The 1998 Faial earthquake, Azores: Evidence for a transform fault associated with the Nubia–Eurasia plate boundary?

Abstract

With very few exceptions, M>4 tectonic earthquakes in the Azores show normal fault solution and occur away from the islands. Exceptionally, the 1998 shock was pure strike-slip and occurred within the northern edge of the Pico–Faial Ridge. Fault plane solutions show two possible planes of rupture striking ENE–WSW (dextral) and NNW–SSE (sinistral). The former has not been recognised in the Azores, but is parallel to the transform direction related to the relative motion between the Eurasia and Nubia plates. Therefore, the main question we address in the present study is: do transform faults related to the Eurasia/Nubia plate boundary exist in the Azores? Knowing that the main source of strain is related to plate kinematics, we conclude that the sinistral strike-slip NNW–SSE fault plane solution is not consistent with either the fault dip (ca. 65°, which is typical of a normal fault) or the ca. ENE–WSW direction of maximum extension; both are consistent with a normal fault, as observed in most major earthquakes on faults striking around NNW–SSE in the Azores. In contrast, the dextral strike-slip ENE–WSW fault plane solution is consistent with the transform direction related to the anticlockwise rotation of Nubia relative to Eurasia. Altogether, tectonic data, measured ground motion, observed destruction, and modelling are consistent with a dextral strike-slip source fault striking ENE–WSW. Furthermore, the bulk clockwise rotation measured by GPS is typical of bookshelf block rotations observed at the termination of such master strike-slip faults. Therefore, we suggest that the 1998 earthquake can be related to the WSW termination of a transform (ENE–WSW fault plane solution) associated with the Nubia–Eurasia diffuse plate boundary.