<p>We used teleseismic waveforms and ground deformation data from GNSS and InSAR to estimate source fault parameters of the M<sub>w</sub>6.4 earthquake that occurred just offshore southwestern Puerto Rico on 7 January 2020. The mainshock was a part of an energetic seismic sequence that started on 28 December 2019 and led to a M<sub>w</sub>5.8 earthquake on 6 January 2020, a day before the M<sub>w</sub>6.4 mainshock. The ground-shaking due to the largest earthquakes of the sequence caused significant damage to buildings and infrastructures in Puerto Rico and one casualty was reported by the local media. The mainshock was followed by a strong aftershock sequence that included four M<sub>w </sub>&#8805; 5 events within the first 3 hours. In the first 40 days of the seismic sequence, data from the Puerto Rico Seismic Network were used to locate ~3800 earthquakes of magnitude > 2, illuminating an east-west elongated 30x50 km<sup>2</sup> area, just offshore the southwestern coast of Puerto Rico. The region affected by this activity was before characterized by relatively low seismicity rates, even if a system of active faults, both onshore and offshore, had been mapped. The sequence is peculiar due to its complex development and many large aftershocks (magnitude > 4.5), with the mainshock releasing only &#160;~60% of the total seismic moment.</p><p>We estimated the key source parameters of the mainshock using teleseismic data, GNSS data from the Puerto Rico Geodetic Network, and InSAR data from the Sentinel-1 and ALOS-2 satellites. The modeled source is consistent with a ~15 km long and ~11 km wide blind fault, oriented roughly east-west and dipping 46<sup>o</sup> towards north, and with up to 1.1 m of oblique normal and left-lateral strike-slip.</p><p>The optimal fault plane source indicates that it is an offshore continuation of the mapped North Boquer&#243;n Bay - Punta Montalva fault zone, supported by the large number of the aftershocks that trend along the same direction. However, most of the aftershocks, even those of magnitude > 5, occurred on other nearby faults, highlighting the complexity of this fault zone area.</p>
