**5.2 X-shaped damage, a seismic evidence**

*Natural Hazards - Impacts, Adjustments and Resilience*

Moroccan territory made communications difficult, and the authorities and rescue teams focused on the effects of the floods rather than the effects of the earthquake. As a consequence, under the impact of two natural phenomena at the same time and along de same disaster area, it is difficult to distinguish which damages are due

The Al Hoceima earthquake of February 24, 2004 took place at 02:27:46 UTC, at coordinates 35.1563 N -3.9841 W, south of the Alboran Plain, in the Mediterranean Sea near the Strait of Gibraltar, with epicenter north of Tamassint (Morocco). The magnitude of this seismic event was Mw = 6.2 (IGN) and there are no data about its depth, so it is estimated that it must have been very superficial <5 km. To have an approximate idea of the superficiality and energy released, the main shock was followed by more than 760 aftershocks over Mw>3 during the following two months.

This earthquake hit the province of Al Hoceima, especially the city so named, the municipality of Imzourem and the villages of Ait Kamra and Izemmouren. The rural houses in this geographical area of Morocco, most of them of vulnerability class A, were built using the traditional construction pattern, very similar to those we have just highlighted in the most rustic areas of the Algarve, with more emphasis in Fonte de Louzeiros: single-story load-bearing walls made with unworked raw materials and unskilled masonry techniques, supporting a brittle roof of wooden logs in parallel under a bed of reeds tied together with rope or wire, all covered by a layer of water-repellent mortar and, occasionally, with a plastic film interposed acting as waterproof element (**Figure 13a**). The load-bearing walls were made up

*Ait Kamra, traditional house pattern: (a) partial collapse showing the doble-leaf load-bearing walls, fieldstone masonry bonded with mud mortar and roof of wooden logs and canes covered with a plastic film (blue) and layer of water-repellent mortar (grade 4, class A); (b) handmade joist of hollow bricks assembled* 

to the floods, to the earthquake or to the combined effects of both episodes.

**5. The Al Hoceima earthquake of February 24, 2004**

**5.1 The failure of traditional construction patterns**

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**Figure 13.**

*with a single steel rod. Source: P Murphy.*

Regardless of the function of the walls (infill, load-bearing, retaining, etc.), type of building materials (adobe, stone, brick, gray concrete block…) and vulnerability class (A, B, C…), not all cracks should be exclusively interpreted as a result of seismic shaking. In general, wall cracks are caused by the effect of five recognized forces: tension, compression, bending, torsion, or shearing. But only when cracks have a diagonal shape it can be inferred that they have been generated by shear stress or a combined effect tension-shearing or bending-shearing. Shear or diagonal cracks may be due to earthquakes, but also―and most frequently―to a slope in the ground, poor foundations, landslide, soil composition (i.e., expansive clays), geological conditions, etc., which can lead to subsidence or settlement processes.

In April 2019, during a brief field visit to the Roman ruins of Baelo Claudia (Tarifa, Spain), an ancient city where some geologists have believed to find evidence of great destruction caused by a sequence of earthquakes between the 1st and 3rd centuries BC, we certainly observed shear cracks in the *summa cavea* of the theatre, bowing of a load-bearing wall in a *vomitorium* arch, and expulsion of voussoirs in another arch near the *parascenium* that looked like piano keys shifted (**Figure 15**). We can firmly state that these structural damages were not induced by earthquakes because they occurred after the reconstruction works of this archeological site carried out in the 1980s and without seismic events to justify them. Rather, these

**Figure 14.** *Ait Kamara, corner failure pulling down the roof slab (grade 4, class A). Source: P Murphy.*

#### **Figure 15.**

*Baelo Claudia, roman ruins (theatre): bowing of a* vomitorium *wall (left) and expulsion of voussoirs near the*  parascenium *(right), which show a tensile stress in the direction of the slope. Source: J A Aparicio.*

effects can be explained by the instability and slope of the ground, as well as a bad combination between the weight of the structure, the insufficient foundations, and the progressive and slow landslide.

A single shear crack only proves the existence of a one-way displacement force. However, X-shaped cracks are unmistakable signs of lateral and reverse forces that result in a very characteristic diagonal crack pattern. The best candidates are earthquakes due to the effect of S-waves and surface waves, particularly Love waves, which transmit loads in opposite directions. In Al Hoceima, this type of damage is found in any vulnerability class buildings (A, B or C). **Figure 16** shows a very striking case, where the X-shaped damage completely surrounds the load-bearing walls on the ground floor of a four-story residential building of vulnerability class B in Imzourem (grade 3). There are no shear cracks on the upper floors, which is a clear indication

#### **Figure 16.**

*Imzourem, X-shaped damage completely surrounding the load-bearing walls on the ground floor of a four-story residential building of vulnerability class B (grade 3). Source: P Murphy.*

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**Figure 17.**

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

of a soft-story damage, which will be discussed later. X-shaped cracks are sometimes

The effects of this diagonal tension cracking are also common in infill walls of RC structures where, except in case of collapse, are clearly visible. They spread from lower to upper floors, with a more severe impact on the ground floor after receiving the loads of seismic shaking. In this case, the positive aspect is that these infill walls are not structural elements, unlike in buildings of class A or B, and must be assessed as damage of grade 3 in buildings of vulnerability class C, unless the RC frame structure has been seriously damaged. When X-shaped cracks cross external infill walls located between discontinuities or openings arranged for windows and doors, and the location and depth of these cracks coincide with a RC frame column that ends up being damaged, it can result in a very characteristic type of damage called "short column" or "captive column" (**Figure 18**). This effect is caused by the modification of the expected proportional distribution along the column body of its deformation ability under the influence of lateral loads [20], due to a partial confining of RC frames and a lesser stiffness of a free portion of the column less supported by partitioning brickworks. The consequence is a shorter column that concentrates

blurred by the loss of outer leaf or toppling of the external walls (**Figure 17**).

most of the shear stress, i.e., a major part of the column ductility is lost.

*Examples of X-shaped damage clearly visible in a single-story class A house (above) and blurred by loss of* 

*outer leaf in a four-story residential block (below). Source: P Murphy.*

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