*Effects of Earthquakes on Buildings in the Ibero-Maghrebian Region DOI: http://dx.doi.org/10.5772/intechopen.94739*

*Natural Hazards - Impacts, Adjustments and Resilience*

information provided by news agencies and newspapers.

**4. The February 28, 1969 earthquake**

Knutsen" while sailing in deep waters [12].

**4.1 Effects in Portugal**

Instituto Nacional para la Reducción de los Desastres (IERD) [11].

The 1969 SW Cape St. Vincent earthquake took place on Saturday, 28th February, at 02.40.32 UTC (t0), and was located in the Horseshoe Abyssal Plain, coordinates 35.98° N 10.81° W, 200 km away to the SW of Cape Saint Vincent, with hypocenter at 20 km depth. Its high magnitude Mw = 7.8 (IGN) caused seismic waves to heavily shake Portugal, Spain, and Morocco. Moreover, this seismic event is associated with a reverse faulting between Azores and the Gulf of Cadiz in the western part of Africa-Eurasia plate boundary, which generated a moderate tsunami recorded in several tide gauges, strongly damaging the Norwegian tanker "Ida

*4.1.1 Algarve: an example of bad behavior of buildings of vulnerability A or B*

The greatest damage to buildings in Portugal was registered in the Algarve region, with total or partial collapses spread over different locations, amplified by the so-called *site effect* [13, 14], that is, the amplification of the seismic waves due to the geological characteristics of soils in superficial layers and the soil-structure interaction of buildings. Settlements built on younger tectonic formations, such as poorly consolidated alluvial soils, felt the shakes more strongly [15], increasing the vulnerability of traditional constructions made with extremely poor materials and techniques. Most of them had one or two floors supported on load-bearing walls made of fieldstone or adobe masonry bound with mud mortar, clay or without mortar. These external walls, quite thick and most often strengthened with doubleleaf shape (two walls facing each other, barely joined by a poor mortar), supported a gable roof made of wooden logs or struts under a bed of reeds covered with tiles

**3. Documentary sources**

grade 4, very heavy damage or partial collapse; and grade 5, destruction or total collapse. Buildings with a grade of damage 1 to 3 do not result in structural or significant structural damage, being easy to repair and recover, while those of grade 4 or 5 have severe or complete structural damage and are difficult or impossible to recover. In this scientific contribution we will explain these effects in detail based on three of the most destructive and best documented earthquakes recorded in the study area: southwest Cape St. Vincent, 1969; Al Hoceima, 2004; and Lorca, 2011.

The analysis and description of damage from these selected seismic events is based on several documentary sources: available unofficial field reports, official reports from public authorities, photographs showing damage to buildings, and

In the case of the 1969 earthquake, our main source for the area of Portugal is the technical report carried out by the seismologist Mário de Vasconcelos Trêpa [9] and, for the case of Huelva (Spain), the official reports of Isla Cristina Town Council. Regarding the 2004 Al Hoceima earthquake, the analysis will be based on the field report made by the technical architect Patrick Murphy Corella [10]. Finally, with respect to the 2011 Lorca earthquake we will take as documentary source our own field report and photographs taken by emergency managers of the

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or zinc plates. Therefore, they must be classified as structures of vulnerability class A, and hence the effects in Castro Marim, Barão de São Miguel, Vila do Bispo, Bensafrim, Fonte de Louzeiros or Silves.

The damage reached greater severity and geographical extension within a radius of 50 km around Cape St. Vincent. In Silves, several houses collapsed, and lateral loads produced many shear cracks, overturning or toppling of load-bearing walls, corner failures and large cracks in the walls of the castle. **Figure 1** shows shear cracks, with detachment of little pieces of plaster and partial toppling of external wall from the upper floor, after loss of connection and possible mutual pounding of adjacent buildings with roofs at different levels, in a three-story building of class B (grade 3). Bensafrim, 26 km from Silves and closer to Cape St. Vincent, was one of the towns hardest hit by the earthquake. More than twenty houses were destroyed and around forty suffered the typical damage of buildings of class A: loss of connection between load-bearing walls, corner failure, overturning or toppling of external walls, detachment and fall of outer leaf, X-shaped or diagonal cracks, fall of plaster, and roof collapse. The primary school, recently built with RC (class C), had cracks in several parts (grade 3). An example of corner failure with partial roof collapse is described in **Figure 2**, which should be assessed as grade 4 in building of class A. The same description can be made for Odiáxere, a *freguesia* (municipality) of Lagos council, at the distance of 7 km from Bensafrim.

