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A vigorous shallow earthquake sequence along the southern coast of Puerto Rico commenced on 28 December 2019 in a region with little prior large seismicity. The largest event in the sequence (MW = 6.4), struck on 7 January 2020 and involved normal faulting. It produced extensive damage in southern Puerto Rico and power disruption across the island. Nearby strong ground motions and static offsets from GPS stations along with teleseismic recordings are inverted for the kinematic rupture process of the mainshock. The ~15‐km‐long rupture is spatially concentrated, with most slip between 3 and 13 km deep and peak slip of ~1.6 m. The static stress drop is high, ~19 MPa, with the rupture locating in the eastern section of a ~30‐km‐long band of seismicity bisected by a near‐orthogonal lineation. Complex faulting and high stress in the intraplate region appears to be responsible for the high earthquake productivity. Plain Language Summary: An extensive earthquake sequence, with thousands of events, occurred near the southern coast of Puerto Rico from December 2019 through April 2020, with the largest event being a damaging MW 6.4 extensional faulting earthquake on 7 January 2020. Analysis of seismic recordings from local and distant stations, combined with static ground deformation recordings, resolves the space–time history of the mainshock rupture. The slip zone extends about 15 km horizontally at depths from 3 to 13 km deep below the southern margin of the island. The overall sequence extends much further, about 30 km parallel to the east–west coast, and there is a roughly north–south lineation of activity that bisects this trend, with focal mechanisms indicating activation of multiple strike‐slip and normal faults during the sequence. The mainshock has a high change in stress due to the concentration of large slip. The high stress intraplate environment and the faulting complexity appear responsible for the intense earthquake activity. Key Points: The 7 January 2020 MW 6.4 normal‐faulting earthquake is the largest in a vigorous seismicity sequence along Puerto Rico's southern coastInversion of strong‐motion, teleseismic, and GPS data for the MW 6.4 rupture indicates concentrated slip of up to 1.6 m from 3 to 13 km deepThe rupture spanned a small region of the overall earthquake sequence which has a cruciform pattern indicating multiple fault activation [ABSTRACT FROM AUTHOR]