Constraining pre-instrumental earthquake parameters from slope stability back-analysis: Palaeoseismic reconstruction of the Güevéjar landslide during the 1st November 1755 Lisbon and 25th December 1884 Arenas del Rey earthquakes

Abstract

Slope stability back-analysis performed for landslides known to have been triggered by an earthquake can provide additional constraints on the size and location of pre-instrumental seismic events. This paper reconstructs the pre-earthquake conditions of a major landslide located in the Granada Basin -the Güevéjar landslide, which was triggered twice by the 1755 Lisbon and 1884 Arenas del Rey earthquakes. For each case the minimum seismic acceleration needed to trigger the instability has been calculated, and from this datum the most likely magnitude and epicentral location have been inferred for each event. The results suggest that the moment magnitude of the 1755 Lisbon earthquake was at least 8.5 and that it was located as far as 580 km from the landslide. For the 1884 Arenas del Rey earthquake, the moment magnitude was at least 6.5 and it was located within 55 km of Güevéjar. These results indicate the Ventas de Zafarraya Fault as the seismogenic source of the event. Apart from these two major events, other significant historical earthquakes which took place in the Granada Basin are also analysed in the paper. It is confirmed that none of these was able to reactivate the landslide, not even the 1806 Pinos Puente earthquake. The Güevéjar landslide is stable under present-day conditions, but it could be reactivated by an earthquake as small as Mw = 4.7 directly at the site.