The destructive 1790 Oran (NW Algeria) earthquake in a region of low seismicity

Tectonophysics 759 (2019) 1–14

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The destructive 1790 Oran (NW Algeria) earthquake in a region of low seismicity

T

E. Buforna, , P. Cocaa, M. Bezzeghoudb, A. Udíasa, Y. Bouhadadc, M. Mattesinia ⁎

a

Dept. Física de la Tierra y Astrofísica, Universidad Complutense, Madrid, Spain Instituto de Ciências da Terra, Departamento de Física, Escola de Ciências e Tecnología da Universidade de Évora, Portugal c National Earthquake Engineering Center (CGS), Algiers, Algeria b

ARTICLE INFO

ABSTRACT

Keywords: Oran 1790 earthquake Damage Source parameters

The Oran earthquake of 9th October 1790 in NW Algeria caused heavy damage in that city, and originated a tsunami observed in Almeria and Cartagena, SE Spain. It occurred in a region of low seismicity in comparison with other regions east of Algeria. The abundant contemporary documentation analysed in the present study, some of it not used previously, confirms a maximum intensity of IX–X (EMS-98) for Oran city. The estimated focal parameters are: origin time 01 h 15 m or 01 h 30 m UT, duration 1-to-2 min, a macroseismic offshore epicentre at 35.9°N, 0.6°W to explain the generation of the tsunami, and estimated magnitude 6.0–6.5. The rupture is complex, formed by two events, and may have been associated with NE-SW oriented thrust faults present in the area.

1. Introduction On the 9th October 1790, an earthquake occurred in NW Algeria which caused heavy damage and major economic losses in the city of Oran. This is one of the largest events to have occurred in the NW of the Mediterranean Basin. It practically destroyed the city of Oran, was felt in several places in Algeria and in southern Spain, and generated a small tsunami and a long series of aftershocks. In the 18th century, the city of Oran and its surrounding area was a Spanish enclave. It consisted of a fortified city, surrounded by major defensive structures including ramparts, bulwarks, towers, and five castles. As a consequence of the earthquake, Spain left Oran and Mazalquivir (a site very close to Oran and also a Spanish enclave) on 28th February 1792. This decision was made due to the heavy damage caused by the earthquake and the high cost estimated for reconstruction (Real Decreto 15 de Diciembre de 1791, Sánchez Doncel, 1991). Northern Algeria is characterized by important seismic activity, with the occurrence of moderate to large shocks (M < 7.5), including those of 1790 Oran (X), 1954 Orleansville (M = 6.7), 1980 El Asnam (M = 7.3), 1994 Mascara (M = 5.7), and 1999 Ain Temouchent (M = 5.7) (Ayadi and Bezzeghoud, 2015) (Fig. 1a and b). The Oran earthquake occurred in the western part of the EurasianAfrican plate boundary, and in a century that was seismically dominated by the large 1755 Lisbon earthquake, the largest that had ever occurred in the Ibero-Maghrebian region, and probably in all of ⁎

Western Europe. The Lisbon earthquake was located offshore of Cape St. Vincent, SW Iberia (Fig. 1a). In the 18th century, in zones of eastern Iberia considered to be of low seismicity, two earthquakes occurred that caused major damage: 1748 Montesa (Buforn et al., 2015) and 1790 Oran (Fig. 1a). The occurrence of recent earthquakes (such as L'Aquila, 2009 and Haiti, 2010) in regions regarded as of low seismic risk but where, in the past, large shocks have occurred shows the importance of carrying out detailed studies of historical earthquakes in this type of region. Today, Oran is the second largest city in Algeria, with > 1,500,000 inhabitants, and large-scale urban development. Hence, any repeat of an earthquake such as that of 1790 would be catastrophic. A detailed study of the damage caused by that earthquake and knowledge about its source would help improve seismic risk assessment in this region. Here we present a re-assessment of the damage and of the occurrence of a small tsunami caused by the 1790 Oran earthquake, using all the information available in the original contemporary documents. The focal parameters and a model of the fracture are estimated on the basis of the distribution of the intensities. 2. Geological setting The city of Oran is located in the western Tell Atlas chain of Algeria, on the northern edge of the Plio-Quaternary basin of Oran, a western continuation of the Cheliff basin which is known for its high seismicity

Corresponding author. E-mail address: [email protected] (E. Buforn).

https://doi.org/10.1016/j.tecto.2019.03.008 Received 22 January 2019; Received in revised form 11 March 2019; Accepted 13 March 2019 Available online 01 April 2019 0040-1951/ © 2019 Elsevier B.V. All rights reserved.

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a)

−12˚

−9˚

−6˚

−3˚

0˚

3˚

IBERIA

39˚

39˚

1829 1884 1755

36˚

1969

2016 1790

MOROCCO −12˚

−9˚

−6˚

36˚

1980

ALGERIA

2004

−3˚

0˚

3˚

b)

Fig. 1. a. Large earthquakes in the Ibero-Maghrebian region for the period 1500–2018. Triangles correspond to historical seismicity (before 1900, Imax > VIII) and discs to instrumental seismicity (after 1900, M > 6). The star is the proposed epicentre of the 1790 Oran earthquake. Symbols are proportional to the earthquake's size. b. Seismicity for the Oran region (area in the square of a) for the period 2000–2018 (Catalogue of the Instituto Geográfico Nacional). The star is the proposed epicentre of the 1790 Oran earthquake.

2

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(caption on next page)

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Fig. 2. a. Main geological features of the north-western Algeria. b. Main geological features of the Oran region (Murdjadjo structure). Active faults are from Meghraoui (1988), Bouhadad (2001), Belabbes et al. (2009), and Benbakhti et al. (2018).

