The Northern Andes boundary is a first-order tectonic structure in Colombia with historically M>7 earthquakes. However, details about the individual sections of the system remain unknown. We illuminate the seismotectonic of the Algeciras fault by investigating an earthquake sequence that started on December 24, 2019. Using recent seismic networks of the region, we estimate focal mechanisms of the foreshocks and aftershocks, local stress field, kinematic slip models of the largest events, and Coulomb stress changes. Two mainshocks (a doublet of Mw 6.0 and 5.8) occurred within 16 minutes, rupturing just a few kilometers from each other. Discrimination of causative faults among the centroid moment-tensor nodal planes is difficult because the focal zone is a complex tectonic environment. We reinterpret local faults using geologic information, geomorphology and combine this new information with seismology results. The relocated aftershocks show a cluster with an L-shaped pattern concentrated in a ~7 km x 7 km area. Our model defines the Algeciras fault with two structural styles merging to the Guaicáramo Fault System and border the Eastern Cordillera to the east, supporting its regional dextral and transpressional kinematics. The NW part is characterized by a duplex-style of right-lateral strike-slip with inner secondary faults of the same sense or movement, and the SE zone by a domino-style system with inner minor faults of sinistral kinematics. The earthquake doublet is a part of the duplex style, whereas, the south part of the aftershocks is located on the domino-style, of the northern termination of the Algeciras Fault System.