Interseismic Strain Accumulation near Lisbon (Portugal) from Space Geodesy

The Lisbon Metropolitan Area, Portugal, has been affected by several destructive earthquakes nucleating both along the offshore Africa-Eurasia plate boundary and on onshore inherited intraplate faults. Using a dense GNSS dataset coupled with PSInSAR analysis, we provide new 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 toward the SW with a relative velocity of 0.96 ± 0.20 mm/yr, whose boundaries are part of a complex and as yet poorly constrained strike-slip fault system, possibly rooting at depth into a simpler basement fault. Comparison between geodetic and seismic moment-rates indicates a high seismic coupling. We show that the contribution of intraplate faults to the seismic hazard in the LMA is more important than currently assumed.