and recent tectonic deformation. The Tell Atlas, a fold-and-thrust E-W to NE-SW trending belt, is the most seismically active region in the Maghreb and western Mediterranean region. The analysis of the spatial distribution of historical epicentres during the last three centuries, including the recent strong earthquakes of Orléanville (09/09/1954, M6.7), El Asnam (10/10/1980, M7.3), and Zemmouri (21/05/2003, M6.8), show several areas of epicentre concentration. Examples are Oran city and its vicinity, the axes of Oran-Mascara-Relizane-ChlefBlida-Alger-Sour El Ghozlane-Boumerdes (Benouar, 1994; Ayadi and Bezzeghoud, 2015) (Fig. 2a). The tectonic regime of this zone is compressional, with a N-S to NW-SE shortening direction associated with the convergence between Nubia and Eurasia (McKenzie, 1972; Bezzeghoud et al., 2014). The shortening rate in northern Algeria (3.7–5.6 mm/year) is conditioned by the occurrence of large earthquakes such as those of 1910 (Sour El Ghozlane-former Aumale, Ms 6.6) (Benouar, 1994); 1954 (Ms, 6.7) and 1980 (Ms 7.3) (El Asnam region) and 2003 (Mw 6.8) (Boumerdes) (Bezzeghoud et al., 2014). This rate is consistent with the 4–6 mm/year shortening rate between the Nubia and Eurasia plates (Demets et al., 2010; Nocquet and Calais, 2004; Serpelloni et al., 2007). The Tell Atlas mainly comprises parallel to sub-parallel thrust faults that underlie asymmetrical folds (faulted-fold structures), 20 to 30 km long and oriented NE-SW in an en echelon structure (Meghraoui et al., 1986; Meghraoui, 1988; Meghraoui, 1991; Aoudia and Meghraoui, 1995; Bouhadad, 2001; Maouche et al., 2011; Bouhadad, 2014; Benfedda et al., 2017; Abbouda et al., 2018; Benbakhti et al., 2018). The active faults may or may not be blind (Meghraoui, 1991; Bezzeghoud et al., 1995; Belabbes et al., 2009). The en echelon system of faulted folds is related to E-W deep strike slips affecting the substratum that characterize the Neogene and Quaternary intermountain basins of the Tell Atlas chain (Meghraoui, 1988; Morel and Meghraoui, 1996; Meghraoui and Pondrelli, 2012; Derder et al., 2013). There is often geological, geomorphic, and morpho-structural evidence of active tectonics observable in Quaternary and Neogene deposits of western Algeria, including the uplift of alluvial and marine terraces and the diversion of hydrographic networks (Abbouda et al., 2018; Benbakhti et al., 2018). Geological evidence for strong seismic shaking events includes seismites in Quaternary deposits of the Oran region, likely the result of historical or unknown pre-historical earthquakes (Boukhedimi et al., 2017; Guessoum et al., 2018). The Oran basin is characterized by E–W to NE–SW trending folds and reverse faults in the Quaternary deposits in the vicinity of Oran city. These make the region susceptible to future moderate-to-large earthquakes (Meghraoui et al., 1996; Meghraoui, 1988; Bouhadad, 2001; Belabbes et al., 2009; Benbakhti et al., 2018; Guessoum et al., 2018). The most important geological feature in the Oran region is the Murdjadjo structure. This is limited to the north and the south by NESW thrust faults, and has short en echelon faults at its southern border (northern edge of the Oran Quaternary basin) (Fig. 2b). This fault system continues to the NE until Arzew on the coast, and some of the faults probably continue offshore of Oran and Arzew. These structures were likely the source of several moderate-to-strong earthquakes, for example, the historic earthquakes of 1790 (Oran, Io IX–X), 1789 (Oran, Io VII–VIII), and 1900 (Oran, Io VII), and those that occurred more recently in 1959 (Oran, Io VII), 1994 (Mascara, Ms5.6), 1999 (Ain Temouchent, M = 5.8), and 2008 (Oran bay, Mw5.5) (Bezzeghoud and Buforn, 1999; Yelles et al., 2004; Belabbes et al., 2009; Ayadi and Bezzeghoud, 2015). During the present century, however, no significant earthquake has occurred in the neighbourhood of Oran city. This would seem to indicate that there exists a significant seismic gap in the city's

vicinity (Bezzeghoud et al., 1996; Ayadi and Bezzeghoud, 2015). 3. Seismicity of the western Mediterranean region The seismicity of Oran is associated with the western part of the plate boundary between Eurasia and Africa. This part of the boundary stretches eastwards from the Azores Islands triple junction (North American, Eurasian, and African plates) to the Strait of Gibraltar in the Atlantic Ocean, to continue along the south of the Iberian Peninsula and the north of Africa to the Sicilian-Calabrian arc in the Mediterranean Sea. The boundary has a clear oceanic nature in its westernmost part, from the Azores to south-west of Cape St. Vincent. At this point and continuing to the east (to about 1°E), there begins a transition from an oceanic to a continental plate boundary, with the interaction of the southern border of the Iberian Peninsula, the northern border of Africa, and the Alboran Basin between them (Fig. 1a). One manifestation of this transition is a change in the characteristics of the occurrence of earthquakes. Seismicity to the east is spread over a wide region corresponding to an extensive zone of deformation. Earthquakes in this zone have different focal depths, focal mechanisms, and average seismic slip velocities (Buforn et al., 2004, 2014; Ayadi and Bezzeghoud, 2015). Further east, the plate boundary recovers its oceanic nature following the northern coastline of Algeria and Tunisia. Fig. 1a plots the distribution of epicentres of large earthquakes: Imax = IX or X in the historical period (before 1900), or magnitude > 6.0 for the instrumental period. Some of these earthquakes caused major damage, and in some cases generated tsunamis. The largest shocks (Imax = X, Mw > 6) were, from west to east, offshore of Cape St. Vincent, 1755 Lisbon and the 1969 (Mw = 7.8) earthquakes, in southern Spain, the 1884 Arenas del Rey and the 1829 Torrevieja earthquakes, and in northern Morocco the 2004 and 2016 shocks (both Mw = 6.3). In northern Africa, the Algerian region is seismically the most active, where large shocks have occurred along the coast. The 1790 Oran earthquake, the subject of the present study, is located in the western part. As one moves to the centre, the number of earthquakes increases: in the El Asnam region in 1934 (Ms = 5.6), 1954 (Ms = 6.7) and 1980 (Mw = 7.1), and, further east, large shocks in 1716 (Io = IX), 1825 (Io = X–XI), 1858 (Io = VI–VII), and 2003 (Mw = 6.8). Earthquakes have generated tsunamis at both ends of the Ibero-Maghrebian region (Cape St. Vincent: 1755 and 1969; Algeria: 1790 and 2003). One observes in Fig. 1a that, in comparison with other regions of Algeria, there has been a lack of large earthquakes in the Oran region since the occurrence of the 1790 event. Fig. 1b shows the distribution of epicentres for the period 2007–2017 (M ≥ 3.0) taken from the Instituto Geográfico Nacional (IGN) Catalogue (https://www.ign.es/ign/ layoutIn/sismoFormularioCatalogo.do). We selected those earthquakes with a well determined focal depth (discs for h < 40 km and squares for 40 < h < 100 km). The star shows the location of the 1790 earthquake. The epicentres have a general distribution that is parallel to the coast in a NE-SW direction. There are three clusters of epicentres. From east to west, these are: first, the El Asnam region with epicentres distributed NNW-SSE, some foci being offshore; second, the Mascara region with epicentres distributed NE-SW, parallel to the coast, but with foci inland; and third, the Oran region with seismic activity being very close to the coast, and foci offshore. The largest earthquake in Fig. 1b is the Oran 6 June 2008 event (Mw = 5.5). It was followed by several aftershocks causing panic among the city's inhabitants (Mw = 4.5), and a shock on 25 August 2008 (Mw = 4.9, USGS). The main shock was felt in many cities in NW Algeria and SE Spain. It was preceded by a foreshock on 9th January 2008 (Mw = 4.8) located south of Oran. 4

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4. Documentary sources of the 1790 earthquake

October to 12th November.

There are abundant contemporary documents providing information about the damage caused by the Oran earthquake of 1790. Some of them were used by Lopez Marinas and Salord (1990), but new ones have been found which are also used in this present study. We divided these documents into three categories: contemporary publications and manuscripts with author, the same but anonymous, and other sources of information. When referring to the sites, we shall use the Spanish denomination with the Arabic name in italics in parentheses.

