Simulations of strong ground motion in SW Iberia for the 1969 February 28 (M_s= 8.0) and the 1755 November 1 (M∼ 8.5) earthquakes - II. Strong ground motion simulations

Abstract

S U M M A R YThis is the second paper of a series of two concerning strong ground motion in SW Iberia due to earthquakes originating from the adjacent Atlantic area. The aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 (M s = 8.0) and 1755 (M = 8.5-8.7) earthquakes.First, we propose a regression to convert simulated values of Peak Ground Velocity (PGV) into Modified Mercalli Intensity (MMI) in SW Iberia, and using this regression, we build synthetic isoseismal maps for a large (M s = 8.0) earthquake that occurred in 1969. Based on information on the seismic source provided by various authors, we show that the velocity model effectively reproduces macroseismic observations in the whole region. We also confirm that seismic intensity distribution is very sensitive to a small number of source parameters: rupture directivity, fault strike and fault dimensions. Then, we extrapolate the method to the case of the great (M = 8.5-8.7) 1755 earthquake, for a series of hypotheses recently proposed by three authors about the location of the epicentral region. The model involving a subductionrelated rupture in the Gulf of Cádiz results in excessive ground motion in northern Morocco, suggesting that the source of the 1755 earthquake should be located further west. A rupture along the western coast of Portugal, compatible with an activation of the passive western Iberian margin, would imply a relatively low average slip, which, alone, would could not account for the large tsunami observed in the whole northern Atlantic ocean. A seismic source located below the Gorringe Bank seems the most likely since it is more efficient in reproducing the distribution of high intensities in SW Iberia due to the 1755 earthquake.

Implementation of Tsunami Evacuation Maps at Setubal Municipality, Portugal

“…Since there are records of the 1969 earthquake, Grandin et al, 2007 [28] were able to validate the earthquake source model. For this reason, in this study, numerical modelling of this tsunami was carried out using the fault parameters proposed by [28] and source dimensions of 82.5 km by 35 km, located on the Horse Shoe Fault.…”

“…The 1969 earthquake, with a magnitude of Ms 8.0 [28], triggered a tsunami. It occurred on 28 February at around 2:40 a.m. [29], and although it was on a winter night, many people felt the earthquake.…”

“…The same tsunami behaviour was reported by the UK witnesses, as compiled by [1]. In addition, sediments were identified as a signature of the 1755 tsunami [32][33][34]; (iii) a seismic moment assessment was conducted by [35] and the author proposed an earthquake of Mw 8.7, with the source model located on the Gorringe Bank, and with dimensions of 200 km by 80 km; (iv) turbidities were found nearby the Gorringe Bank by two different teams of researchers [36,37] who concluded that the records were associated with the 1755 earthquake; (v) seismic intensity modeling was carried out by [28], who were able to validate the observed intensity published by [31]; (vi) by re-analyzing tsunami travel times reported by the witnesses, [38] used wave ray analysis and determined the tsunami source area. The authors concluded that it could be located on the Gorringe Bank; (vii) in this same study ([38]), the tsunami numerical model was carried out at the regional scale.…”

Seismo-electromagnetic phenomena in the western part of the Eurasia-Nubia plate boundary

“…Parallel to the San Vicente canyon, the Horseshoe scarp and the Marquês de Pombal scarp since, at least, the Miocene have undergone deformations. This hypothesis is corroborated by the incidence of oceanic earthquakes with abnormally large magnitude inside the area of dispersed seismic activity alike the 1969 earthquake, with M s = 8.0, and the historical 1755 Lisbon earthquake, Grandin et al (2007aGrandin et al ( , 2007b. On the other hand, historical earthquakes with maximum intensity of IX or X occurred W of San Vicente Cape (Lisbon earthquake in 1755), southern Iberia (1829 and 1884) and in northern Morocco (1909).…”

Development of a methodological framework for the assessment of seismic induced tsunami hazard through uncertainty quantification: application to the Azores-Gibraltar Fracture Zone

“…However, source location and contemporary effects of such tsunami are not precisely identified. Several earthquake scenarios have already been published in the literature since last decades (Baptista et al, 1998;Grandin et al, 2007;Gutscher et al, 2006;Horsburgh et al, 2008;Johnston, 1996;Zitellini et al, 1999), mainly based on a comparison of several fault hypothesis based on geological properties of the Azores-Gibraltar fault zone (AGFZ). All of 10 these studies show how variable parameters of the seismic source can be, depending on the studied fault and focal mechanisms.…”