A network of 15 short-period seismograph stations in the northeastern Caribbean became fully operational in late 1975. The signals from the 15 seismometers are transmitted via FM radio to a central recording station on St. Thomas in the Virgin Islands. A Geotech Develocorder is being used as the principal recording device with several Helicorder records available for quick scanning for events.
The network spans the region of changing tectonic style at the northeastern boundary of the Caribbean plate. On the eastern boundary, the American plate is underthrust beneath the northern Lesser Antilles; on the northern boundary the relative motion between the plates is characterized by oblique thrusting. A principal objective in placing the network in this tectonic transition zone is to understand the nature of this transition and its impact on the problem of earthquake hazard evaluation through the analysis of microearthquake data.
The spatial pattern of small and microearthquakes recorded by the network is similar to that observed using 25 years of teleseismic data. Most of the earthquakes recorded locally and teleseismically are located between the axis of the Puerto Rico trench and the Virgin Islands platform. A few shallow earthquakes are located beneath the platform itself. The north wall of the Anegada trough, to the south of the platform, is also the site of shallow earthquake activity. As with the teleseismic data, the new hypocenters, in general, form an inclined seismic zone dipping to the south beneath the island platform and reach a depth of 125 to 150 km. There is a clear spatial correlation between the distribution of earthquakes and bathymetric features on the inner wall of the Puerto Rico trench; in particular, a prominent submarine ridge coincides with a marked change in the spatial density of earthquakes.
There are striking variations in the daily rate of earthquake occurrence. A significant portion of the earthquake activity occurs as swarms. During these swarms the daily number of events may jump one order of magnitude or more (e.g., 5 events/day to events/day); most of these events are very shallow (<20 km), spatially clustered, and occur to the north of 19°N.