4.2. Anonymous contemporary documents The first anonymous document is a report dated 17th October 1790 with a description of the damage caused by the earthquake in Oran (Anonymous, 1790a), including a diary describing what happened in Oran. There is another anonymous diary (Anonymous, 1790b) with a description of the earthquakes felt in Oran from October 9th to 31st (this document has not previously been used by any study of the Oran earthquake).

4.1. Contemporary documents with author

4.3. Other contemporary sources

The first document is a letter of Joaquín Mayone y Ferrari, Count of Cumbre-Hermosa (Cumbre-Hermosa, 1790a) sent to the Military Governor of Cartagena (Spain) to which at that time Oran depended, dated the day of the earthquake (9th October) giving general information about the damage caused by the shock. Cumbre-Hermosa was a brigadier. He had taken command in Oran as a result of the death during the earthquake of the Governor General, Manuel Pineda de la Torre, Marquis of Campo Santo. The second document is of Pedro María Legallois Grimarest, a military officer (Grimarest, 1790). It is dated 29th October. It gives information on the damage in Oran, on how the seismic sequence was felt, and on its effects on the Oran springs. The third, of Cumbre-Hermosa (Cumbre-Hermosa, 1790b), is a letter addressed to Charles IV, King of Spain, on 2nd November, with detailed information about the time of the earthquake and the damage in Oran due to the main shock and its aftershocks, from 9th October to 2nd November. Other documents are two journals, the first by N. Antoñano, dated on 3rd November, which reports the earthquakes felt from 22nd October to 3rd November (Antoñano, 1790) during a boat trip from Mazalquivir (Al-Marsa al-Kibir) to Arseo (Arzew) (Figs. 3 and 4). The second was written by Rafael Adorno (Adorno, 1790), a brigadier of the Oran garrison, with information about what happened from 9th

Other documents used are the Council Minutes of October 1790 (Actas Capitulares o de Cabildo) corresponding to 16 Spanish towns of SE Spain (from Yecla to Málaga, Fig. 3). The Almería Council Minutes of the 12th, 21st, 22nd, and 26th October (Actas Cabildo Almería, 1790) give additional information about the earthquakes. In particular, the 26th October document reports that earthquakes are continuing, and that some people of Oran injured by the earthquake are arriving in the city. The Council Minutes of Cartagena for the 9th of October (Actas Cabildo Cartagena, 1790) report the occurrence of a large earthquake giving information on the time of occurrence, and that this is followed by other smaller shocks. Those of the 11th, 16th, 22nd, and 30th of October report the continuation of earthquakes, but without causing appreciable damage. The last information corresponds to the 18th November: “Thanks are given for God's mercy in the long season of earthquakes experienced in this city without causing any damage”. The last Council Minutes are from the town of Vera (Actas Cabildo Vera, 1790) of 27th October with an order to help the injured people who arrive in Vera from Oran (this document is not included in Lopez Marinas and Salord, 1990). There are Council Minutes for October 1790 from another 13 towns in SE Spain, but none of them mention the Oran

39˚ Yecla

Spain

Cartagena

Granada

Vera

Santa Fe Rioja

Malaga

Baños

Almeria

1790

Alboran Sea

36˚

Mostagem

Arzew Mazalquivir Mascara

Melilla Tremecen

Africa −6˚

−3˚

0˚

Fig. 3. Geographical sites where the 1790 earthquake was felt or produced damage (discs), while the solid squares are towns with no information about the earthquake. The star is the epicentre proposed in this study. 5

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Fig. 4. Top: limits of the city of Oran (present day: dashed line; 1790: solid line). Bottom: engraving of the city of Oran and the Mazalquivir castle.

earthquake. Other sources of information come from newspapers and journals. A Madrid newspaper (La Gazeta de Madrid, 1790) published on the 19th and 26th November detailed information of the Oran earthquake. But these articles are based on the Cumbre-Hermosa report (CumbreHermosa, 1790b), and this information is also given by another Madrid newspaper, the Mercurio de España of November 1790 (Mercurio de España, 1790). In the Memorial Literario (a journal also published in Madrid) of November and December 1790, there is information about the Oran earthquake and the Spanish towns where the shock was felt (Memorial Literario, 1790a,b). There is a short note about the occurrence of the Oran earthquake in the Le Moniteur Universel, also known as Gazette National, a journal published in Paris, of 30th October and 2nd November 1790 (Le Moniteur Universel, 1790). A very important information source is a collection of maps prepared by Antonio Hurtado, an army engineer in Oran. In these maps, Hurtado shows the main buildings, bulwark, walls, and external defence towers, and the damage caused by the earthquake. This information is available in the Archivo General Militar de Madrid (Hurtado, 1791a,b), Archivo del Museo Naval de Madrid (Hurtado, 1791c), and Centro Geográfico del Ejército (Anonymous, w.d.). We searched Ottoman archives for information because at this time the region was part of the Ottoman Empire, but could find no documents regarding the 1790 earthquake (Dr Karin Sesetyan, personnel

communication). Seismologically, the 1790 Oran earthquake is found in three early seismic catalogues: Cotte (1807), Huot (1837) and von Hoff (1840) (Udías, 2017). These catalogues note the occurrence of the shock in the Oran region, and report some of the damage caused and that the event was felt in Spain. Later catalogues (Perrey, 1847; Mallet and Mallet, 1858; de Prado, 1863; Taramelli and Mercalli, 1886 and Milne, 1911) include the names of cities and towns where the earthquake was felt, information about the largest aftershock on 25th October 1790, and effects at sea. Sanchez Navarro-Neumann (1917, 1921) gives further information, including the number of victims, origin time of the earthquake, and the principal damage. Galbis (1932) summarized the information of previous catalogues, including the number of victims – 2000 – and the sites where the earthquake was felt — in Malta, and Cartagena, Santa Fé (Granada), and Almería in Spain. Munuera (1963) assigned coordinates on land to the macroseismic epicentre (35.7°N, 0.6°W), and a maximum intensity of IX–X. These values are used in other later catalogues (Roussel, 1973; Mezcua and Martínez-Solares, 1983; Benhallou, 1985; Mokrane et al., 1994; El Mrabet, 2005; Ayadi and Bezzeghoud, 2015). Martínez-Solares and Mezcua (2002) do not include the Oran earthquake in their catalogue, but they list instead an earthquake on the same day, 9th October 1790, in Melilla (another Spanish territory, west of Oran) (intensity V), and classified it as a false event. The most thorough study is that of Lopez Marinas and Salord 6

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Table 1 The 1790 Oran seismic series. Date

Local time

9th October 9th October

Events

Reference

01:15

Main shock > 20 > 34 in 8 h > 33 (four very strong) Continuation

10th October 11th 12th 13th 15th 16th

October October October October October

17th 18th 19th 20th

October October October October

21st October 23rd October 24th October 25th October

27th October

Early morning

00:30 19 – 11

6–7 6–7? No time At night?