In Vila do Bispo, 8 km from Sagres, many houses of class A were partially or completely collapsed and turned into rubble (**Figure 3**), with a level of destruction similar to Bensafrim. **Figure 4** shows an example of façade overturning pulling down the roof and causing a complete collapse of the structure (grade 5). In other photographs we can clearly observe damage of grade 3 in corner failures (**Figure 5**) and grade 4 in the toppling of external wall, seriously affecting the roof (**Figure 6**). Also, the upper body of the church tower of Nossa Senhora da Conceição, built with massive stone, ashlars and burnt clay/sand bricks showed a progressive X-shaped

#### **Figure 1.**

*Silves, building of class B with damage of grade 3: (a) shear cracks with detachment of little pieces of plaster; (b) partial toppling of external wall. Source: IPMA.*

#### **Figure 2.**

*Bensafrim, building of class A with damage of grade 4: (a) advanced corner failure and toppling of external wall with partial roof collapse; (b) loss of connection and drift between load-bearing walls; (c) generalized detachment of plaster. Source: IPMA.*

#### **Figure 3.**

*Vila do Bispo, general overview: (a) loss of the outer leaf of load-bearing walls and detachment of plaster; (b) partial toppling of façade wall, pulling down part of the roof; (c) good behavior of a recent construction (class B); (d) simple overturning of a freestanding masonry wall; (e) adobe house without apparent damage. Source: IPMA.*

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**Figure 5.**

*Google Earth.*

**Figure 4.**

*Effects of Earthquakes on Buildings in the Ibero-Maghrebian Region*

crack and fall of pinnacles [15], which means damage of grade 3 in buildings of class B. In summary, the traditional construction techniques based on adobe masonry

*Vila do Bispo, building of class A with damage of grade 3 due to loss of connection between load-bearing walls and corner failure (left). This two-story house was repaired and is currently in use (right). Sources: IPMA/*

*Vila do Bispo, total collapse of building of class A made of mixed fieldstone and adobe masonry, mud or clay mortar and roof supported by wooden logs and bed of reeds (grade 5). See the doble-leaf shape on an interior* 

Several houses were totally collapsed in Sagres and many others suffered damage of grade 2 or 3 in buildings of vulnerability class A. In **Figure 8**, one single-story house with gable roof shows shear cracks, detachment of cornices and tiles, partial roof collapse, loss of connection between load-bearing walls and drift from the façade with risk of overturning (grade 3). Also, in Raposeira, 5 km east from Vila do

and clay mortar showed a high vulnerability and weakness (**Figure 7**).

*DOI: http://dx.doi.org/10.5772/intechopen.94739*

*load-bearing wall remaining standing. Source: IPMA.*

*Effects of Earthquakes on Buildings in the Ibero-Maghrebian Region DOI: http://dx.doi.org/10.5772/intechopen.94739*

#### **Figure 4.**

*Natural Hazards - Impacts, Adjustments and Resilience*

*Bensafrim, building of class A with damage of grade 4: (a) advanced corner failure and toppling of external wall with partial roof collapse; (b) loss of connection and drift between load-bearing walls; (c) generalized* 

*Vila do Bispo, general overview: (a) loss of the outer leaf of load-bearing walls and detachment of plaster; (b) partial toppling of façade wall, pulling down part of the roof; (c) good behavior of a recent construction (class B); (d) simple overturning of a freestanding masonry wall; (e) adobe house without apparent damage. Source:* 

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**Figure 3.**

*IPMA.*

**Figure 2.**

*detachment of plaster. Source: IPMA.*

*Vila do Bispo, total collapse of building of class A made of mixed fieldstone and adobe masonry, mud or clay mortar and roof supported by wooden logs and bed of reeds (grade 5). See the doble-leaf shape on an interior load-bearing wall remaining standing. Source: IPMA.*

#### **Figure 5.**

*Vila do Bispo, building of class A with damage of grade 3 due to loss of connection between load-bearing walls and corner failure (left). This two-story house was repaired and is currently in use (right). Sources: IPMA/ Google Earth.*

crack and fall of pinnacles [15], which means damage of grade 3 in buildings of class B. In summary, the traditional construction techniques based on adobe masonry and clay mortar showed a high vulnerability and weakness (**Figure 7**).

Several houses were totally collapsed in Sagres and many others suffered damage of grade 2 or 3 in buildings of vulnerability class A. In **Figure 8**, one single-story house with gable roof shows shear cracks, detachment of cornices and tiles, partial roof collapse, loss of connection between load-bearing walls and drift from the façade with risk of overturning (grade 3). Also, in Raposeira, 5 km east from Vila do

#### **Figure 6.**

*Vila do Bispo, building of class A with damage of grade 4: (a) loss of connection between load-bearing walls; (b) toppling of external wall; (c) collapse of roof made of wooden logs and bed of reeds. Source: IPMA.*

#### **Figure 7.**

*Vila do Bispo, typical house of vulnerability class A built with raw materials: load-bearing walls made with adobe bricks, doors and windows with lintels, and wooden logs supporting a tiled roof over a bed of reeds. Source: Google Earth.*

Bispo, a crack arose on the façade of the hermitage of Nossa Senhora de Guadalupe, running from the gable to the apex of the ogival arch of the entrance, from top to bottom, crossing the rose window [15]. This chapel dates from 14th century and was built with load-bearing walls made of massive stone, with pillars of ashlars embedded and supporting a ribbed vault built with the same material. The entire structure was reinforced with solid buttresses around the side walls and toothed ashlars in the corners; therefore, it should be assessed as damage of grade 2 in a building of vulnerability class C.

