Potential for geologically active faults Department of Antioquia, Colombia

Potencial de fallas geológicas activas Departamento de Antioquia, Colombia

Luis Hernán Sánchez-Arredondo ^a & Orlando Giraldo-Bolivar ^b

^a Departamento de Materiales y Minerales de la Facultad de Minas, Universidad Nacional de Colombia, Colombia. lhsanche@unal.edu.co
^b Departamento de Ingeniería Civil de la Facultad de Minas, Universidad Nacional de Colombia, Colombia. ogiraldo@unal.edu.co

Received: November 7^th, 2012. Received in revised form: March 13^th, 2013. Accepted: April 11^th, 2014.

Keywords: seismology, geostatistics module, active faults, Colombia.

Resumen
Con base en estudios geoestadísticos de estimación global de los datos de sismicidad reportados por la Red Sismológica Nacional de Colombia (RSNC), se determinó un módulo geoestadístico (mg₉₅) que permitió categorizar el nivel de actividad del fallamiento cortical en el Departamento de Antioquia (DA), Colombia. E lmg₉₅ relaciona los valores de estimación con krigeage poligonal y su error correspondiente para cada uno de los municipios antioqueños con un parámetro de Student al 95% de confianza, que depende del número de microsismos registrados localmente. Para determinar los niveles de fallamiento activo en cada municipio, se propone la siguiente escala de categorización: fallamiento activo probadomg₉₅∈[0-0.2], fallamiento activo probablemg₉₅∈[0.2-0.3] y fallamiento activo posible mg₉₅ > 0.3.

Palabras clave: sismología, módulo geoestadístico, fallas activas, Colombia.

1. Introduction

The first line of each paragraph is indented 0.5 cm. The Department of Antioquia is located in the north western corner of South America. It is one of the world's least understood regions because its geology is the result of the interaction of multiple geological processes and of tectonic plates and microplates that meet there. The seisms generated in this region show drastic changes in the focal mechanisms [1]. In the last 130 years, cortical active faults have generated earthquakes such as the one in Turbo 1882² and the one of in Murindo 1992[3].

The seismicity analysis reported by the RSNC from June 1, 1993 to June 30, 2009, indicates that the Department of Antioquia has registered 2429 epicentres, of which 78% (1897) corresponds to superficial activity, (77% of the data is located at a depth of 3 Km.) with a local magnitude (M_L) of 2.7, and 25% of the data in the interval 3 ≤ M_L ≤ 5.3.

The quantile - quantile chart (Q-Q plot) of Fig. 1, indicates the strong tendency of superficial microseismic activity in the DA, to behave statistically as a log-normal distribution for the data of local magnitude.

3. Structural Geostatistics

A semivariogram whose graph assimilated an exponential type model (Fig. 2), with an influence range for seismic data of 5 kilometres and a real variance (plateau) of 0.31 developed as the main geostatistical tool to evaluate the spatial behaviour of the superficial seismic activity in the Department of Antioquia.

The semivariogram model for the M_L variable was validated regionally with 1725 seisms, a standardized error

average of 0.005, and a standardized error variance of 0.88. The model rejected 0.4 % of the data inside a 100 Km radius. The equation representing the model is the following.

The global estimate was based on the geostatistical technique known as "polygonal kriging". This procedure was designed to provide an estimated value of seismicity, inside the areas delimiting the polygonal coverage of each one of the municipalities of the DA. Each municipality receives a unique global estimate value with its corresponding error. The number of seisms considered for the estimates inside each municipality, is equivalent to the local epicentres, plus those located outside the municipal polygon but inside a 5 km outline, which corresponds to the influence range deduced from the semivariogram (Fig. 2).

4. Geostatistics Module And Categorization Of Geologically Active Faults

The level of geological fault activity, or segments of geological faults belonging to a system, was defined with a confidence factor of 95% labelled "geostatistics module (mg₉₅) to characterize the level of activity of geological faults".

t depends on the number of seisms considered.

