Evidence of Surface Rupture Associated With Historical Earthquakes in the Lower Tagus Valley, Portugal. Implications for Seismic Hazard in the Greater Lisbon Area

Carolina Canora,Pedro Pina,Yolanda De Pro-Diáz,Sandra Heleno,Susana P Vilanova

doi:10.3389/feart.2021.620778

Abstract

The Lower Tagus Valley Fault, Portugal, has long been associated with the damaging earthquakes that affected the Greater Lisbon Area in historical times. These include a poorly documented earthquake that occurred in 1344, the relatively well-documented 1531 earthquake, and the most recent M6.0 1909 earthquake. In this work, we use a 0.5 m resolution LiDAR-based digital elevation model and a 0.5 cm resolution digital surface model based on UAV photogrammetry to accurately locate the fault scarps in the northernmost portion of the western fault strand and to select sites to perform paleoseimolological investigations. The paleoseismological and geochronological analysis performed in the Alviela trench site document the fault activity in the last 3000 years, including two earthquakes during historical times. We performed ground motion scenarios for 20 km, 40 km, and 60 km ruptures including the trench site. The ground motion fields obtained for the 40 km and 60 km ruptures are in agreement with most macroseismic intensity data available for the 1531 earthquake, implying a magnitude in the range M6.8–7.4. However, the degree of deformation preserved in the trench suggests a value closer to the lower magnitude bound. The intensity level observed in Lisbon in 1531 (IX) is lower than the modeled intensities for all considered scenarios and could be related to a particularly high level of vulnerability of the building stock.

Highlights

The association of historical events with specific faults is enormously important when defining seismic sources for seismic hazard analysis
We focused on the northernmost section of the western strand of the LTVFZ (Figure 1)
We present results for peak ground velocity (PGV) because that parameter is better correlated with macroseismic intensity for higher intensity levels than peak ground acceleration (PGA) (e.g., Boatwright et al, 2001; Caprio et al, 2015)

Summary

INTRODUCTION

The association of historical events with specific faults is enormously important when defining seismic sources for seismic hazard analysis. The dispersion includes both the aleatoric nature of the ground motions, and distinct site conditions within the localities, and the horizontal lines indicate the 1-sigma bounds for intensity IX according to Caprio et al (2015). The macroseismic intensities for the 1909 earthquake are presented in Figure 13 together with the ground motion model of Kotha et al (2016) Both the distribution of high intensities around Benavente, 45 km away from the Alviela trench site, and the moderate magnitude of the event (M6.0–6.2 estimated using early instrumental records, Dineva et al, 2002; Stich et al, 2005) make it most unlikely that the earthquake ruptured through the trench site. It is unlikely that a triggered local shock associated with the Great 1755 Lisbon Earthquake would reach the trench site since the highest intensities (VIII–IX, IX, and IX–X) were located South of Vila Franca de Xira, 40 km away from the trench site (see https://www. emidius.eu/AHEAD, last visited December 2020), and the ground deformation was reported for the left river margin (Vilanova and Fonseca, 2007)

DISCUSSION

Findings

CONCLUSION

DATA AVAILABILITY STATEMENT