The Azores‐Gibraltar Plate Boundary: Focal mechanisms, depths of earthquakes, and their tectonic implications

Abstract

We have analyzed the focal mechanisms and depths of 10 moderately sized earthquakes along the Azores‐Gibraltar plate boundary by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases. Our data, together with a compilation of results reported for very large events from the past 30 years and for two recent events in 1983, place new constraints on the present‐day deformation of this boundary separating the Eurasian and African plates. The slow (∼2 to 3 mm/yr) divergent plate motion near the Azores Islands at the western end of the boundary appears to be very similar to that of a typical ridge‐transform‐ridge configuration. A currently aseismic segment with presumed transcurrent motion separates the western end from a broad zone of ocean‐ocean convergence to the east. This convergence zone is characterized by scattered seismicity, complex bathymetry, and large positive geoid and gravity anomalies. The dispersed locations of the earthquakes and the lack of consistency in the orientation of their nodal planes and slip vectors all suggest that a single major plate boundary is not present there. We note that a regional stress field of north‐northwest‐south‐southeast compression can be inferred from the consistent P axis directions. The focal depths for earthquakes in this region reach a maximum of 50±5 km beneath the seafloor. These events are among the deepest oceanic earthquakes which are apparently unrelated to a Benioff zone and their maximum depths are approximately limited by the position of the 600°C isotherm. We interpret the regional stress field and the focal depths as the result of the interaction of two sections of strong, cold oceanic lithosphere with nearly identical thermal structure. As the plate boundary crosses the passive continental margins of Iberia and Africa, the zone of ocean‐ocean convergence becomes that of continental collision. Two unusually deep events, one at 100 and another at 640 km, have occurred beneath southern Spain since the occurrence of the deep Spanish earthquake of 1954 in the same area. The configuration of a seismic upper crust and uppermost mantle straddling an aseismic lower crust resembles the distribution of focal depths in other areas of recent continental convergence. Based on the distribution of focal depths along the entire plate boundary and recent reconstructions of the history of relative motions between Eurasia and Africa, the very deep events beneath southern Spain are likely to have occurred within a detached piece of lithosphere which sank to its present depth during the current phase of plate convergence.