Relocated aftershocks of the March 10, 1988 Trinidad earthquake: Normal faulting, slab detachment and extension at upper mantle depths

Abstract

We relocated aftershocks of the March 10, 1988 earthquake off the east coast of Trinidad in order to determine the fault plane for the main shock and to understand this anomalous earthquake series. The March 10 main shock (M W 6.6) was the first significant instrumentally recorded earthquake to occur in the epicentral region east of Trinidad. The reported depth of the main shock, 53 km (by the International Seismological Centre, ISC), is unusual, but is consistent with observations of pP– P times. The main shock Harvard centroid moment tensor (CMT) solution, nearly pure normal faulting on nodal planes striking ENE, is also unusual given the depths of the seisms and the rarity of extensional faulting at these depths. Using P arrival times tabulated by the ISC, we relocated the aftershocks using the joint hypocentral determination method to find new locations relative to the main shock. In map view, the relocated epicenters form a tight elliptical cluster, with major axis trending WNW–ESE and minor axis NNE–SSW. Virtually all the seisms in the earthquake series occurred at depths ranging from 30 to 60 km, deeper than normal crustal earthquakes, in an area previously not considered a locus of subducting lithosphere. The aftershocks define a distinct plane, dipping 34°NW. This dip is consistent with the Harvard CMT focal mechanism of the main shock: one CMT nodal plane dips 38°N and strikes ENE, very close to our estimate of the active fault plane. Thus, the March 10 aftershocks are consistent with slip on the north-dipping plane of the main shock. Because the earthquake series occurred 100 km SE of oceanic South American slab subducting beneath the Caribbean plate, at a depth generally inconsistent with shallow normal faulting outside of slabs, we interpret the main shock as having occurred deep (50 km) in South American lithosphere currently moving NW into the subduction zone. The occurrence of this earthquake indicates an extensional stress applied to the lithosphere of the South American plate, likely related to a combination of slab pull to the NW as the slab subducts beneath the Caribbean plate, and to detachment of the subducting lithosphere from buoyant continental South America.