INTERMEDIATE-DEPTH SEISMICITY IN NORTHERN COLOMBIA AND WESTERN VENEZUELA AND ITS RELATIONSHIP TO CARIBBEAN PLATE SUBDUCTION

The recent intermediate-depth seismicity in northern Colombia and western Venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern South America. The area included in this study is located to the north and east of the Bucaramanga nest, which is a particular region beneath Bucaramanga, northern Colombia, that presents a high concentration of intermediate-depth earthquakes. To the north of the nest, the seismicity of the area is sparse, and most of the events are of low magnitude (m(b) less than or equal to 5.1). Thus only 23 earthquakes were large enough to be investigated using teleseismic data. The focal parameters of the two largest events (m(b) greater than or equal to 5.4) were obtained from the formal inversion of long-period body waves recorded at teleseismic distances. The focal mechanisms of 10 more events were determined from first-motion data. In total, the focal mechanisms of 12 events were determined from both the inversion of P and S H waveforms and the polarities of first arrivals. For the smaller earthquakes, the focal depths were calculated by measuring the observed pP-P interval time and comparing it to the theoretical travel time tables. The isodepth curves reflect a slab striking in a NNE-SSW direction and dipping approximately at 25 degrees-32 degrees to the southeast. This observation is corroborated by the direction and plunge of the T axes of the focal mechanisms, which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters. These results indicate that the intermediate-depth earthquakes in western Venezuela and northern Colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of Colombia. The two largest earthquakes, located at a significant distance from the Bucaramanga nest, present similar fault plane solutions. Moreover, they also agree with those of the two largest earthquakes reported inside the nest. This similarity suggests that the Bucaramanga nest lies on the same subducted slab where the other earthquakes occur. There is not enough shallow seismicity to define the location where the Caribbean lithosphere subducted beneath the South American plate. However, the extension of the slab toward the surface, inferred from the intermediate-depth seismicity, suggests that the subducted lithosphere may still be attached to the Caribbean plate.