Interseismic Strain Accumulation near Lisbon (Portugal) from Space Geodesy

Abstract

The southern sector of the Lusitanian basin, SW Portugal (Figure 1a), has been the locus of relevant seismicity in historical time (Moreira, 1989;Stucchi et al., 2013). The list of known destructive earthquakes affecting the region and the adjacent continental shelf ranges in time from 1344 to 1909, with catastrophic occurrences in 1356, 1531, and 1755 (https://www.emidius.eu/SHEEC). The existence of important seismogenic structures offshore SW Portugal was recognized at an early stage, but the 1909 earthquake, with epicenter ∼40 km to the NE of Lisbon and estimated magnitude in the range M6.0-M6.5, had a clear intraplate nature, and it is widely accepted that the M7 1531 earthquake also nucleated onshore, in the active structures of the Lower Tagus Valley (Canora et al., 2021;Justo & Salwa, 1998). All these features point toward a diffuse zone of deformation involving both onshore and offshore active structures, which accommodate the Nubia-Eurasia plate convergence through a relevant seismicity release, as recently proposed by Palano et al. (2015). The relative importance of the contributions of onshore versus offshore sources to seismic hazard in Portugal is largely debated. On one hand, in view of the modest NW Africa-SW Iberia convergence rate (∼4 mm/yr in a NW-SE direction; Fernandes et al., 2003), it has been argued that most of the cumulated crustal deformation is fully released by 1969-type offshore earthquakes of the Gulf of Cadiz, implying that intraplate faults account for very small slip-rates. It follows that destructive intraplate earthquakes are deemed very rare events with limited contribution to the probabilistic hazard (e.g., Ramalho et al., 2020). This view is supported by very low intraplate slip-rate estimates derived from geological studies (0.005-0.3-0.5 mm/yr; Cabral, 2012). On the other hand, seismic hazard disaggregation studies have led