Abstract
The twin island nation of Trinidad and Tobago is located in the actively deforming Caribbean-South American (Ca-SA) plate boundary zone. Geodetic GPS work over the past decade has accurately determined present-day Ca-SA relative plate motion. This work (Weber et al., 2001; Perez et al., 2001; Lopez et al., 2006) clearly shows the Caribbean plate today moves approximately eastward relative to the South American plate at ~20 mm/yr. Earthquakes do not mark the active faults in Trinidad; however, a low-precision triangulation-to-GPS comparison at 23 sites showed that significant strikeslip faulting is probably occurring on the Central Range Fault (CRF) (Saleh et al., 2004). The lack of recent seismic activity on the CRF might indicate that the fault is elastically locked. Locked faults tend to store motion and then release it in infrequent big earthquakes, whereas creeping faults produce more continuous small earthquakes. A future large earthquake is therefore possible in our study area. Our research looks at Trinidad’s neotectonics using new GPS data from 19 high-stability sites that were built and measured in 2005, then measured again in 2007, and also includes data from a few sites that go back to 1994. These new data allow us to refine previous results from the lowerprecision geodetic work. We intend to better quantify the rate of slip across the CRF and its mechanical behavior. We used campaign-style GPS field measurements collected by coauthor Weber and associates at the University of the West Indies. We compiled these data and processed them 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 the overall Caribbean plate motion due to a major (magnitude 6.7) earthquake that had occurred off Tobago’s south coast in 1997. Our results support the hypotheses that the CRF is an active strike-slip fault. We find that sites in Tobago and northern Trinidad are on the Caribbean plate and move at a rate of about 20 mm/ yr eastward (relative to South America). Sites in southern Trinidad move slowly if at all relative to South America. We are fitting locked fault (elastic dislocation half-space) models to these data; this will allow us to look more closely at the mechanical behavior of the CRF and to establish whether it is locked or creeping to help evaluate its seismic hazard. This could be an important factor in determining the safety of citizens living in Trinidad and Tobago.