Huérfano, Victor¹, Pulliam, Jay², ten Brink, Uri S.³ Lopez-Venegas, Alberto³ and von Hillebrandt-Andrade, Christa¹

¹Red Sísmica de Puerto Rico, Dpto. de Geología, Universidad de Puerto Rico, Mayagüez, PR USA
²Institute for Geophysics, Jackson School of Geoscience, University of Texas at Austin, Austin, TX USA
³U.S. Geological Survey, Woods Hole, MA USA

Ocean bottom seismographs (OBSs) were deployed in the Northeast Caribbean in 2005 and 2007 to record earthquakes in the Sombrero Seismic Zone (SSZ). Our goals were to better constrain earthquake locations, to identify crustal features, including a possible rupture in the subducting North American Plate, that could cause tsunamigenic events and to clarify the tectonics that lead to larger earthquakes and tsunamis. Data were analyzed jointly with records from permanent stations of the Puerto Rico Seismic Network (PRSN), whose stations are located on islands to the south of the SSZ and are therefore biased and incomplete without offshore complements. Our goal was to identify, locate, and estimate focal mechanisms for small-magnitude seismic events in this corner of the Caribbean.

In the 2005 data set we identified and analyzed thirty-five events on at least four OBSs and one or more PRSN stations and an additional fifteen events that were observed on four or more OBSs only. Upon relocation, the jointly recorded events revealed tighter clustering and clear biases with respect to original PRSN locations. Changes in epicenters were greatest in the northeastern limit of activity, while there was a consistent progression toward deeper focal depths for events closer to the island of Puerto Rico. The average change in focal depth for the cluster of five events closest to Puerto Rico was 35 km. Events located solely with OBSs were consistent with the relocated events.

Most of the relocated events are consistent with the depth and dip of the subducted North American plate in this region and may therefore be associated with active subduction, rather than the shallow tectonics suggested by original locations. Events recorded during the longer 2007 OBS deployment are being analyzed to explore this association and its implications for tearing of the North American Plate lithosphere during subduction.