Fault sources of the 2020 southern Puerto Rico seismic sequence and the regional tectonic structure

The 2020 southwestern Puerto Rico seismic sequence occurred in an area not known for seismic activity and included multiple normal and strike‐slip focal mechanisms. The largest earthquake, M_w 6.4, caused significant damage and permanent shoreline subsidence. The locations of the ruptured faults have been explored extensively using seismological and geodetic evidence, but no consensus has been reached. Here, we report the results of a high‐resolution seismic reflection survey that identified a north‐verging 20 km long curved normal fault spanning the upper reach of the Guayanilla canyon, which we call the Guayanilla Canyon fault. Part of the fault likely ruptured during the M_w 6.4. The concave‐to‐the‐north fault bounds a south‐dipping ∼1 km deep asymmetric basin. This long‐lived structure, identified also on sediment isopach and magnetic anomaly maps, appears to control the formation of the Guayanilla canyon system, the only submarine drainage system in southern Puerto Rico, suggesting a relationship among seismic activity, geomorphology, and reef development. Our seismic reflection data do not support the existence of previously published major seismogenic faults in the area, such as the north‐northeast–south‐southwest Guayanilla fault zone, the Northern Boqueron Bay‐Punta Montalva fault, or a northwest–southeast strike‐slip fault, suggested to have ruptured as part of the M_w 6.4 earthquake. We identified smaller faults in the survey area, some of which can be correlated with seafloor expressions, and/or with changes in sediment thickness and magnetic anomalies, and/or with moderate‐sized earthquakes. Multiple seismic images of Investigator fault, a prominent mid‐slope feature parallel to the south coast of Puerto Rico, show buried low‐angle thrusts and an abrupt change in the attitude of the overlying sediments, leading us to interpret this feature as the expression of a boundary between the now‐inactive upper accretionary prism of Muertos trough and a buried back‐arc basin that overlies the arc basement of Puerto Rico.