Analyses of the size distribution of earthquakes and their relocation for central-western Colombia

Abstract The Red Sismológica Nacional de Colombia-RSNC catalog for the period 1993-2012 was used to relocate shallow earthquakes (0.4 < ML < 6.7) in central-western Colombia and determine spatial variations in b-value, the slope of the frequency-magnitude distribution-DFM. From an initial set of 18,327 earthquakes a subset of 11,553 earthquakes was selected using the ML > MC criterion (where MC = 2.7 is the magnitude of completeness). Precise relocations were obtained with the double difference method and DFM analyses were performed using the maximum likelihood method. Relocations resulted in: reduction of formal errors in hypocenter parameters form initial averages of 15.42 km horizontally and 39.34 km vertically to final average errors nearing 4 m and 6 m in the horizontal and vertical directions respectively; concentration of earthquakes along fault zones reported in recent geological campaigns and a better delineation of geological structures such as the Murindó, Algeciras-Altamira, and Ibagué faults. Earthquake size distribution analyses indicate b-values in the range 0.6 - 1.5 with volcanic areas that are not detected as b-anomalies. Map of the study area a) with the earthquake RSNC catalog, b) relocated earthquakes.