P-wave tomography and seismotectonics of the Puerto Rico subduction zone

Abstract:

The Puerto Rico (PR) trench is a complex transition of a progressively more oblique convergence from east to west. North of the PR and the Virgin Islands, the seismicity is characterized by frequent earthquake swarms and by a change from a typical fore-arc morphology to a collapsed fore-arc. This study incorporates the data from six ocean bottom seismometers (OBS) deployed by the USGS offshore of PR (Mid-2015 to Mid-2016) with data from land-based PR Seismic Network (1964-2019, M>4) for local and regional events. The OBS data substantially improves the azimuthal coverage for seismicity that predominantly occurs offshore. The processing involves earthquakes relocations, determination of source mechanisms, and P-wave seismic imaging (using the tomography code of Zhao D. et al., 2012). A total of 186 teleseismic events from ISC-EHB catalog were added to the inversion along with the 3515 local and regional earthquakes. Preliminary results show that significant portion of the seismicity is within the crust of the overriding plate (20-km-thick). To the southeast of the NW-trending Main Ridge seamount, the NW trend of seismicity agrees with the focal mechanism for multiple large earthquakes with thrust faulting both at shallow and deeper depths. Deeper seismicity is not uniform (denser at –64.6° and –66.4°) and in some profiles is correlated with low velocity zones. The PR platform is underlain by a low velocity at 20 km with bordering high velocities to the north. The northern Mona Rift is underlain by a high velocity anomaly below 20 km. About 40-60 km away from the PR trench, an intermittent high velocity anomaly extends parallel to the trench that we interpreted as the slab at 40-km depth.