SAR IMAGING OF THE COSEISMIC AND POSTSEISMIC DEFORMATION FROM THE 2020 SOUTHWEST PUERTO RICO SEISMIC SEQUENCE

We analyzed synthetic aperture radar (SAR) images from Copernicus Sentinel-1A and -1B satellites operated by the European Space Agency and the Advanced Land Observation Satellite-2 (ALOS-2) satellite operated by Japanese Aerospace Exploration Agency for the earthquakes near the southwest coast of Puerto Rico in January 2020 and for later deformation in the area. We use SAR interferometry (InSAR) measurements of displacements in the radar line-of-sight directions and combine data from different radar look directions to estimate two components of the surface displacement for the events. The InSAR measurements are much less sensitive to the north component of displacements so we concentrate on the east and vertical components. The combined measurements reveal large-scale deformation due to slip on faults at depth is dominated by vertical downward motion in and around the Guayanilla Bay with an amplitude of about 20 cm, accompanied by westward motion on the east side and eastward motion on the west side, all consistent with normal fault slip on a generally north-dipping fault plane. The InSAR deformation on land does not constrain strongly the strike of the main fault, but the north-west dipping nodal plane of the USGS moment tensor for the main magnitude 6.4 is consistent with the data. A north-dipping plane is also consistent with relocated hypocenters (HypoDD) of early aftershocks. In addition to the larger displacements likely caused by the magnitude 6.4 mainshock, the ALOS-2 InSAR pair, which spans the time from September 2019 to 20 January 2020, shows that there was fault slip of part of the Punta Montalva Fault between Punta Montalva and Punta Brea, southwest of Guánica. If we assume purely horizontal deformation, the InSAR line-of-sight signal is consistent with 8 cm of left-lateral slip at the surface. This may represent slip during one of the large foreshocks or aftershocks. The longer radar wavelength of the ALOS-2 (24 cm) enables InSAR measurements in the more vegetated Punta Montalva-Punta Brea area that is low coherence with the Sentinel-1 radar (6 cm wavelength). Additional analysis of Sentinel-1 images acquired between January and June show that the Punta Montalva Fault likely had additional slip at the surface after mid-January that may be related to large aftershocks.