29th October

5:15

31st October

20

2nd November 7th November 8th November 9th November 10th November 11th November

4 shocks 2 shocks, one fairly long 3 shocks 2 shocks, one fairly long Continuation 3 shocks 2 shocks 3 shocks, no damage 4 shocks, no damage Strong, underground noise With underground noise & damage to S. Roque tower 2 shocks, one “crecido” Strong, similar to the main shock Underground noise Continuation Several shocks The rest of the Royal Hospital and several houses ruined Strong earthquake Walls of ruined houses fell Shock almost like the main shock Arch of the Contaduria house and nearby buildings fell Short time duration Two small shocks Two shocks Moderate shock 2 shocks 4 shocks no damage 2 shocks, one strong 6 shocks at the night 2 shocks Continuation Ball of fire followed by an earthquake

(1990) who use abundant contemporary documents. They report the damage caused by the earthquake in detail, and assign it a maximum intensity X, but do not explain how they have arrived at that assignation. They propose an offshore epicentre, near Oran, without giving its coordinates. Chimouni et al. (2018) reappraised the seismic intensity, taking the contemporaneous social and political situations into account, mainly using the information given by Lopez Marinas and Salord (1990), Sanchez Navarro-Neumman (1917, 1921), and Galbis (1932).

Cumbre-Hermosa 1790a Anonymous 1790 Anonymous 1790b Anonymous 1790b Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790 Anonymous 1790a Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790 Anonymous 1790a,b Anonymous 1790a Adorno 1790 Anonymous 1790a Adorno 1790 Anonymous 1790a Anonymous 1790a Anonymous 1790a Antoñano (1790) Anonymous 1790b Adorno 1790 Anonymous 1790b Adorno 1790 Anonymous 1790b Adorno 1790 Anonymous 1790b Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790 Adorno 1790

Adorno (1790) gives it at night on the previous day. On 27th October, another shock occurred which caused the collapse of the Public Accounting Office (La Contaduria) and several other houses. The aftershocks are reported to have continued until 11th November. Adorno (1790) reports a strange phenomenon, difficult to explain: the observation of a ball of fire preceding the 11th November aftershock. From these descriptions, we conclude that the aftershock series extended over about one month from the occurrence of the main shock on 9th October until 11th November, with > 80 quakes reported (Fig. 5). Underground noise is reported to have been heard in some of the largest shocks (main shock, and the 20th and 21st October aftershocks).

5. The main shock and its aftershocks The date and time of the main shock are given by most of the contemporary accounts as 9th October 1790 at 1:15 local time (Cumbre-Hermosa, 1790; Adorno 1790; Grimarest, 1790), o during the night of the 8th to the 9th October (Anonymous, 1790a), with a duration of 1–2 min (Anonymous, 1790b; Grimarest, 1790). Anonymous (1790a) and Adorno (1790) describe a “frightening” underground noise. All accounts refer to the occurrence of aftershocks, mainly until the 11th November (Table 1 and Fig. 5). The number of aftershocks reported for the 9th October varies according to the different authors, for example, from 20 to 34, with four of them being very strong (Anonymous, 1790b). The aftershocks continue, with shocks felt every day from 10th October to the beginning of November. On 20th October, two shocks were felt with accompanying underground noise. One caused damage to the S. Roque tower (Fig. 6). Aftershocks caused damage on 25th October (ruining the Hospital which had already been damaged by the main shock and several houses). There is discrepancy about the time of the largest shock of 25th October. Anonymous (1790a) and Antoñano (1790) give 6 h or 7 h in the morning, but

6. Oran city At the time of the 1790 earthquake, Oran was a fortified town of the Spanish Crown, with most of its population comprised by military personnel of different corps and their families. In 1772, the total number of inhabitants was 9317 (Hontabant), and in 1790 it was approximately 9000 (Roel, 1790). Fig. 4 shows the limits of Oran and the coastline in 1790 and today (top), and an engraving (bottom) with the location of Oran and Mazalquivir, a fort located to the west of the city. Oran is situated on a hill, surrounded to the west by steep mountainous terrain, and to the south and east by ravines (Zappino, 1785). Most of the buildings were inside the walls. The most important was the “Alcazaba” (Citadel), a type of castle (Fig. 6). Inside the Alcazaba were the Royal Hospital, the Governor General's Palace, the Public Accounting Office (La Contaduría), Treasury House (Tesorería), and the house of the Royal Treasury Minister (Ministro de la Real Hacienda). 7

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Fig. 5. Temporal distribution of aftershocks. Numbers indicate the earthquakes. MS = main shock, C = continuation, SMS = similar to main shock.

Some stores and the Asturias military quarters were also located in the Alcazaba. Outside the Alcazaba, but inside the city walls, were another two military quarters (“Fixo” and “Desterrado”) and four churches, the Iglesia Mayor and La Merced, both in good condition, and Santo Domingo and San Francisco which had been repaired in 1783, seven years before the 1790 earthquake (Roel, 1790) (Fig. 6). In the town, the “Conducto Real” was a solid leat (watercourse) construction in good condition (Roel, 1790). To the north, limited by the sea and outside the walls, there was the Marina, a fishing district (Fig. 7). The town was surrounded by defensive forts and towers. Most of the buildings, including those of the Marina neighbourhood, were military premises. The number of family houses was 535, and another 49 houses were property of the Crown. The building types of these houses were of inferior quality, mostly of stone and brick masonry (Hontabant, 1772). Most of the town walls were of “tapia” (a material similar to adobe), with concrete lining and masonry in bad conditions (maltratada). Zappino (1785) proposed how to improve and strengthen the defences of the town. Roel (1790) gives a similar number of inhabitants, 9000, and characteristics of the buildings, but he describes the good quality of the new buildings. Improvements in the Alcazaba, were made, probably as a consequence of Zapino's report, where the Palace of General Commander, the Treasury House (Tesorería and Contaduría), and the house of the Royal Minister of Finances (Ministro de la Real Hacienda) were located. From these descriptions, one can conclude that the walls, buildings, and monuments in Oran were in solid condition due to the maintenance work that had been carried out some years before the occurrence of the earthquake. One problem in the study of historical earthquakes is that contemporary documents do not present the description of damage in any great detail. In documents in Spanish, houses and buildings are said to be, from greater to lesser damage, “destroyed” (destruidas), “collapsed”

(desplomadas), “ruined” (arruinadas), “sunken” (hundidas), “damaged” (dañadas), or “impaired” (consentidas, resentidas). These descriptions may in some way be comparable with the five grades of classification of damage to masonry buildings of the EMS-1998 (Grünthal, 1998; Buforn et al., 2015), but of course with a certain ambiguity in the assignation of those grades. Another problem is that, in the present case, the damage also includes that due to the largest aftershocks which occurred on the 21st and 25th October. 7. Damage in Oran city The city of Oran was seriously damaged by the 1790 earthquake, with most buildings completely destroyed or collapsed due to the quakes (Cumbre-Hermosa, 1790a,b; Anonymous, 1790a; Grimarest, 1790; Anonymous, 1790b). According to these documents, the damage may be summarized as follows: The Alcazaba (Fig. 6) was ruined (Anonymous, 1790a). Grimarest (1790) describes the buildings in the Alcazaba, including the Royal Hospital, as having suffered great damage (ruined) despite the thickness of their walls (Fig. 6), and the 25th October aftershock ruined the rest of the Hospital. The Iglesia Mayor and Santo Domingo churches collapsed (Grimarest, 1790; Anonymous, 1790b). The other two churches were ruined. The military quarters of Fixo, Desterrados, and Asturias collapsed (Anonymous, 1790b; Grimarest, 1790). Some parts of the city wall fell and were ruined. Other parts were broken and seriously damaged (Cumbre-Hermosa, 1790a; Anonymous, 1790a; Grimarest, 1790). Grimarest (1790) describes gaps that were left in part of the city wall, again despite the wall's great thickness (about 2.5 m) “no obstante de ser sus murallas de tres varas de grueso” (1 vara = 0.835 m). The damage inside the city walls is depicted in great detail in the maps of Hurtado 1791a,b) and in Anonymous (w.d.) and Cumbre8

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Fig. 6. Intensity distribution (EMS-98) in Oran city for the 1790 earthquake. (Modified from Hurtado (1790a).)

