Crustal deformation near Lisbon, Portugal, from GNSS and PSInSAR data

The Lisbon Metropolitan Area, in the southern sector of the Lusitanian basin, SW Portugal, has been affected by relevant seismicity. Known destructive earthquakes affecting the region range in time from 1344 to 1969, and include catastrophic occurrences in 1356, 1531, 1909 and 1755. Modern instrumental data are available for the M7.8 Cadiz Gulf earthquake of 1969 only, which reached EMS-98 intensity 5 to 6 in the study area. While several of these earthquakes nucleated offshore, the 1909 earthquake, with 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. 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), 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. This view is supported by very low intraplate slip-rate estimates of 0.005 to 0.3-0.5 mm/yr derived from geological studies. However, seismic hazard disaggregation studies indicate that the dominant scenario is the rupture of an intraplate fault.

Using a dense GNSS dataset coupled with PSInSAR analysis, we characterize the style of crustal deformation in the Lisbon Metropolitan Area and estimate the associated fault slip rates. We provide evidence of sinistral simple shear driven by a NNE-SSW first-order tectonic lineament. PSInSAR vertical velocities corroborate qualitatively the GNSS strain-rate field, showing uplift/subsidence where the GNSS data indicate contraction/extension. We propose the presence of a small block to the W of Lisbon moving independently towards the SW with a relative velocity of 0.96±0.20 mm/yr. Comparison between geodetic and seismic moment-rates indicates a high seismic coupling. We conclude that the contribution of intraplate faults to the seismic hazard in the Lisbon Metropolitan Area is more important than currently assumed.