School of Sciences Faculty of Geosciences Undergraduate Thesis: Seismotectonic Characterization of the Colombian Pacific Region: Identification of Tectonic Patterns Through Geostatistical Analysis María Daniela Gracia 201222439 Director: Fabio Iwashita ____________________ Co-Director: Jean Baptiste Tary ____________________ November 24/2017 - 1 - I wish to thank my mother Pilar, father Daniel and sisters Manuela and Sara for being a constant source of help and encouragement. Special thanks to my lovely boyfriend Jesse for supporting me and helping me believe in myself and to friends who have supported me throughout the process. And finally, I wish to thank the faculty of geosciences and my professors Fabio Iwashita and Jean Baptiste Tary for their motivation, disposition to help and great knowledge. - 2 - Abstract Earthquake occurrence is a consequence of many processes within the Earth such as tectonic stress loading, fluid diffusion or static stress triggering. As a result of this, patterns of spatial and temporal distribution, that earthquakes have historically displayed, keep the footprint of the mechanisms that give them origin. Quantifying these patterns and the extent of the causality and correlation between seismic events is then an interesting and useful subject of study. It is useful because it opens the doors to more accurate estimations of the behavior of earthquakes, which inherently decreases the risk of catastrophes. The Colombian Pacific region is a zone that presents a high degree of geological complexity, it lies parallel to a trench where the Nazca plate subducts below the South American Plate, which inherently results in increased seismic activity and rupture. In this study a sequence of 134 events that happened in this zone in a period of 68 months is studied. These earthquakes are organized in complex spatial structures that were separated trough clustering analysis and then subjected to geostatistical analysis. The geostatistical analysis consisted of: an evaluation of the distribution of events as a function of time and a semivariogram analysis, by which the degree of correlation between events was studied. The interaction within these events is highly complex but, to some extent, some system wide correlations are observed. As opposed to what was initially expected, the semivariogram analysis did not manage to measure the degree of correlation for this particular sequence. What this means is that all semivariograms lie within a zone that denotes no-correlation and present a generalized uncorrelated form. - 3 - Resumen La ocurrencia de terremotos es la consecuencia de los múltiples procesos que ocurren en el interior de la Tierra como lo son la carga tectónica por esfuerzo, la difusión de fluidos o el desencadenamiento por esfuerzo estático. Como resultado de esto, los patrones de distribución espacial y temporal que los terremotos han mostrado históricamente guardan la huella de los mecanismos que los originan. Cuantificar dichos patrones y la extensión de la causalidad y correlación entre eventos sísmicos es por lo tanto una materia de estudio útil e interesante. Es útil ya que abre las puertas a un área de estudio donde las estimaciones del comportamiento de los terremotos adquieren mayor precisión, lo cual inherentemente reduce el riesgo de catástrofes. La región Pacífica de Colombia es una zona que presenta un alto grado de complejidad en su geología, yace paralela a una trinchera donde la placa Nazca subduce por debajo de la placa Sur Americana, lo que automáticamente resulta en una mayor actividad sísmica y en ruptura. En este estudio se considera una secuencia de 134 eventos que ocurrieron en esta zona en un periodo de 68 meses. Estos terremotos se organizan en estructuras espaciales complejas, dichas estructuras fueron separadas por medio de análisis de clusters y luego analizadas usando métodos geoestadísticos. El análisis geoestadístico consistió de: La evaluación de la distribución de los eventos como función del tiempo y un análisis de semivariogramas, por medio del cual el grado de correlación entre los eventos fue estudiado. La interacción entre los eventos estudiados es de alta complejidad, sin embargo, hasta cierto punto es posible observar correlaciones a lo largo de todo el sistema. A diferencia de lo que se esperaba, el análisis por medio de semivariogramas no logró medir el grado de correlación para esta secuencia particular. Lo que esto significa es que todos los semivariogramas experimentales obtenidos se encuentran dentro de una zona que denota ausencia de correlación y en general presentan una forma que no muestra relación entre la causalidad de los eventos. - 4 - Table of Contents 1. Chapter 1. Introduction (6) 2. Chapter 2. Geology of the Colombian Western