Applying earthquake cycle concepts in complex plate boundary zones: An example from the Caribbean-South American plate boundary zone, Trinidad and Tobago
John Weber, weberj@gvsu.edu
Halldor Geirsson, University of Iceland
Peter La Femina, The Pennsylvania State University
Joan Latchman, The University of The West Indies, Seismic Research Centre
Richard Robertson, The University of the West Indies, Seismic Research Centre
We studied active faults in Trinidad and Tobago in the Caribbean-South American (CA-SA) transform plate boundary zone using episodic GPS (eGPS) data from 19 sites and continuous GPS (cGPS) data from 8 sites, and then modeling these data. Our best-fit model for interseismic fault slip requires: 12-15 mm/yr of right-lateral movement and very shallow locking (0.2 ± 0.2 km; essentially creep) across the Central Range Fault (CRF); 3.4 ± 0.3mm/yr across the Soldado Fault in south Trinidad, and 3.5 ± 0.3 mm/yr of dextral shear on fault(s) between Trinidad and Tobago (see below). Faults in Trinidad show very little seismicity (1954-current from local network). Paleoseismic studies indicate that the CRF ruptured between 2710 and 500 yr. B.P. Together, these data suggest spatial and/or temporal fault segmentation on the CRF. The CRF marks a physical boundary between rocks associated with thermogenically generated petroleum and overpressured fluids in south and central Trinidad, from rocks containing only biogenic gas to the north, and a long string of active mud volcanoes align with the trace of the Soldado Fault along Trinidad's south coast. Fluid (oil and gas) overpressure may thus cause the CRF and Soldado Faults to creep.
On 22 April 1997 the largest earthquake recorded in the Trinidad-Tobago segment of the Caribbean-South American plate boundary zone (Mw 6.7) ruptured a shallow, ENE striking, shallowly dipping (~28°) dextral-normal fault ~10 km south of Tobago. We studied this earthquake and related foreshocks and aftershocks. We derived coseismic offsets using GPS data, and modeled fault rupture and coseismic slip. Tobago moved NNE and subsided. This earthquake was anomalous and is of interest because: (1) its large component of normal slip and ENE strike are unexpected given the active E-W dextral shearing across the Caribbean-South American plate boundary zone, (2) it ruptured a normal fault plane with a low (~28°) dip angle, and (3) it reactivated and inverted the preexisting Tobago terrrane-South America ocean-continent (thrust) boundary that formed during early Tertiary oblique plate convergence.