Abstract The 2019–2026 southwestern Puerto Rico earthquake sequence has the characteristics of an earthquake swarm with over 9,000 events of Mw 2.5 or greater over an area of 1,500 km 2 , and the largest event is an Mw 6.4 on 7 January 2020. Previous studies have proposed two structural interpretations: (a) an E‐striking, N‐dipping normal fault hosting the largest event with a left‐lateral oblique slip; superimposed on this fault is an X‐shaped conjugate strike‐slip faults reflecting NE‐SW shortening; and (b) a NE‐striking, SE‐dipping normal fault of the largest event with a right‐lateral oblique slip. Based on focal mechanism solutions, global navigation satellite system (GNSS) and satellite radar interferometry data, we propose four fault families that define a slightly different X‐shaped pattern: (a) left‐lateral WNW‐ESE faults including two parallel faults that define a 12‐km‐wide stepover area and pull‐apart basin; (b) an WSW‐ENE left‐lateral fault; (c) right‐lateral NNE‐SSW faults; and (d) E‐W left‐lateral oblique‐slip normal faults including the one hosting the largest event inside the pull‐apart basin. This active fault kinematics closely reflects measured faults in Late‐Miocene‐Pliocene rocks, indicating that this deformation pattern has been active for several million years. The proposed fault kinematic model is consistent with geodetic data and an overall crustal deformation model of coupled NW‐SE extension and NE‐SW compression produced by oblique convergence across the North America‐Caribbean plate boundary. The occurrence of prolonged earthquake sequence on crustal conjugate systems poses a seismic hazard for this oblique plate boundary.
Sun et al. (Sun,) studied this question.