Abstract
The Lower Tagus Valley (LTV) has been the source of several local earthquakes that affected the region of Lisbon, in the central part of Portugal. In spite of absence of strong earthquakes during the last 100 years, past events have produced large damage in this area. We present the state of the art concerning the last two major earthquakes, which occurred in 1531 and 1909. The effects of both events are described, based on coeval reports already interpreted by several authors. The source parameters determined by different authors are also presented. The characterization of the seismogenic potential of the LTV is very important to better estimate the seismic hazard and risk of the region of Lisbon and Tagus Valley.
Highlights
The Lower Tagus Valley (LTV) region is located in the central part of Portugal mainland, bordering the Atlantic Ocean
The Lower Tagus valley (LTV) is sited in the Lower Tagus Cenozoic Basin (LTB; central-western mainland Portugal) which consists of a NE-SW elongated tectonic sink that developed as a complex left-lateral strike-slip basin in the Paleogene, acted by an approximately N-S compression
Current seismicity monitored by the National seismic network, as well as the source parameters estimated for the 1909 earthquake, indicate that the seismogenic sources extend through the upper crust and comprise faults in the Paleozoic basement, located beneath the Mesozoic rocks of the Lusitanian Basin and the overlaying Cenozoic sedimentary cover that outcrops in the area (Cabral et al 2013)
Summary
The Lower Tagus Valley (LTV) region is located in the central part of Portugal mainland, bordering the Atlantic Ocean. In 1910, a Mainka seismograph was installed in the Observatory of Lisbon University, followed by the installation of Agamennone seismoscopes in three meteorological stations in Portugal mainland (Penhas Douradas, Évora and Lagos) and one in Madeira Island (Funchal) (Mendes-Victor 2003) This is why many authors state that the instrumental seismicity in Portugal began in 1910. Due to the concentration of the population, lifelines, critical infrastructures and the proximity to Lisbon, the scenario of occurrence of an earthquake similar to the 1909 event represents a high risk for the whole Lisbon Metropolitan Area (Figure 1), where more than one quarter of the Portuguese population lives (almost 3 million people, according to the last census of 2011) During these last 100 years the seismic activity of the LTV has been low with about one felt earthquake per year. In this work we will describe the present knowledge on the above referred two main LTV earthquakes, based on the up to date information collected from published papers of various authors
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