Observations from a seismic network in the Virgin Islands Region: Tectonic structures and earthquake swarms

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Abstract
Hypocenters of earthquakes located by a telemetered seismic network in the Virgin Islands provide a detailed picture of the seismicity along the northeast boundary of the Caribbean plate. The Virgin Islands‐Puerto Rico group represents an island arc characterized by plate motion highly oblique to the arc. The network data indicate a complex distribution of epicenters that is not homogeneous along the strike of the plate boundary. Earthquake foci between 25‐ and 120‐km depth align along a downgoing seismic zone that dips to the south beneath the Virgin Islands, indicating the presence of a subducted portion of the North American plate. Shallow events above 20‐km depth are concentrated between 64.5° and 65.1°W and appear to be associated with the boundary between the North American plate and the Caribbean plate. Twenty swarms of earthquakes (localized surges of shocks of nearly equal size) were detected by the network during the 2½ year period of study. Three of the swarms consisted of over a hundred events between magnitude 2 to 4½. Certain areas produced more than one swarm within this time interval. Some of the environments that generated the swarms include the region beneath the inner wall of the Puerto Rico trench near 19°N, 65°W, one portion of the outer wall of the trench, and a zone 40–50 km deep within the North American plate at about 19°N. During the period of observation, each swarm that occurred near 19°N between 64° and 65°W was followed by a second swarm in that area within 28 days. Results from the network indicate that the north wall of the Virgin Islands basin, which forms a portion of the Anegada passage, is a significant locus of seismicity within the Caribbean plate. This activity was largely manifested as swarms to the south of the islands of Vieques and St. Thomas during the time of network observation.