Hermosa (1790b) and Grimarest (1790). In the south of the city wall, the La Campana and Santa Isabel bulwarks were severely damaged, with some parts that collapsed leaving gaps in the wall (the Tremecen and Canastel gates) (Fig. 6). The Santiago and Rosario bulwarks suffered less damage, with portions of the concrete and masonry covering becoming detached and flaking off. The Los Leones and Santa Isabel bulwarks sank. The San Francisco bulwark collapsed. The number of people who died in the earthquake was estimated at more than 2000 (Grimarest, 1790; Anonymous, 1790b). The Gazeta de Madrid (Gazeta, 1790) gives with great detail the number and affiliations of the dead military men (more than 250), but it does not give any information about civilian deaths, noting that it was impossible to identify civilian victims due to the loss of the census in the earthquake, and it considers that the number of 2000 victims is only a “prudential” estimate.

parts of the walls and bulwarks, namely Los Leones, La Campana, S. Isabel, and S. Francisco bulwarks. These maximum intensities (IX–X) are concentrated in the S and W parts of the fortified perimeter, with the exception of the S. Francisco bulwark which is located in the SE and is assigned that same intensity. The wall and towers located NW to N (the Los siete vientos and S. Roque towers) and S to SE (Santiago and Del Rosario) have lower intensities — VII to IX. The greater intensities observed for the walls and towers of the fortified perimeter (the parts from W to S) may be due to the effect of the soft river sediments. Grimarest (1790) describes the earthquake's effects on the city's two springs. For the Nacimiento spring, which has the higher flow rate of the two, the flow decreased by half, and the water was hot and changed in colour to red.

In accordance with these descriptions, we assigned a maximum intensity of IX–X to the Alcazaba, including the Hospital, the four churches (Iglesia Mayor, S. Francisco, S. Domingo, and La Merced), and most of the houses inside the city (Fig. 6). The number of victims, > 2000 of a total of the 9000 inhabitants of Oran (22%), supports the high intensity of the 1790 earthquake. A maximum intensity of IX–X is assigned to some

To the south, inland, Oran city had an external fortified perimeter formed by a wall that connected forts and defensive towers (Fig. 7). To the north, on the coast were located the Royal Store (Almacén del Rey, a solid building) and the fishing neighbourhood of La Marina and port of El Banquillo (Roel, 1790). These northern parts suffered heavy damage (“fueron arruinados”) (Anonymous, w.d.; Grimarest, 1790; Sánchez Cisneros, 1799). We have assigned them intensity IX–X. To the east, the

8. Damage to the external fortifications

9

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Fig. 7. Assigned intensities for the Oran perimeter and defences in 1790. (Modified from Hurtado (1790c).) Table 2 Damage to external fortifications. External fortification damage

Intensity

In the northern part, the El Banquillo harbour, the Royal Store, and the La Marina district, suffered major damage (“fueron arruinados”). S. Andrés and S. Felipe forts and the Nacimiento tower were damaged, but still operational. The Campo Santo, Algarrobo, Yfre, and S. Pedro towers were ruined and left in very bad condition. The Santiaguillo tower had large cracks. The Santa Cruz fort, a solid stone construction rebuilt shortly before the earthquake, also had large cracks.

S. Miguel and S. Barbara towers presented large cracks (Anonymous, w.d.), while the S. Andrés fort was damaged, but was still operational (Adorno 1790; Cumbre-Hermosa, 1790b; Sánchez Cisneros, 1799). Left in similar conditions were the S. Felipe fort and the Nacimiento tower to the south. We assigned intensity VII–VIII to these sites. To the west,

IX–X VII–VIII VIII–IX VII–VIII

Reference Anonymous w.d.; Grimarest, 1790; Sánchez Cisneros, 1799 Adorno 1790; Sanchez Cisneros, 1799; Cumbre-Hermosa, 1790b Anonymous, w.d.; Cumbre-Hermosa, 1790b Zapino 1790; Sanchez Cisneros,1799; Cumbre-Hermosa, 1790b

damage was greater. The Campo Santo, Algarrobo, Yfre, and S. Pedro towers (this last very close to the Alcazaba) were ruined or left in very bad condition (Anonymous, w.d; Cumbre-Hermosa, 1790b). We assigned intensity VIII–IX to these sites. The Santiaguillo tower had large cracks. The Santa Cruz fort, a solid stone construction rebuilt shortly 10

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main shock and the largest aftershocks were felt in Spain in Cartagena, Almería, Baños de Rioja, and Sierra Alhamilla. Table 3 summarizes the damage in southern Spain and northern Africa that has been discussed in this section.

before the earthquake (Zapino 1790), also suffered large cracks (Sánchez Cisneros, 1799). The S. Gregorio fort had only some light damage, and was still operational after the earthquake (Cumbre-Hermosa, 1790b). We assigned a lower intensity, VII–VIII, to these sites. In Fig. 7, one observes that the high intensities are located to the north, on the coast (El Barquillo harbour, the La Marina district, and the Royal Store). We assigned a similar intensity (VII–VIII) to the eastern and western external fortifications, but greater intensity (VIII–IX) to the SW part at the S. Pedro, Yfre, Algarrobo, and Campo Santo towers. Table 2 summarizes the damage to the external fortifications that has been discussed in this section.

10. The tsunami Several authors mentioned the earthquake's effects at sea. According to Grimarest (1790), the shock was felt on boats offshore of Oran, in some of them very strongly, and small boats almost capsized with some sailors falling into the sea. The sea retreated with such violence that, in an instant, the shore was left dry for some distance (and could not be fished for many days), but it returned shortly to cover the beach. Óbanos (1912) reproduces a letter by Francisco Javier EverardoTilly, Marquis of Casa-Tilly, Captain-General of the Spanish Navy, dated 9th October 1790, stating that in Cartagena there was a strong motion of the sea in the port and dock, and that the water level had risen by 6 ft. In the port, some boats became untied and were lost and some parts of the port were briefly left dry. Near Almería, the sea moved inland 40 varas (1 vara = 0.835 m, so about 33 m). Together, this information makes it clear that the 1790 event generated a small tsunami that affected Oran and reached the Spanish coast at Cartagena and Almeria (Fig. 3). The generation of a tsunami also supports the assignation of a high value to the maximum intensity.

