Geotechnical Impacts of the January 7, 2020, Mw 6.4 Puerto Rico Earthquake and Associated Seismic Sequence

A seismic sequence along the south coast of Puerto Rico (PR) initiated on December 28, 2019. The strongest event, an M_w 6.4 earthquake with epicenter just offshore (17.9578, −66.8113) near Guayanilla on January 7, 2020. The ongoing (as of January 2021) sequence has involved hundreds of additional felt seismic events, many of which registered above M_w 5.0. The earthquakes resulted in moderate to locally severe structural damage in several south coast municipalities. Several specialized teams (e.g., USGS, GEER, STEER) were deployed and worked synergistically to assess and document perishable structural and geotechnical damages after the M_w 6.4 event. This paper summarizes geotechnical failure documented by the GEER reconnaissance mission. The locations of common liquefaction, ground extension, settlement, and mass wasting features are concentrated in the immediate epicentral area in the municipalities of Guánica, Yauco, Guayanilla, Peñuelas, and Ponce. Abundant small to medium sized sand boils and longitudinal blows were documented mostly in low-lying areas very near the coastline in Guánica and Guayanilla. Ground extension was observed almost always parallel to a nearby coastline or river channel. Several of these sites were noted to have reactivated during the episodic seismicity. Along the coastline in Ponce, the widest spreading features at La Guancha were measured to have 20 cm separation. The dominant mass wasting mechanism was rock fall, which was common across the south coast limestone bedrock exposed in the epicentral area. Similar failures in limestone of northwestern Puerto Rico were reported after the 1918 M 7.0 Mona Passage event. Failures along roadways were almost exclusively small, but temporarily blocked some routes. Rock falls and slumps on unmodified hillslopes were generally larger and involved much longer run out distances, especially for boulders commonly larger than 2 m across.