Seismic Potential of Thrust-Wrench Tectonic Interference between Major Active Faults Offshore SW Iberia: A New Explanation for the 1755 Great Lisbon Earthquake?

We propose a new explanation for the origin of the historical and instrumental very high magnitude (Mw > 8.5) earthquakes offshore SW Iberia (NE Atlantic). We investigate the occurrence of thrust-wrench tectonic interference and stress transfer between two major active oceanic faults as a possible cause for (1) successive (cascading) seismic multirupture along different nearby fault segments, or (2) whole-length single fault rupture along a unique continuous fault. We use semiempirical scaling laws to determine the potential magnitude associated with both rupture scenarios, and hence to gain some quantitative insight into the seismic potential of the investigated area. After reviewing the main inconsistencies of previous proposals for the seismogenic sources of the local very high magnitude events (including the 1755 Great Lisbon earthquake), we discuss the plausibility of the multirupture interference scenario by comparing it with other similar well-known cases. We further explain how the proposed thrust-wrench fault interference can better accommodate most of the identified discrepancies of previous interpretations by including the potential to generate either a series of time succeeding (cascading) earthquakes with magnitudes between Mw 7.7 and 8.3, or a single event with magnitude between Mw 8.3 and 8.6 accounting for the simultaneous whole rupture of the fault interference zone.