The Bucaramanga Nest: A natural laboratory for understanding the physics of intermediate-depth earthquakes

The mechanism for intermediate depth and deep earthquakes is still under debate. The temperatures and pressures are above the point where ordinary fractures ought to occur. Nevertheless these earthquakes represent a significant portion of global catalogs and various mechanisms have been proposed to explain their presence, including dehydration embrittlement and runaway shear instabilities. In order to better understand the rupture mechanism and the underlying physics of intermediate earthquakes we have started a thorough investigation of the seismic records on the Bucaramanga Nest, a unique pocket of seismic activity with a depth concentration around 160 km depth. The Bucaramanga nest has a larger activity rate, a smaller volume and a clearer isolation from nearby activity than any other concentration of intermediate-depth earthquakes in the world, and is thus an ideal natural laboratory for studying intermediate depth seismicity. We have found a significant number of nest earthquakes with extreme similarity (CC > 0.9), but that in some cases the waveforms are polarity reversed. These curious “anti-repeat” events may suggest common rupture planes but differing slip direction (overshooting?) or sub-parallel planes limiting an “extruding block”. We use relocation algorithms to better decipher the underlying reason for these events as they are not known for shallow, crustal earthquake populations. In addition we study the earthquake sources and the behavior of earthquakes as a function of size – also known as earthquake source scaling. Stress drops and radiated seismic energies for a range of magnitudes are compared to typical values observed for shallow earthquakes. Preliminary results suggest that Bucaramanga Nest earthquakes present fairly large stress drops (<10 MPa) and low apparent stress values of around (~1 MPa), but contrary to their shallow counterparts. This is reflected in small rupture areas and very low seismic efficiencies.