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**Figure 8.**

*Effects of Earthquakes on Buildings in the Ibero-Maghrebian Region*

A similar description could be made for Portimão, located 40 km to the east. According to Trêpa's report, this town was "undoubtedly, the most affected city in the whole territory". However, there is no specific information of damage that would allow us to confirm this statement. **Figure 9** shows the partial detachment and toppling of the outer leaf of the external wall from the second floor of a two-story building of class B, with the subsequent fall of a cornice section and detachment of tiles (grade 3) due to a hammering effect between the wooden joists

*Sagres, building of class A with damage of grade 3: (a) loss of connection of load-bearing walls with drift of* 

*façade, and (b) detachment of cornices and tiles with partial roof collapse. Source: IPMA.*

*4.1.2 Fonte de Louzeiros: the maximum intensity in the whole macroseismic area*

Fonte de Louzeiros is a small rural hamlet located 7 km at east of Silves. In this settlement of sixteen houses at the time, the most of buildings were total or partially collapsed. The earthquake caused all type of damage mentioned above: X-shaped cracks, corner failures, overturning of load-bearing walls and roof collapses; almost no house could be rebuilt. As an exceptional example, in **Figure 10** we can see one of the few buildings left standing and showing the patched damage of a corner failure. About 95% of these single-story buildings of vulnerability class A suffered damage of grade 5. Some seismologists have assigned to this place intensity VIII, the maximum level of destruction in the whole macroseismic area [16], although other researchers extend this isosist, in general, to the Algarve region [4] or enclose it to the Cape St. Vincent and surroundings areas [9, 15, 17, 18]. But this particular case of Fonte de Louzeiros, with a total destruction of buildings of vulnerability class A, does not correspond to an area of intensity VIII but rather to intensity IX, or at least VIII-IX [19]. Therefore, it is consistent to raise the damage recorded in Fonte de Louzeiros to intensity VIII-IX, where "a bombing would not have been worse", literally "wiped

supporting the roof and the top of the load-bearing walls.

off the map", as defined by the *Diario de Lisboa* on March 2, 1969.

*4.1.3 Alentejo coast, Setúbal and Lisbon: the transition to vulnerability class C*

along the Atlantic coast between Aljezur and Setúbal, where the effects reached

Outside the Algarve region, the intensity of the damage decreases slightly, except

*DOI: http://dx.doi.org/10.5772/intechopen.94739*

*Effects of Earthquakes on Buildings in the Ibero-Maghrebian Region DOI: http://dx.doi.org/10.5772/intechopen.94739*

#### **Figure 8.**

*Natural Hazards - Impacts, Adjustments and Resilience*

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**Figure 7.**

*Source: Google Earth.*

**Figure 6.**

building of vulnerability class C.

Bispo, a crack arose on the façade of the hermitage of Nossa Senhora de Guadalupe, running from the gable to the apex of the ogival arch of the entrance, from top to bottom, crossing the rose window [15]. This chapel dates from 14th century and was built with load-bearing walls made of massive stone, with pillars of ashlars embedded and supporting a ribbed vault built with the same material. The entire structure was reinforced with solid buttresses around the side walls and toothed ashlars in the corners; therefore, it should be assessed as damage of grade 2 in a

*Vila do Bispo, typical house of vulnerability class A built with raw materials: load-bearing walls made with adobe bricks, doors and windows with lintels, and wooden logs supporting a tiled roof over a bed of reeds.* 

*Vila do Bispo, building of class A with damage of grade 4: (a) loss of connection between load-bearing walls; (b) toppling of external wall; (c) collapse of roof made of wooden logs and bed of reeds. Source: IPMA.*

*Sagres, building of class A with damage of grade 3: (a) loss of connection of load-bearing walls with drift of façade, and (b) detachment of cornices and tiles with partial roof collapse. Source: IPMA.*

A similar description could be made for Portimão, located 40 km to the east. According to Trêpa's report, this town was "undoubtedly, the most affected city in the whole territory". However, there is no specific information of damage that would allow us to confirm this statement. **Figure 9** shows the partial detachment and toppling of the outer leaf of the external wall from the second floor of a two-story building of class B, with the subsequent fall of a cornice section and detachment of tiles (grade 3) due to a hammering effect between the wooden joists supporting the roof and the top of the load-bearing walls.
