New GPS Evidence for Continental Transform Fault Creep, Central Range Fault, Trinidad, Caribbean-South American Plate Boundary and Geological/Hazard Implications

John Weber, Grand Valley State University
Chris Churches, Grand Valley State University
Richard Robertson, UWI-SRC
Pete La Femina, PSU
Halldor Geirsson, PSU

Most continental plate boundary transform faults exhibit stick-slip behavior and lock elastically. Important exceptions include segments of the Hayward, Parkfield, and central San Andreas Faults. Studying how and why fault creep occurs has implications for our understanding how faults work in general and how to better assess their associated hazards. Trinidad is located in the Caribbean-South American (Ca-SA) dextral transform plate boundary. Weber et al. (2001, 2011), using triangulation-to-GPS geodetic measurements, discovered that the 072°-trending Central Range Fault (CRF) is the active transform in Trinidad, accommodating ~two-thirds of the total ~20 mm/yr ~E-W Ca-SA plate motion with an aseismic, interseismic slip rate of 12 ± 3 mm/yr. We build on this earlier work using new high-precision data collected between 1994 and 2014 from a set of ~25 continuous and episodic GPS stations that we processed using GIPSY-OASIS II. 2-D elastic fault dislocation modeling of the new GPS velocity field gives a best-fit (reduced χ2 = 4.119) interseismic slip-rate of 14 ± 1 mm/yr and a fault locking depth of 0.083 ± 0.153 km (model parameters given at 1σ uncertainty determined via empirical bootstrapping method). The shallow locking depth is well resolved, indicating that the CRF largely creeps. However, a sparsity of data in the east leaves open the possibility of some/more locking there. The new results are consistent with the one prehistoric (~2.7 - .55 ka; Prentice et al., 2010) earthquake trenched along the CRF being of small magnitude or possibly related to a slow-slip event, and with the low permanent neotectonic, strike-slip strain recorded/observed in the walls of the fault (Giorgis et al. 2011). We propose that a rigid northern block bounds a soft, weak, thick, Paleogene-shale-rich covered southern block along a sharp 072°-oriented inherited (?) boundary along which the neotoectonic strain concentrates. The lack of sharp, conclusive creep offsets that were searched for, but not observed on the ground (Weber et al. 2011) could mean that the CRF creeps across a relatively broad (dm-scale?) zone. We next plan to establish a series of creep arrays and a network of microseismometers and additional cGPS stations to better quantify the behavior of the CRF.