TECTONIC SETTING OF THE 2019-2020 PUERTO RICO EARTHQUAKE SWARM BASED ON INTEGRATION OF ACTIVE FAULTS, EARTHQUAKE FOCAL MECHANISMS, AND TECTONIC GEOMORPHOLOGY

The islands of Puerto Rico and Virgin Islands (PRVI) are exposed as land areas on the axis of EW-trending arch that folds 20-km thick, Cretaceous to Oligocene island arc crust capped by a 3 km-thick cap of Oligocene to recent clastic and carbonate rocks. The PRVI arch formed as the consequence of Eocene to recent southwestward amagmatic subduction of Atlantic oceanic crust (North America Plate) along the Puerto Rico Trench and northward amagmatic subduction of Caribbean oceanic plateau crust (Caribbean Plate) along the Muertos Trench. The 2019-2020 earthquake swarm occurred at intraplate depths of mostly 4-16 km in the locations about 15-40 km south of the PRVI arch axis and 10-20 km above the underlying subducted Caribbean slab. In this study, we show that the complex faulting within the 1,500 km² area of the earthquake swarm closely reflects the more regional 8,700 km² pattern of active faults and tectonic geomorphology. The deformation on both a regional scale and the swarm area scale can be conceptualized as a strain ellipse that is shortening in a NE-SW direction and simultaneously lengthening in a NW-SE direction. On a regional scale, NE-SW compression is expressed by the E-W trending PRVI arch while concurrent NW-SE extension is expressed by NE-SW striking normal faults. Within the swarm area, earthquake focal mechanism solutions from 204 events show an X-shaped pattern of conjugate strike-slip faults with WNW-ESE left-lateral faults and NNE-SSW right-lateral faults as well as NE-SW normal faults intersecting the NNE-SSW faults. Tectonic geomorphology shows that the tectonic uplift is slow with gentle longitudinal river profiles and low normalized steepness indices. These results are consistent with longer term studies of uplifted coral terraces indicating minimum uplift rates between 0.03 and 0.05 mm/yr over the past 4 million years. Despite these slow uplift rates, the Puerto Rico earthquake swarm demonstrates that the deforming island arc crust is under a uniform stress related to its unusual subduction tectonic setting and is capable of producing damaging intra-plate earthquakes.