9. Damage in southern Spain and northern Africa The 1790 earthquake was felt in Mazalquivir (Al-Marsa al-Kibir), a major Spanish harbour guarded by a castle, located to the west of Oran (Figs. 3 and 4) (Anonymous, 1790b). The damage was lighter than in Oran city and its external fortifications, and there were no victims (Cumbre-Hermosa 1790a; Sánchez Cisneros 1799). The castle was “cracked” (cuarteado) (Anonymous, w.d.). The earthquake was also felt in Mostagem (Mostaganem), Mascara, and Tremecén (Tlemecen) (Fig. 3), but there is no information about damage or victims, only an ambiguous phrase about possible damage in Mascara (Grimarest, 1790). The main shock was felt in Melilla, a Spanish enclave in northern Morocco (de Morales y Mendigutia, 1920). In Cartagena (SE Spain, Fig. 3) at 214 km from Oran, the main shock and some aftershocks were felt. Especial reference is made to the aftershocks of 11th October, but with no damage or victims (Memorial literario, 1790a; Actas Cabildo de Cartagena, 1790). The 25th October aftershock is reported as causing major damage in a document describing this damage street by street (buildings “broken”, “quebrantados”) (Ayuntamiento de Cartagena, 1790). It is difficult, however, to separate the damage caused by the main shock from that of aftershocks. In this case, we think that the aftershocks, especially the main one, affected buildings that had withstood the main shock with an apparently very low level of damage, and were finally “broken” by the aftershocks. We assigned Cartagena intensity IV–V for the main shock. In Almeria, on the coast of Spain at 207 km NE of Oran (Fig. 3), the main shock was felt very strongly at 01 h 30 m, with a duration of 32 s (Memorial Literario, 1790b). Many people left their houses which had been “impaired” (resentidas) by the shock. The San Francisco convent church was seriously damaged and had to be demolished. We assigned Almeria intensity V–VI. Several aftershocks were felt in Almeria, especially that of 9th October at 8:30, but with lower intensities, and in Rioja, very close to Almeria. The 23rd October aftershock was felt in Almeria, Rioja, and Baños de Sierra Alhamilla, and the large aftershock of 25th October was felt by many people in Almeria (Memorial Literario, 1790b, gives the time as 07 h 06 m). The main shock and the aftershock of 25th October were felt in Melilla, but there is no description of damage (de Morales y Mendigutia, 1920). The main shock was felt in Santa Fé (Granada) and Malaga (Huot, 1837; Perrey, 1847; Sanchez Navarro-Neumann, 1921; Galbis, 1932), but this information was not to be found in all the local archives such as the minutes of the cities' council meetings (Actas Cabildo Granada, 1790; Actas Cabildo Málaga, 1790). For this reason, one can only conclude with certainty that the

11. Discussion The date and time of the main shock is given in many contemporary documents as 9th October 1790 at 01:15 or 01:30 local time, with a duration in Oran that varies from 1 to 2 min (Adorno, 1790; Grimarest, 1790; Anonymous, 1790b; Cumbre-Hermosa, 1790a). In Almeria, the duration was 32 s. There is no mention of any foreshock. All the accounts refer to aftershocks felt until the beginning of November. > 80 aftershocks were reported (Table 1 and Fig. 5), some of them very strong (being described as “of the same size as the main shock”). The maximum intensity assigned to Oran city, IX–X (EMS-98), is in agreement with other studies (Bonaz, 1988; Lopez Marinas and Salord, 1990). The estimate is based on the descriptions of damage given in the original contemporary documents: in Oran city most buildings collapsed or were completely destroyed by the earthquake (Cumbre-Hermosa, 1790a,b; Anonymous, 1790a; Grimarest, 1790; Anonymous, 1790b). Some of these buildings were family houses with poor structures, but there were other robust constructions such as the Alcazaba (with walls of great thickness) or the city walls themselves with thicknesses > 2.5 m (Grimarest, 1790). The four churches, which were in good condition due to the repairs to them that had been carried out in 1783 (Roel, 1790), collapsed or were ruined. We considered Grade 5 damage to the four churches, the military quarters, the Alcazaba, Hospital, and other official buildings, structures corresponding to Class B of EMS-98. The great number of victims of the earthquake, > 2000 out of a total of 9000 persons, is another piece of information that supports an intensity of IX–X. We therefore do not agree with Chimouni et al. (2018) who propose a maximum intensity of VIII (EMS-98) for Oran. In their study, those authors only used Lopez Marinas and Salord (1990), not the original Spanish documents, together with Algerian sources

Table 3 Damage in southern Spain and northern Africa. Cities damaged Cartagena, 214 km from Oran, in SE Spain Almería, 207 km NE of Oran on the Spanish coast Melilla

Intensity

Reference IV–V

V–VI Several aftershocks were felt in Almeria The main shock and the 25th October aftershock were felt, but no description of damage.

11

Memorial literario, 1790a; Actas Cabildo de Cartagena, 1790; Ayuntamiento de Cartagena, 1790 Memorial Literario, 1790b de Morales y Mendigutia, 1920

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Table 4 Focal parameters. Date and time

Duration

9th October 1790; 01:15 or 01:30

1–2 min

Seismic source Complex event formed by two subevents

Motion Thrusting

containing information about the earthquake. The most important of those Algerian sources is Al-Rachidi (1791) who was part of an expeditionary force against the Spanish occupiers. However, Chimouni et al. (2018) state literally that this author and other “local sources that are contemporary to the event repeat the damage description reported in the city of Oran in the European sources”, so that there is in effect no new information provided. We do not agree with those authors' conclusion that the damaging effects of the shocks were exaggerated in order to justify Spain's leaving Oran and Mazalquivir. We think that the reason was the opposite: the great cost of repairing the damage in Oran together with the cessation of the attacks of pirates based in ports along the Algerian coast. This decision to leave was preceded in Spain by a broad-ranging debate, and was made in the framework of a period of political struggle in the country. Chimouni et al. (2018) explain the heavy damage as being due in part to landslides and in part to the attacks of Algerian troops in the context of military conflict in the region. It is clear from the original documents consulted in the present work (some of them never having been used in previous studies) that the attack on the city occurred two days after the earthquake when damage had weakened the defences. The report of victims and damage due to the earthquake was written on 9th and 10th October, i.e., the day of the main shock and the following day, before the attack. There is further confirmation of the assigned intensity (IX–X) in the effects on the rivers, with changes in colour and flow rate (Grimarest, 1790). Finally, the generation of a tsunami implies an at least moderate size for the earthquake. Small shocks do not generate tsunamis. As mentioned above, the tsunami was observed on the Spanish coast in Cartagena and Almeria (Fig. 3), located at 214 km and 207 km from Oran, respectively. Chimouni et al. (2018) propose that the tsunami in Cartagena occurred on 21st September (Rodríguez de la Torre, 1979; Bonaz, 1988) due to the occurrence of a foreshock on this date. But this is a mistaken interpretation of Rodríguez de La Torre (1979) and is unsupported by the contemporary documents in which the date of the observation of this phenomenon is very clear: 9th October. Comparison of the damage of the Oran earthquakes of 1790 (Imax = IX–X) and 6th June 2008 (Imax = VII) shows the area where the latter was felt to be smaller: in Almeria and Cartagena with intensity II (http://www.ign.es/web/ign/ portal/sis-catalogo-terremotos/-/catalogo-terremotos/open30Spain) as against an intensity of V–VI for that of 1790. The heavy damage in Oran city may be explained by the quake having a shallow focus with a macroseismic epicentre located offshore, as is supported by the generation of the tsunami observed on the coast at Oran, Almeria, and Cartagena. The catalogues of Munuera (1963), Mokrane et al. (1994), and El Mrabet (2005) assign epicentral coordinates (35.7°N, 0.6° W) to the main shock which correspond to the city of Oran, but this is incompatible with the generation of a tsunami, a tsunami for which there is sufficient evidence. We propose a macroseismic epicentre with coordinates 35.9°N, 0.6°W, in agreement with Lopez Marinas and Salord (1990). Grimarest (1790) describes the earthquake beginning with a loud underground noise, followed by the shock being experienced with two motions: the first upwards, and the second in the SW-NE direction. This description could be interpreted as the rupture corresponding to a complex event formed by two sub-events, with a reverse fault focal mechanism. Near the coast NW of Oran, there is a SW-NE thrust fault system (Benbakhti et al., 2018; Guessoum et al., 2018) (Fig. 2). These faults may be the structure responsible for the Oran earthquake, and

