The 2019-2020 Indios, Puerto Rico earthquake sequence: seismicity and faulting

The 2019-2020 Indios, Puerto Rico earthquake sequence ruptured on multiple faults with several moderate magnitude earthquakes. Here we investigate the seismotectonics of this fault system using high precision hypocenter relocation and inversion of the near-field strong motions of five largest events in the sequence (5.6≤Mw≤6.4) for kinematic rupture models. The Mw6.4 mainshock occurred on an NE striking, SE dipping normal fault. The rupture nucleated offshore ~15 km SE of Indios at the depth of 8.6 km and extended SW-NE and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 seconds. The Mw5.7 and Mw5.8 events on 6th and 7th January 2020, respectively, occurred on two E-SE striking, near-vertical, left-lateral strike-slip faults. However, the 7th January 2020 Mw5.8 aftershock which occurred only 10 minutes after the mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ~8 km further E, forming a set of parallel faults in the fault system. On 11th January 2020, a Mw6.0 earthquake occurred on a N-NE striking, W dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We also apply template matching for the detection of missed, small magnitude earthquakes and to study the spatial evolution of the main part of the sequence. These detections show multiple examples of acceleration phases before moderate earthquakes and reveal migration patterns within the sequence. Using the template matching results along with GPS analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to an inferred tectonic transient that most likely originated on the Muertos subduction as a slow-slip event.