OVERVIEW OF PRELIMINARY INVESTIGATIONS OF THE 2020 SOUTHWEST PUERTO RICO SEISMIC SEQUENCE (Invited Presentation)

The 2020 Southwest Puerto Rico Seismic Sequence commenced on December 28, 2019 along an offshore extension of the Punta Montalva fault with a series a left-lateral strike slip events including a M_L 5.0 event on December 29, 2019 and a Mw5.8 event on January 6, 2020. The current hypothesis is that this triggered an adjacent normal fault via stress transfer, which induced the Mw 6.4 mainshock earthquake on January 7, 2020. This represents the largest magnitude damaging event felt in Puerto Rico since the 1918 7.3M Mona Canyon earthquake. For the 2020 mainshock, shaking intensities were as high as VIII nearest the epicenter. Strong Motion Stations recorded PGAs at the Yauco Fire Station of 40%g at 8.7 km epicentral distance and 45%g in Utuado UPR at 34.2 km epicentral distance. This destructive earthquake caused significant structural damage across southwestern Puerto Rico homes and infrastructure and also generated a local small tsunami of ~0.04m recorded at the Magueyes Island sea level station. Widespread rock falls and scattered coastal liquefaction were also triggered by the shaking. The 6.4Mw event generated a permanent onshore subsidence up to 0.20 m based on interferometric synthetic aperture radar (InSAR) measurements. Additionally, thirteen stream gauges in western Puerto Rico recorded up to a 40% increase in stream height following the event, and local hot springs in Coamo recorded a 5°C increase in water temperature.

Over 10,000 earthquakes related to the sequence have been manually reviewed by the geophysical data analysts at the Puerto Rico Seismic Network through the end of July 2020. Ninety-eight of these were M4.0 or greater. Preliminary relocations of earthquakes using HypoDD indicate that the sequence evolved from a simple stress transfer between the left-lateral Punta Montalva fault and the adjacent normal fault into a more complicated sequence involving a suite of multiple interacting strike-slip and normal faults in a previously unnamed fault zone. This interpretation is consistent with results of active source seismic surveys performed by the USGS in collaboration with the University of Puerto Rico at Mayagüez in the Spring of 2020. The intricacy of the fault zone and ongoing seismic sequence highlight the tectonic complexity of the northeastern Caribbean region in general.