Puerto Rico’s 2019-2020 earthquake swarm: Conjugate strike-slip and normal faults within an upper-crustal intra-plate setting

Abstract:

The 2019-2020 southern Puerto Rico’s earthquake swarm initiated with a pre-shock of Mw 4.62 on December 28, 2019, that was followed 11 days later by the Mw 6.40 mainshock on January 7, 2020. Four lives were lost and the estimated economic loss of the prolonged seismicity totaled 3.1 billion USD.

The earthquake swarm consisted of more than 4000 aftershocks of Mw 2.55 or greater that have declined to the present-day across a 1500 km² area of on land and offshore southern Puerto Rico. Most hypocenters rupture at 4-16 km depths as determined by the Puerto Rico Seismic Network (PRSN). In this study we use the distribution of aftershocks and 204 focal mechanisms reported by PRSN to map three fault families that define a large X-shaped pattern of conjugate strike-slip faults: 1) WNW-ESE faults are left-lateral; 2) NNE-SSW faults are right-lateral; and 3) NE-SW faults that intersect NNE-SSW faults are right-lateral oblique-slip normal faults. Focal mechanism principal axes show ENE-WSW compression and NNW-SSE extension and GPS geodetic data show an overall right-lateral NW-SE extensional co-seismic displacement during the mainshock. The majority of the earthquake slip vectors show N-S slip along both the normal and right-lateral strike-slip fault families. The overall fault pattern and displacements within the interior of the X-shaped conjugate faults approximate a left-stepping pull-apart basin with the WNW-ESE left-lateral faults forming the northern and southern edges of the stepover. For the first two weeks, earthquakes of the swarm were confined within this interior stepover area but eventually breached the southern WNW-ESE left-lateral fault and expanded the swarm areas 13 km to the SW. The overall effect of this fault assemblage is to shorten the upper crust in the NE-SW direction and to extend the same crust in the NW-SE direction. This structural pattern is several million years old based on our previous study of the NE normal faults measured at 10 localities in southern Puerto Rico that span the 2019-2020 aftershock zone.