Imaging a diffuse block boundary: Preliminary results from a multichannel seismic reflection survey to identify the fault sources of the southwestern Puerto Rico seismic sequence

Abstract:

The ongoing seismic activity in southwestern Puerto Rico which started on December 28, 2019, appears to have activated a zone of diffuse deformation, where rupture may be transitioning from one fault to another. This activity may reflect the establishment of a new block boundary between Puerto Rico and Hispaniola, where Hispaniola is moving west relative to Puerto Rico. Earthquake focal mechanisms are mixtures of NW-SE striking normal ruptures and E-W and N-S oriented strike-slip ruptures, with many of the epicenters located offshore. The largest earthquake, a M6.4 on January 7, 2020, caused ≤20 cm of shoreline subsidence around Guayanilla. In order to track the fault geometry offshore, 250 line-kilometers of high-resolution multichannel sparker seismic reflection data were collected during March 7-13, 2020, aboard the University of Puerto Rico’s R/V Sultana. Data imaged the top 0.5 s (~0.5 km) of the sub-seafloor sedimentary section. Numerous faults offsetting all but the shallowest unconsolidated(?) sediments were identified along an ENE trend ~5 km seaward of the shelf edge and parallel to it. Additional faults were identified ~15 km from the shelf edge and intersect the surface projection of the M6.4 earthquake, whose focal mechanism indicated normal motion down to the NW. A fault, identified within Guayanilla Bay, may be a continuation of a fault mapped on land. The offshore continuation of Punta Montalva Fault was not identified in deep water and may be masked by acoustic reverberations on the shelf. Bathymetry of the area is dominated by the Guayanilla canyon system, the only significant submarine drainage on the south coast of Puerto Rico. The canyon system has eroded into the insular shelf, forming an asymmetric amphitheater with its long axis sub-parallel to the presumed strike of the normal faults. Right-angle meandering of submarine channels and NE-oriented bathymetric lineaments that are shallower to the SE, suggest recurrent down-to-the-NW normal displacement over geologic time on SW-NE striking faults, which facilitated shelf collapse and the development of the canyon system.