Estimating Coseismic Deformation of Southwestern Puerto Rico from the 7 January 2020 Mw 6.4 Earthquake: Constraints from Campaign and Continuous GPS

The Puerto Rico-Virgin Islands (PRVI) block lies within the Northern Caribbean Plate Boundary Zone-a zone accommodating stresses between the larger North America and Caribbean plates. Data from Global Positioning System (GPS) sites throughout the PRVI block have been used to confirm the existence of a distinct microblock in the southwest. It is no coincidence that this portion of the PRVI block is the epicentral region of the 7 January 2020 Mw 6.4 earth-quake and the ensuing seismic sequence. Prior to the mainshock, the southwestern Puerto Rico (SWPR) region exhibited most of the onland seismic activity. The 2020-2021 SWPR earth-quake seismic sequence has been characterized by having an atypical aftershock decay dis-tribution occurring along multiple faults. As a result, fault parameters of the 7 January 2020 mainshock have been poorly defined by conventional seismic methods. Here, we present results from campaign and continuous GPS sites in SWPR, and compare GPS-derived displace-ments to those computed from the U.S. Geological Survey National Earthquake Information Center (NEIC) focal mechanism. We conclude that irrespective of which nodal plane is used, the observed coseismic displacements from GPS differ from those predicted using a simple elastic model and the NEIC focal mechanism. We infer based on these observations that the complex mainshock rupture resulted in a suboptimal double-couple solution.