The study case of the municipality of Murindo (Fig. 3) will be used as an example.

If the number of epicentres increases, the estimation error and the Student factor decrease and, therefore, the level of threat by active faults increases.

Thus the mg₉₅ is used to categorize the level of active faults of the DA, based on the fact that the presence of cortical seismic activity implies the presence of active geological faults.

Thus the mg₉₅ is used to categorize the level of active faults of the DA, based on the fact that the presence of cortical seismic activity implies the presence of active geological faults.

The classification levels for geological fault activity are proposed considering the mg₉₅ values obtained for each one of the municipalities of the DA, (Table 1).

5. Discussion

Regionally the territory of the DA is affected by the triple union of the convergence of the Caribe, Nazca and Suramericana tectonic plates. This contact zone is represented by two microplates called bloque Andino and bloque de Panamá Baudó [4]. As a consequence of this geotectonic activity, three geological fault systems control the seismological activity in the DA: The Cauca Romeral System, cartographed since the start of the 20th century [5], the Palestina Fault System [6], and the Mutatá-Murindó Fault System (Suture of Dabeiba).

For some authors [7], seismicity in Colombia's Northwest is diffused and complex and caused by the fault generating compression east-west. This implies that these hypotheses suppose that seisms generated in the region are not caused by subduction, but the result of the convergence tectonic plates and microplates.

The geostatistical results obtained in this research indicate that 42% of the municipalities of Antioquia are controlled by geological faults with a proved level of activity. 24% of the municipalities determined in this level are related geotectonically to the Mutatá-Murindó Fault

The highest potential for active faults in the DA is registered in the municipalities of Ituango (Cauca Romeral Fault System, Suture of Dabeiba), Dabeiba, Urrao, Uramita, (Palestina Fault System).

Cañas Gordas, Murindó (Suture of Dabeiba) and Remedios

The cortical fault presenting the largest threat for the DA is related to the Mutatá-Murindó Fault System (Suture of Dabeiba). Considering the external seismic threat presented by this suture for the Valle de Aburra (3,317,166 inhabitants) [8], home of the city of Medellín, the segments that pass through Urrao are the neotectonic features presenting the highest danger. They present potential for generating superficial seisms with a PGA (Peak Ground Acceleration) of 0.96g (units of gravity), which may arrive at Medellín, with a acceleration of 0.25g. Historically, one of the most representative seisms in the zone, related to the Suture of Dabeiba, is the one that occurred on September 7, 1882, with an intensity of X in the Mercalli Scale and an Ms magnitude estimated between 6.5 and 7.2. Likewise, this seism was felt intensely in the Isthmus of Panama and in a large part of the Departments of Antioquia and Choco¹. Similar to the seism of 1982, most of the Uraba Region was affected by the seisms of October 17 and 18 of 1992, which brought about countless damage to the environment and the urban centres of Colombia's Northwest₃. The event of October 18 registered, in an accelerograph located in ISA in the city of Medellín, a maximum horizontal acceleration of 0.03g. This is a pretty low value considering the damage registered [3].

In conclusion geostatistics can be used as a valuable tool for supporting seismology. It complements neotectonics and geochronology (carbon-14) studies and can be used to take decisions when instrumenting and monitoring the places with the greatest seismic threats.

Acknowledgements

This article is a contribution to project 9536 "Seismic Threat of the Department of Antioquia Based on Microseismic Activity of the RSNC" Phase I, financed by the Office of Research of Universidad Nacional de Colombia, Sede Medellín (DIME).

References

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O. Giraldo-Bolivar, received the Bs. Eng in Civil Engineering in 1981, the Sp degree in structures in 2000, all of them from the Universidad Nacional de Colombia. Medellin, Colombia. He is a Full Professor in the area of Materials and Structures in the department of civil engineering at the National University of Colombia, Medellín. His research interests include: Architectural precast concrete panels, Evaluation of chemical additives for concrete, concrete quality control of civil works.