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.

The October 30, 2020, Mw7 Samos earthquake ruptured a north-dipping offshore normal fault, bounding the homonymous basin. Genetically is related to the rapid southward motion of the Aegean, contributing to significant extension and the development of active graben structures within a dextral shear zone. It will be recalled as among the deadliest (118 fatalities) that affected the Greece-Turkey cross border region, generated a strong tsunami, and caused a co-seismic uplift of 20 to 35 cm of the NW part of the Samos Island. Using broadband, strong-motion and geodetic data, we constrain the location and source geometry of the mainshock. A multiple-point source model suggests three sequential subevents providing 20 s of source duration. Our finite-fault kinematic model confirms the prevalence of large slip amplitudes (~2.4 m) along the entire ruptured area and the up-dip and westward rupture propagation. This directivity is independently confirmed by Apparent Source Time Functions inferred from regional recordings using a herein developed empirical Green’s function method. Static GNSS displacements from inland stations yield a near-surface co-seismic slip of ~1 m amplitude, breaking the sea bottom and contributing to any interpretation of the observed island uplift. The 2020 Samos event dramatically showed that in the spatially heterogeneous oblique transtentional regions in the back-arc Aegean region, normal faults bounding the basins are capable to rupture in M7 earthquakes, provoke tsunami generation, and constitute a constant threat for the nearby coastal areas of both Greece and Turkey.

The 30 October 2020 M7 Samos earthquake occurred offshore the Greece-Turkey cross border region, and will be recalled as among the deadliest (118 fatalities) that affected both countries. It generated a strong tsunami and caused coseismic uplift of 20 to 35 cm of the NW part of the Samos Island. It ruptured a ~60 km long, north-dipping normal fault, related to the back-arc extension of the Aegean Sea area. Using picks from regional strong motion and broad-band waveforms we relocated the mainshock and the aftershocks, applying suitable velocity models. The closest strong-motion recordings, constrained the finite fault slip model, suggesting up-dip and westward propagation of the rupture. The westward rupture propagation is independently confirmed by the apparent source time functions inferred using the empirical Green’s function method from near-regional broad-band and strong-motion waveforms. Static displacements measured by GNSS stations constrain near surface slip of ~1 m, explaining the tsunami and the island uplift. The 2020 Samos event showed that normal faults bounding the basins in the back-arc Aegean region can host M7 earthquakes and when combined with tsunami generation, constitute a constant threat for the nearby coastal areas of both Greece and Turkey.