Comparison of the Aftershock Sequences of Shallow and Intermediate-Depth Earthquakes in Colombia

While observations of shallow (<70 km) and deep (>70 km) earthquakes show that they have many similar properties, well-developed aftershock sequences are less frequently observed for deep earthquakes than their shallow counterparts. We compare the productivity and temporal decay of the aftershock sequences of two recent earthquakes in Colombia, the 23 March 2019 Mw 6.1 Versalles earthquake (126 km depth) and the 14 September 2016 Mw 5.9 Mutatá earthquake (24 km depth). To investigate the characteristics of these events and their aftershock sequences, we analyze the earthquake catalog and waveform data from the Servicio Geológico Colombiano (SGC). The network allows the resolution of a complete earthquake catalog down to at least ML 2.0 at the depth of the Versalles earthquake; the magnitude of completeness is lower for the shallow earthquake. The normal-faulting, intraslab Versalles earthquake is one of the largest intermediate-depth earthquakes in the Cauca Cluster, a region in southwestern Colombia with a higher rate of earthquakes than the surrounding region. In the month following the earthquake, the SGC catalog reports 7 earthquakes with ML ≥ 2.0 near the main event, including 3 ML ≥ 3.0 earthquakes. The largest aftershock, which occurred 16 hours after the mainshock, had ML 3.7. The shallow, thrust-faulting Mutatá earthquake occurred on the Uramita Fault, the approximate boundary between the Panama-Choco block and the Western Cordillera in northwestern Colombia. In the month following the earthquake, the SGC catalog reports 144 earthquakes with ML ≥ 2.0 near the main event, including 27 ML ≥ 3.0 earthquakes. The largest aftershock, which occurred 5.8 hours after the mainshock, had ML 4.5. Relative to the shallow earthquake, the intermediate-depth earthquake had a larger magnitude differential between the mainshock and the largest aftershock. In addition, the intermediate-depth earthquake had more than an order of magnitude fewer aftershocks above its magnitude of completeness. The poorly-developed aftershock sequence for the Versalles event and its occurence in the relatively young (~10 Myrs) subducting Nazca oceanic plate may support a link between a subducting plate's thermal structure and aftershock productivity.