Aftershocks > 80

Macroseismic epicentre coordinates 35.9°N 0.6°W

Io (EMS-98) IX–X (Oran city)

M 6.0–6.5

this structure may continue under the sea, thus explaining the occurrence of the tsunami. But the 6th June 2008 earthquake (Mw 5.5) suggests that there may also be other offshore faults in the Bay of Oran. Comparison with the central Tell Atlas (Algiers and Boumerdes suggests that there are active reverse faults located either onshore or offshore which are south or north dipping (Meghraoui et al., 2004; Benfedda et al., 2017). There are interesting similarities between the tsunami generated along the Spanish coast by the Zemmouri 2003 (Mw 6.8) earthquake and that described in 1790. The source region can by defined by the area with intensity IX–X. It corresponds to Oran city (Figs. 6 and 7) and probably continues off-shore. The lack of information for other sites close to Oran makes determination of the extension of the source area difficult (Lopez Marinas and Salord, 1991). The duration of the earthquake in Oran, which different documents give as times ranging from 1 to 2 min, is another piece of information confirming the shock size. Based on these observations, we propose a magnitude of 6.0 to 6.5 for the main shock. This is the minimum that can generate a tsunami, and it is important to bear in mind that the tsunami was observed in Almeria and Cartagena on the coast of SE Spain, at 207 km and 215 km distance, respectively. The proposed focal parameters are summarized in Table 4. The 1790 Oran earthquake has similar characteristics to those of another historical event which occurred in 1748 in Montesa, SE Spain. Both locations are in zones of low seismicity in the eastern part of the Ibero-Maghrebian region, zones in which such large (maximum intensity IX–X) shocks are isolated events. The great 1755 Lisbon earthquake occurred in the interval between these two events, 12. Conclusions With an estimated maximum intensity of IX–X (EMS-98), the earthquake of 1790 in Oran is probably the largest to have occurred in the western part of Algeria. This estimate of the intensity was made on the basis of information in original contemporary documents reporting the damage in Oran city and its surrounding fortified perimeter. In particular: - The original contemporary documents give very detailed information about the buildings which collapsed or were left severely damaged, and the number (and in some cases the names) of victims from the 9th October to mid November 1790. Some of these documents are diaries. - The great damage was due to the earthquake, not to the attack on the city by the Algerian army which took place two days after the main shock. - The main buildings and the defensive walls were in good condition before the earthquake because of the repair work that had been carried out beginning in 1783. - The maximum intensity reached has been estimated as IX–X (EMS98). This value may be influenced by the structural complexity of monumental buildings, different type of soils present and the accumulation of damage due to the aftershocks. - The occurrence of a tsunami observed on the coast of both Oran and southern Spain (in Cartagena and Almeria) is further confirmation of the large size of the earthquake. - Given the heavy damage observed in Oran, the seismic source 12

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corresponds to a shallow focus very close to the city. The magnitude of 6.0 to 6.5 corresponding to the maximum intensity felt is the minimum magnitude that can generate a tsunami. - The rupture corresponds to a complex event with thrusting motion, and may be associated with the NE-SW fault system in the Oran area. This system comprises short en echelon faults that perhaps continue offshore, which would explain the generation of the tsunami.

