Source analysis of the February 12th 2007, MW 6.0 Horseshoe earthquake: Implications for the 1755 Lisbon earthquake

While very large earthquakes are generally confined to subduction zones, the SW Iberian margin –setting of the famous Mw 8.5–8.7, 1755 Lisbon tsunami earthquake- may be an exception to this rule. Evidence for active subduction is not conclusive here, but instead plate convergence in old oceanic lithosphere with large brittle layer thickness can account for the occurrence of great earthquakes along moderate-length faults. We estimate the source parameters of the February 12th 2007, Horseshoe earthquake. Regional moment tensor inversion yields an Mw 6.0, reverse to strike-slip faulting source in the upper mantle. Modelling teleseismic, surface-reflected body waves (pP, pwP, sP) indicates a source depth of 40 km beneath the seafloor. Analysing apparent source time functions allows identifying the preferred fault plane (strike N245°E/ dip 55°/ rake 50°), and estimating rupture area (53 km2) and average slip (0.27 m). Scaling the source characteristics to the size of the 1755 earthquake suggests a fault length of 230–315 km, being compatible with the length of mapped faults in the area.