Margin (8) 2.1 Geological Setting (8) 2.2 Tectonic Setting (10) 2.2.1 Tectonic Background (14) 2.3 Main fault systems (17) 2.4 Seismicity (21) 2.4.1 Colombian National Seismic Network (Red Sismológica Nacional de Colombia) (21) 2.4.2 Historical seismicity (21) 2.4.3 General Characteristics of Seismicity (24) 3. Chapter 3: Theoretical Framework (31) 3.1 Seismology Framework (31) 3.1.1 Focal Mechanisms (31) 3.2 Geostatistical Framework (34) 3.2.1 Geostatistics (34) 3.2.2 Preliminary Definitions (36) 3.2.3 Semivariogram Analysis (38) 3.2.4 Cluster Analysis: K-Means (40) 4. Chapter 4. Data Selection, Processing and Methodology (42) 4.1 Data Selection, Variable Overview and Exploratory Analysis (42) 4.2 Spatial and Temporal Classification of Earthquakes: Clustering of the Data (44) 4.2.1 Spatial Clustering Results (49) 4.2.2 Temporal Clustering Results (51) 4.3 Distribution of Earthquakes as a Function of Time (52) - 5 - 4.4 Semivariogram Analysis of the Data (53) 4.4.1 Correlation of Earthquakes on Individual Features (55) 4.4.2 Correlation of Earthquakes in the System (57) 4.5 Focal Mechanisms (58) 5. Chapter 5. Results, Discussion and Conclusions (58) 5.1 Clustering of the Data (58) 5.2 Fault Interaction - Earthquakes as a Function of Time (60) 5.3 Correlation of Earthquakes on individual Faults and in the System: Evaluation of Semivariogram Functions (62) 5.4 Conclusions (63) 6. Appendix A. Spatial Clustering Characteristics (66) 7. Bibliography (69) - 6 - 1. Chapter 1. Introduction. Subduction zones have four main types of associated events: (1) shallow events occurring in the crust, (2) intraplate events due to bending of the subducting slab ahead of the trench, (3) large intraplate events and (4) deep events associated to the Wadati-Benioff zone (Scawthorn & Chen, 2002). The seismicity of the Western Colombian Margin (WCM), where the Nazca Plate subducts below the South American Plate, has been predominantly studied in terms of the associated Wadati-Benioff zones, large intraplate events, seismic nests (Cauca and Bucaramanga),current state of stress, seismic hazard and slab geometry (Barazangi & Issacks, 1976; Suárez, Molnar & Burchfiel, 1983; Wysession, Okal & Miller, 1991; Taboada,2000; Chen, Bina & Okal, 2001; Rietbrock & Waldhauser, 2004; Pedraza Garcia, Vargas & Monsalve, 2007; Pararas-Carayannis, 2012; Castilla & Sánchez, 2014; Salcedo-Hurtado & Pérez, 2016, Wagner et al, 2017.) Although it is known that seismicity in the region surrounding the trench (type 1 and 2 events) is mostly associated to active tectonic features, a more in depth study, that properly groups the events, associates them to different structures and quantifies their characteristics, is yet to be developed. It is well known that earthquake occurrence is not randomly distributed, instead it is a phenomenon that when observed over long temporal and spatial scales behaves in a coherent and structured manner (Walsh and Watterson, 1991; Nicol et al., 2006.). As a realization of this, historical seismicity has shown evidence of both spatial and temporal clustering (Plafker & Savage, 1970; Stein et al, 1997). This behavior is directly linked to the conduct of - 7 - the mechanisms triggering the seismic events (tectonic static stress triggering, fluid migration and extraction, etc.). Therefore, given that seismic behavior does not occur randomly, geostatistics becomes a great tool in order to quantify its characteristics. Geostatistics have been developed in order to model and evaluate natural resources and phenomena, its main assumption is that spatial auto correlation exists (Olea, 2006). It is a useful approach at quantifying seismic information, given that it helps measure the extent and behavior of spatial correlation through tools such as the semivariogram. Some authors have applied this methodology to different areas of seismic study; Şen (1998) used semivariograms in order to identify heterogeneities in regional seismicity of Turkey and Shaefer et al. (2014) used clustering algorithms in order to separate background seismicity form triggered seismicity. Mouslopoulou & Hristopulos (2011) analyzed an entire earthquake sequence through semivariogram analysis and manage d to identify and measure system wide correlations. The latter study is particularly interesting given that it can lead to the quantification of interesting and useful spatio-temporal variables in a broad variety of scenarios. For this reason, the methodology proposed by Mouslopoulou & Hristopulos (2011) is the one that guides this project. In the present study the goal is to identify and explore the existence of spatiotemporal patterns in seismic data from the Colombian Western Margin and based on this answer questions pertaining: the (1) structure of earthquake activity in