Global Positioning System (GPS) Determination of Motions, Neotectonics, and Seismic Risk in Trinidad and Tobago

The twin island nation of Trinidad and Tobago is located in the actively deforming Caribbean-South American (Ca-SA) plate boundary zone. Recent GPS (Global Positioning System) studies have accurately determined the relative plate motion between the Caribbean plate and South American plates: ~east-west, dextral motion at 20 mm/yr. Earthquakes do not clearly mark many of the active faults in Trinidad. A low-precision triangulation-to-GPS comparison at 23 sites and paleoseismology showed that significant strike-slip faulting is probably occurring on the Central Range Fault (CRF). The lack of recent seismic activity on the CRF may indicate that it is elastically locked and storing motion. We study Trinidad's neotectonics using new GPS data from 19 high-stability campaign sites that were built and measured in 2005, then measured again in 2007; for a few additional sites we have data that go back from 2007 to 1994. These new data will allow us to test and refine the previous lower-precision geodetic results to better quantify the rate of slip across the CRF and its mechanical behavior. We compiled, and then processed the data using GIPSY/OASIS II (Release 5.0) software at the University of Miami RSMAS Geodesy Lab. We find that, in a South American reference frame, sites north of the CRF move at about 20 (±1-5) mm/yr; sites south of the CRF are stationary (±1-8 mm/yr). Tobago site velocities are slightly oblique to overall Caribbean plate motion due to a major (Magnitude 6.7) earthquake that occurred off Tobago's south coast in 1997. Our new results support the hypotheses that the CRF is the principal active strike-slip fault in Trinidad (i.e., is the current Ca-SA plate boundary). We are fitting locked fault (elastic dislocation half-space) models to the data which will allow us to look more closely at the mechanical behavior of the CRF to further test whether it is locked or creeping to help evaluate its seismic risk.