Source Analysis of Bucaramanga Nest Intermediate-Depth Earthquakes

Intermediate-depth earthquakes are those that occur at depths of 50 to 300 km in subducting lithosphere and can occasionally be destructive. Despite their ubiquity in earthquake catalogs, their physical mechanism remains unclear because ambient temperatures and pressures at such depths are expected to lead to ductile flow, rather than brittle failure, as a response to stress. Intermediate-depth seismicity rates vary substantially worldwide, even within a single subduction zone having highly clustered seismicity in some cases (Vrancea, Hindu-Kush, etc.). One such places in known as the Bucaramanga Nest (BN), one of the highest concentration of intermediate-depth earthquakes in the world. Previous work on these earthquakes has shown 1) Focal mechanisms vary substantially within a very small volume. 2) Radiation efficiency is small for M<5 events. 3) repeating and reverse polarity events are present. 4) Larger events show a complex behavior with two distinct rupture stages. Due to on-going efforts by the Colombian Geological Survey (SGC) to densify the national seismic network, it is now possible to better constrain the rupture behavior of these events. In our work we will present results from focal mechanisms based on waveform inversion as well as polarity and S/P amplitude ratios. These results will be contrasted to the detection and classification of repeating families. For the larger events we will determine source parameters and radiation efficiencies. Preliminary results show that reverse polarity events are present and that two main focal mechanisms, with their corresponding reverse polarity events are dominant. Our results have significant implications in our understanding of intermedaite-depth earthquakes and the stress conditions that are responsible for this unusual cluster of seismicity.