Blaž Vičič , Seyyedmaalek Momeni, Alessandra Borghi, Anthony Lomax , Abdelkrim Aoudia

The 2019-2020 Southwest Puerto Rico earthquake sequence ruptured 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 6th of January, 2020 (10:32:23) Mw5.7 and the 7th of January, 2020 (11:18:46) Mw5.8 events occurred on two E-SE striking, near-vertical, left-lateral strike-slip faults. However, the 7th January, 2020 (08:34:05) Mw5.6 normal faulting aftershock which occurred only 10 minutes after the Mw6.4 normal faulting 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 apply template matching for the detection of missed, small magnitude earthquakes to study the spatial evolution of the main part of 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 a tectonic transient that most affected the whole Puerto Rico island.