The area of Lisbon has been struck by destructive earthquakes in the past and with very intense consequences. As of today, two main areas host active faults with concern for the region: offshore with the still unclear source of the famous and catastrophic 1755 earthquake and inland with the Lower Tagus Valley where unknown fault(s) have produced the 1909 and 1531 events with estimated magnitudes ranging from 6 to 7. Those latter events are of particular importance due to their location within an area that is now densely populated. The repetition of such a shock today would have a barely imaginable impact on the population and economy of Portugal.
An apparent paradox is that in spite of the high stake and expected impact on the Greater Lisbon area, little is known about the source fault(s) of the 1531 and 1909 earthquakes in terms of location, dimensions, maximum magnitude, slip rate and recurrence period.
Throughout the world, major faults are studied and characterized by paleoseismological investigations. However, the particular setting of Portugal, which combines a low deformation rate (< 1 mm/yr) and a temperate climate make such kind of study extremely arduous. Indeed, under such conditions the fault’s signature in the landscape is easily eroded or buried by sediments. For that matter, the Lower Tagus Valley is definitely emblematic. Recently, a novel approach has been successfully applied in intraplate Europe with the characterization of the Breefault in Belgium and the Basel-Reinachfault in Switzerland , which display similarly low slip rates (0.1 – 0.3 mm/yr).
Today, with new and up-to-date competences joining CGE/UE, we are able to propose a novel approach to that crucial problem with the deployment of an integrated paleoseismologicalapproach. Starting with regional observations in instrumental and historical seismicity, geology and geodesy, we will identify and characterize active fault scarps susceptible to have produced historical earthquakes.
The main goal of the project was to improve the assessment of seismic hazard to the region of Lisbon by identifying and characterizing potential sources of destructive earthquakes. Therefore, we addressed five specific issues:
Identify major active faults susceptible to produce destructive earthquakes with relevance to the Greater Lisbon Area, i.e. in the Lower Tagus Valley regions.
Identify and characterize recent earthquakes produced by those faults during historical and pre-historical times in terms of
time of occurrence, magnitude and affected area.
Determine rupture length and return periods for seismic hazard calculations.
Contribute to the construction of a 3D geological model for those regions in order to improve ground shaking calculations.
Propose realistic earthquake scenarios on the basis of observed faulting behavior.