September 12, 2007 9:54 GeophysicalJournalInternational gji3571 Geophys. J. Int. (2007) doi: 10.1111/j.1365-246X.2007.03571.x 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 ,∗ , , , Rapha¨el Grandin,1 Jos´e Fernando Borges,1 2 Mourad Bezzeghoud,1 2 Bento Caldeira1 2 and Fernando Carrilho3 1Centro de Geof´ısica de Evora,´ Universidade de Evora,´ Evora,´ Portugal 2Departmento de F´ısica, Universidade de Evora,´ Evora,´ Portugal 3Instituto de Meteorologia, Lisbon, Portugal Accepted 2007 July 30. Received 2007 July 30; in original form 2006 November 24 SUMMARY This 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 subduction- related rupture in the Gulf of C´adiz 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 GJI Seismology 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. Key words: Iberian region, ocean–continent transition, strong ground motion, waveform modelling. aries are not unequivocally defined by bathymetry, and where the 1 INTRODUCTION existence of an active subduction zone is not clearly supported by Most great tsunamigenic earthquakes are related to well-defined seismological evidence. Thus, despite the large size of the fault that interplate convergence zones: the circum-Pacific seismic belt, the is responsible for this extreme event (several hundred kilometres), Sunda arc, the Hellenic arc and the Antilles arc. An exception is the location of the epicentral region of the 1755 earthquake is poorly the massive earthquake that struck Iberia and Morocco on 1755 known; many hypotheses have been made by various authors for its November 1, was felt in a large part of Europe, and produced a source. Managing to discriminate between acceptable and unac- powerful tsunami that crossed the Atlantic Ocean. This earthquake ceptable scenarios for this earthquake is a fundamental step in the occurred along a passive margin, in a region where plate bound- improvement of seismic risk assessment. In this paper, we concentrate on the comparison of macroseismic observations with synthetic seismic intensity for a set of possible ∗Now at: Institut de Physique du Globe de Paris (IPGP), Laboratoire de source parameters. For this purpose, we have to take into account Tectonique et M´ecanique de la Lithosph`ere, 4 place Jussieu, 75252 Paris, the major heterogeneities that affect, at various scales, the crustal France. E-mail: [email protected] structure in the region, and that largely condition the distribution of C 2007 The Authors 1 Journal compilation C 2007 RAS September 12, 2007 9:54 GeophysicalJournalInternational gji3571 2 Raphael¨ Grandin et al. seismic intensities on land. A realistic velocity model, embedded quake source. This will enable us to determine which source fits best into a wave propagation code, is an appropriate way to study source the observed intensity distribution of the ‘Lisbon’ earthquake. parameters of the 1755 earthquake, provided there is sufficient data. The finite difference method is well adapted to this strategy, since it 2 SEISMO-TECTONIC SETTING is computationally efficient enough to enable a large number of tests to be run, and accurate in the low frequency range (f < 0.5 Hz), 2.1 Tectonic context which is the most significant for the study of the destructive effects of large earthquakes at a regional scale. The velocity model used The current tectonic regime at the boundary of the African and here was proposed and validated in the companion paper (Grandin Eurasian plates varies with longitude (Fig. 1), as a result of the et al. 2007). For this purpose, it is necessary to introduce a regres- rotation of Africa, with respect to Eurasia, around an Euler pole lo- sion between simulated values of Peak Ground Motion Parameter cated offshore Morocco, close to (20◦N; 20◦W) (Argus et al. 1989; (PGMP) and Modified Mercalli Intensity (MMI). The 1969 earth- DeMets et al. 1994). This boundary is commonly divided into three quake (M s = 8.0) constitutes the most obvious test to calibrate this sections (Buforn et al. 1988). To the west, between the Africa– regression: source parameters are reasonably well constrained, and Eurasia–North America triple junction (35◦W) to the eastern end the distribution of macroseismic observations is uniform and of good of the Terceira Ridge (24◦N), the regime is transtensional with an quality. extension rate of 4.4 mm yr–1 (Borges et al. 2007), and is respon- After a short introduction to the seismo-tectonic setting in SW sible for the active volcanism found in the Azores archipelago. In Iberia, we will establish this regression, and calibrate it using the the central section, the relative motion of the two plates seems to 1969 earthquake. Finally, we will extrapolate to the case of the 1755 be accommodated by a single right-lateral fault, the Gloria fault, earthquake, and test three hypothetical models for the 1755 earth- although significant seismic activity is observed in a broad region Figure 1. Bathymetric map of the region of study. Inset shows the location of the region of study, and a sketch of the kinematic regime at plate boundaries. Isobaths are traced with a 1000 m interval below −1000 m, with a 200 m interval above (data provided by the British Oceanic Data Centre 2003). Dots show the location of earthquakes epicentres recorded from 1995 January to 2005 December (data provided by the Instituto de Meteorologia, Lisbon). Small light grey dots are earthquakes with 3 < M w < 4, medium dark grey dots are earthquakes with 4 < M w < 5, and large black dots are earthquakes with M w > 5. White triangles indicate the location of large historical earthquakes (the possible location of the source of the 1755 ‘Lisbon’ earthquake is discussed in the text). Major offshore active fault are also shown (from Hayward et al. 1999; Gr`acia et al. 2003a). Submarine canyons are identified by dashed lines. C 2007 The Authors, GJI Journal compilation C 2007 RAS September 12, 2007 9:54 GeophysicalJournalInternational gji3571 Strong motion in SW Iberia: Part 2 3 off the fault (Lynnes & Ruff 1985). East of 16◦W, the bathymetric pattern of focal mechanisms, and the depth of earthquakes, between continuity of the Gloria fault cannot be followed, and the defini- 14 and 23 km, suggests that deformation is controlled by tectonic tion of the boundary is less clear. A transpressive tectonic regime activity deep in the basement (Fonseca & Long 1991). Seismic dominates, with a very low convergence rate of 4 mm yr–1 (Argus activity in the region of Evora´ is associated with unmapped struc- et al. 1989; McClusky et al. 2003) trending NW–NNW, consistent tures (Borges et al. 2001). In the Monchique range, the steady with the observed maximum horizontal stress direction (Ribeiro microtremor activity probably involves pre-existing tectonic fea- et al. 1996; Borges et al. 2001; Stich et al. 2003; Carrilho et al. tures located at depth (Dias 2001). Further south, the activity of the 2004). Deformation is distributed over an increasingly large area Portimao˜ Fault can be inferred from the N to S trending active zone that can reach a N–S width of 300 km near the continental mar- (Carrilho et al. 2004), consistent with average focal mechanisms gin of Iberia (Chen & Grimison 1989). In this section, seismicity is showing N–S strike-slip fault-planes (Bezzeghoud & Borges 2003), scattered, most events concentrating along a 100-km wide band that as suggested by Terrinha (1997). In the Guadalquivir foreland basin, trends ESE–WNW from 16◦Wto9◦W (Fig. 1). A progressive shift seismicity is low, and seems to be limited to the blind Subbetics of focal mechanisms, from strike-slip mechanisms in the west, to frontal thrust. predominant reverse faulting in the east, has also been reported by Offshore, the poor azimuthal coverage and the lack of stations at Buforn et al. (1988), and can be interpreted as an increasing plunge short distances from epicentres do not allow the depth distribution of the minimum compression axis. In the Gulf of Cadiz,´ seismicity of earthquakes, even close to the coast, to be constrained.