Microseismicity and focal mechanisms of the intermediate‐depth Bucaramanga Nest, Colombia

Abstract

The Bucaramanga nest of intermediate‐depth seismicity in northern Colombia was investigated from microearthquakes (mb ≤ 4.3) recorded from June 20 to July 6, 1979, by a local array of 12 digital three‐component seismographs. Of 161 events studied, 142 are from the nest and 19 define a thin Wadati‐Benioff (W‐B) zone, mainly from 110 to 190 km depth. The W‐B zone, part of the Bucaramanga segment, strikes N10°E with a dip increasing with depth from 30°E to near‐vertical beneath the nest. The 89 best located events from within the nest define a volume of about 8 km (NW) by 4 km (NE) by 4 km (depth) centered at 161 km depth, with relative location accuracies of ±0.5 to ±1.0 km. Double‐couple fault plane solutions of 59 events were determined from an objective procedure using P wave polarities and S/P amplitude ratios. There is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data. The subduction and assimilation of a linear feature (such as an island arc undergoing a phase transformation) may explain the existence of the Bucaramanga nest. However, the extraordinarily high rate of seismicity suggests that the nest may be moving at a rate several orders of magnitude faster than the rate of relative plate motion alone. The intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids, perhaps brine or magma, that will ultimately result in the initiation of volcanic activity over this recently emplaced W‐B zone.