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These characteristics ought to be considered in any reliable study of seismic risk in this region in order to avoid underestimating that risk. The generation of tsunamis is an additional hazard for earthquakes in this region. They cannot be regarded as unusual since they occurred in the 2003 Boumerdes earthquake. Because of today's major urban development of the city of Oran, the repetition of an earthquake such as that of 1790 could result in a catastrophe. Acknowledgements This work was partially supported by the Spanish Ministerio de Economía, Industria y Competitividad, project CGL2017-86097-R. References Abbouda, M.; Bouhadad, Y.; Benfedda, A. and A. Slimani. 2018. Seismotectonic and seismological aspects of the Mostaganem (Western Algeria) May 22, 2014 (Mw 4.9) seismic event. Arab. J. Geosci. 11, 57 (DOI:https://doi.org/10.1007/s12517-0183404-y). Aoudia, A., Meghraoui, M., 1995. Seismotectonics in the Tell Atlas of Algeria: the Cavaignac (Abou El Hassan) earthquake of 25.08.1922 (Ms = 5.9). Tectonophysics 248, 263–276. Ayadi, A., Bezzeghoud, M., 2015. Seismicity of Algeria from 1365 to 2013: Maximum Observed Intensity Map (MOI2014). Seism. Res. Lett. 86 (1), 237–244. https://doi. org/10.1785/0220140075. Belabbes, S., Meghraoui, M., Cakir, Z., Bouhadad, Y., 2009. InSAR analysis of the moderate size Ain Témouchent (Algeria) blind thrust earthquake (22/12/1999, Mw = 5.7). J. Seism. 13, 421–432. https://doi.org/10.1007/10950-008-9135-x. Benbakhti, I.M., Maouche, S., Belhai, D., Harbi, A., Ritz, J.F., Rabai, G., Rezouk, A., Doumaz, F., 2018. Characterizing the active tectonics in the Oran region (Algeria) and recasting the 1790 earthquake. J. Seism. https://doi.org/10.1007/s10950-0189784-3. Benfedda, A., Abbes, K., Bouziane, D., Bouhadad, Y., Slimani, A., Larbes, S., Haddouche, D., Bezzeghoud, M., 2017. The August 1st, 2014 (Mw 5.3) moderate earthquake: evidence for an active thrust fault in the bay of Algiers (Algeria). Pure Appl. Geophys 174 (3), 1503–1511. https://doi.org/10.1007/s00024-017-1481-6. Benhallou, H. (1985). Les Catastrophes Séismiques de la Région d'Echéliff dans le contexte de la Séismicité Historique de l'Algérie. Thèse de Doctorates-Sciences, Institut des Sciences de la Terre. Université des Sciences et de la Technologie Houari Boumediène (IST-USTHB), Algiers, Algeria, 294 pp. Benouar, D., 1994. Materials for the investigation of the seismicity of Algeria and adjacent regions during the twentieth century. Ann. Geofis. 37, 459–860. Bezzeghoud M. and E. Buforn (1999). Source parameters of the 1992 Melilla, (Spain, Mw = 4.8), Alhoceima (Morocco, Mw = 5.8), and 1994 Mascara (Algeria, Mw = 5. 7) earthquakes and seismotectonic implications. Bull. Seism. Soc. Am. 89,2, 359–372. Bezzeghoud, M., Dimitrov, D., Ruegg, J.C., Lammali, K., 1995. Faulting mechanism of the El Asnam 1954 and 1980 earthquakes from modeling of vertical movements. Tectonophysics 249, 249–266. Bezzeghoud, M., Ayadi, A., Sebaï, A., Aït Messaoud, M., Mokrane, M. and H. Benhallou (1996). Seismicity of Algeria between 1365 and 1989. Map of maximum observed intensities (MOI). Avances en Geofísica y Geodesia I, IGN (Madrid) 107–114. Bezzeghoud, M., Adam, C., Buforn, E., Borges, J.F., Caldeira, B., 2014. Seismicity along the Azores-Gibraltar region and global plate Kinematics. J. Seism. 18, 605–620. Bonaz, R., 1988. The historical seismicity of western Algeria and the earthquake of 1790 at Oran. Eur. Earthq. Eng. 3–7. Bouhadad, Y., 2001. The Murdjadjo, western Algeria, fault-related fold: implication for seismic hazard. J. Seismol. 5, 541–558. https://doi.org/10.1023/A:1012039900248. Bouhadad, Y., 2014. Occurrence and impact of characteristic earthquakes in Algeria. Natural Hazard Journal 72, 1329–1339. https://doi.org/10.1007/s11069-0130704-0. Boukhedimi, M.A., Louni-Hacini, A., Bouhadad, Y., Ritz, J.F., Machane, D., Benhammouche, A., Bourenane, H., 2017. Evidence of seismites in coastal Quaternary deposits of western Oranie (northwestern Algeria). J. Seism. 21, 539–549. https://doi.org/10.1007/s10950-016-9616-2. Buforn, E., Bezzeghoud, M., Udías, A., Pro, C., 2004. Seismic sources on the Iberia-African plate boundary and their tectonic implications. Pure Appl. Geophys. 161, 623–646. Buforn, E., Udías, A., Pro, C., 2014. Large earthquakes at the Ibero-Maghrebian Region: basis for an EEWS. Pure Appl. Geophys. 171, 9. https://doi.org/10.1007/s00024014-0954-0.

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BNE MSS/3422. Anonymous (w.d.). “Vista de la ruina de Oran”. CEGET. ArQT10C2-93. Antoñano, N., 1790. Diario que comprende desde 22 de octubre al 3 de noviembre sobre lo acaecido en el viaje dese Mazarquivir al puerto de Arseo en jabeque ibizenco S. Antonio de Padua. AHN Estado Leg. 2040, exp 42 (3 pp), 93. Ayuntamiento de Cartagena (1790). Expediente obrado sobre composición y reparos que necesitan algunas de las casas de esta población mediante reconocimiento practicado por los peones de albañilería. Cumbre-Hermosa, 1790a. Carta del Conde de Cumbre Hermosa (Oct. 9) al gobernador de Cartagena. In: BNE MSS/18547/5/2. Cumbre-Hermosa, 1790b. Parte elevado al Rey por el General Conde de Cumbre Hermosa Gobernador de Orán. Nov. 2 1790. AHN Estado Leg. 2040, exp. 44. El Mercurio de España, 1790. November. In: 1790. Grimarest, V. (1790). Línea de Orán, Oct. 29, 1790. Relación y Breves reflexiones sobre los terremotos de Orán de V. de Grimarest. AHN Estado Leg. 4818, exp. 32. Hontabant, H., 1772. Consistencia de las fortificaciones de Oran y Mazalquivir. In: AGMM 5-3-2-3. Hurtado, A. (1791a). Plano de El Recinto de la Plaza de Orán que manifiesta las ruinas de las murallas ocasionadas por el terremoto del 8 al 9 de octubre de 1790 y los edificios según se hallan al presente. Antonio Hurtado. 25 de septiembre de 1791. AGMM – ORA-04-08. Hurtado, A., 1791b. Vista del frente más avanzado al enemigo de la plaza de Orán, compreendido entre la tenaza de la campana y la puerta de Tremecén. Antonio Hurtado. 4 mayo de 1791. In: AGMM-ORA-05-07. Hurtado, A. (1791c). Plano General de la plaza, castillos, fuertes y baterías de Orán. Antonio Hurtado. 26 de enero 1791. AMN-P11E. La Gazeta de Madrid, 19 and 26 November, 1790, p. 766–770 y p. 786–787. Le Moniteur universel, (1790), 30 octubre y 2 de noviembre de 1790. Memorial literario, 1790a. Noviembre 1790, vol. In: CXXI. Memorial literario, 1790b. Diciembre 1790, vol. In: CXXIII. Roel, L., 1790. Descripción particular de la ciudad de Oran, 284–286. Sánchez Cisneros, J. (1799). Carta africana. Real Universidad, Alcalá. Zappino, B., 1785. Relación de las Plazas de Orán y Mazalquivir, y estado actual de sus fortificaciones. Año 1785. In: AGMM 5-3-2-9.

Contemporary documents AGMM-Archivo General Militar de Madrid; AHN – Archivo Histórico Nacional (Madrid); AMN - Archivo del Museo Naval (Madrid); BNE - Biblioteca Nacional de España (Madrid); CEGET – Centro Geográfico Ejército de Tierra (Madrid). Actas del Cabildo Almería (1790), del 12, 21, 22, 26 de Octubre. Actas del Cabildo Cartagena (1790), del 9, 11, 22 y 30 de Octubre, y 8, 11 y 18 de noviembre. Actas del Cabildo de Granada (1790), del 8 al 19 de octubre. Actas del Cabildo de Málaga (1790), del 11, 18, 21 y 25 de Octubre y 1, 8 noviembre. Actas del Cabildo Vera, 1790. Lib. In: 0133 del 28 de octubre y 27 octubre. Adorno, R. (1790). Diario de D Rafael Adorno a Conde Floridablanca de lo acaecido del 9 de Oct a 12 Nov de 1790. AHN Estado Leg. 4818, exp. 32. Al-Rachidi, A., 1791. The Jamanian Mouth in the Oranian Smile, Heritage Series. Publications of the Ministry of Education and Religious Affairs (in Arabic). Anonymous (1790a). Línea de Orán, 17 Octubre 1790. Escrito detallando las desgracias ocurridas por los terremotos en la guarnición y vecindario y demás particularidades empezadas en el parte n° 1. AHN Estado, Leg. 2040, exp. 46. Anonymous (1790b). Diario histórico-trágico acaecido Oran 9 al 31 de Octubre 1790.

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