Tectonic inversion in the Caribbean-South American plate boundary: GPS geodesy, seismology, and tectonics of the Mw 6.7 22 April 1997 Tobago earthquake

On 22 April 1997 the largest earthquake recorded in the Trinidad-Tobago segment of the Caribbean-South American plate boundary zone (M-w 6.7) ruptured a shallow (similar to 9km), ENE striking (similar to 250 degrees azimuth), shallowly dipping (similar to 28 degrees) dextral-normal fault similar to 10km south of Tobago. In this study, we describe this earthquake and related foreshock and aftershock seismicity, derive coseismic offsets using GPS data, and model the fault plane and magnitude of slip for this earthquake. Coseismic slip estimated at our episodic GPS sites indicates movement of Tobago 1356 to 686mm NNE and subsidence of 79 to 0mm. 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 (similar to 28 degrees) 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.