demonstrating its applicability to site-specific response times and population characteristics.
The methodology proposed has been applied to the area of Barra de Santiago – Acajutla in El Salvador. This area is characterized by a 9 km-long sand spit that protects the estuary (Estero El Zapote) of the Aguachapío, Guayapa and El Naranjo rivers. The wetland includes an important mangrove area and belongs to the Complejo Barra de Santiago ANP (Protected Natural Area). According to the census (VI Censo de Población y V de Vivienda, DIGESTYC, 2007) and the hazard modelling results, the number of people located in the tsunami-flooded area is around 3300, 75 % being located on the sand spit (Barra de Santiago canton), which was affected by the tsunami of 1902 and where, according to the local knowledge, only 5 persons survived the event.
The evacuation modelling for the current response time of 45 min (i.e. warning time 30 min, reaction time 15 min) highlighted that improvements to the warning process must be made to ensure the success of the evacuation, as most of the coastal communities would be reached by the tsunami before being warned about it. A reduction of 15 min in the response time (a response time of 30 min) showed that a higher percentage of populations evacuates (proving the importance of working on this issue); however, the communities located closer to the coastline would not be able to reach the safe areas. For these communities an attempt to identify alternative measures to ensure their evacuation is proposed, such as building new evacuation routes and new vertical shelters. These combined measures (reducing response time and reducing distances to travel) have been demonstrated to be useful for achieving the desired results.
Researcher in active tectonics, earthquakes, tsunamis…