The reviewed literature was focused on bibliographic research of thesis, scientific articles, scientific reports and all other documents relative to the geology and tectonics of the studied zone. This step led to retrace the great features of tectonic history of the studied zone and compare them with the results obtained by the present study.

The interpreted seismic and magnetic maps are respectively from High Authority of Nuclear Emery of Niger Republic (HANEA, Niger) and Project for Reinforcement and Diversification of the mining sector in Niger (PRDSM, Niger). These geophysical data were integrated with the Software “ArCGis 10.1” when seismic and magnetic lineaments were extracted and interpreted ( Figure 2 and Figure 3 ). This step led to determining the main directions of crack propagation in the deep and subsurface.

The field investigation consisted of prior identification of the most affected sectors by cracking events and in situ structural measurement of crack planes. This stage was carried out using geological materials, such as GPS (for the location of the site) and Compass (for direction and dip acquisition of a structure).

Processing and statistical analysis of measured structures was carried out using the software MS Excel 2013, which is well known for its statistical analysis. Field photographs were processed by the Software of computer-assisted design “Canvas 11”. The measured planes of cracks were inputted into MS Excel 2013 software and statistically analyzed with this software. The step allowed the classification of all measured crack planes by the direction intervals of 45˚ from the North ( Table 2 and Figure 5 ).

Structural analysis of seismic cartographic data [5] highlighted two regional lineament systems: the NNW-SSE trending seismic lineament affecting both the Aïr basement and Tenere basin in the East and N-S fault system extended in the Tim Mersoi basin in the West ( Figure 2 ). In addition, some localized seismic lineaments oriented NEE-SWW and cutting the regional lineaments were identified in the western region of Agadez. This regional lineament would control the setup of the uranium mineralization in Arlit region [6] , and in Tenere basin, it constitutes a NNW-SSE petroleum system of Termit basin [7] .

Lineaments extraction (in the case of this study) from aeromagnetic cartographic data [8] of Agadez region at 1/200,000th ( Figure 3 ) showed two main directions of magnetic lineaments according to their size: minor and major. The major part of these magnetic lineaments is oriented in NW-SE direction (mainly N135˚ to N140˚). In addition, a major magnetic lineament oriented N-S ( Figure 3 ) corresponding to In-Azaoua-Arlit fault system revealed by previous geological works was identified ( Figure 1 ). These similar directions were also previously identified by seismic cartographic data ( Figure 2 ). These observations reveal that the subsurface seismicity directions of Agadez region are mainly NW-SE trending. Those cannot be recommended or avoided for the construction of big buildings.

Field investigations and measurements of crack planes were carried out on Agadez sandstone formations and buildings ( Figure 4 ). The obtained microtectonic data are presented in Table 1 and Table 2 . The structural and statistical analysis of the obtained data showed that the cracking events affect both the Agadez sandstone formations and buildings ( Figure 4 ). To determine the major direction of their propagation, the 220 measured planes were statistically analyzed by direction intervals of 45˚ from the North ( Table 2 and Figure 5 ). The obtained results of statistical analysis by the percentage of directions show that:

North to Northeast (N0˚-N45˚) directions: 25%,

Northeast to East (N45˚-N90˚) directions: 30%,

East to Southeast (N90˚-N135˚) directions: 12.73% and

Southeast to South (N135˚-N180˚) directions: 32%.

According to these observations, the directions ranging from NW-SE (N135˚) to N-S (N180˚) are the majority with 32%, followed by NE-SW (N45˚) to E-W (N90˚) directions with 30%. Therefore, the buildings cracking events have a stronger propagation component according to the mean directions which are NW-SE (N135˚) trending.

Note: S: South, N: North, W: West, E: East, V: Vertical.

1) Tectonic origin

Analysis of seismic and magnetic cartographic data showed two main directions: N-S and NWW-SSE directions. According to the previous geological and structural works, the N-S regional lineament systems corresponding to the In-Azaoua-Arlit fault system extended in the Tim Mersoi basin in West. Another N-S regional direction called “Raghane fault system” affects similarly the Precambrian Aïr basement and Tenere basin in East ( Figure 1 ). The NNW-SSE lineaments represent the relay of N-S regional fault systems and cause the building cracking in Agadez region [1] . According to several authors, these two fault systems were the focus of three seismic events (4^th July 1969, 19^th May 1967 and 18^th January 2017 [1] - [3] [5] ). According to geological data obtained in Hoggar [9] - [13] , these regional fault systems derive from the Pan-African reactivation of three major sub-meridian shear zones in Hoggar: East Ouzzal shear zone (3˚30'), Tekouyat shear zone (4˚50') and Raghane shear zone (8˚30'). In the Aïr mountain of Niger, this Pan-African reactivation caused the formation of the juxtaposed terranes from West to East Assodé terrane, Barghot terrane and Aouzgueur Terrane, all limited by the N-S major shear zones [12] - [14] . A recent study carried out by [15] on the studied region has shown the Evidence of brittle tectonic continuity between the basement of Aïr Mountain and its overlying sandstone formations on which Agadez City was built.

2) Anthropic origins

The first anthropic origin of building cracking in Agadez region is the uranium coal mining operation carried out in the three different mining sites in the region (Somaïr, and Cominak for uranium mining in Arlit sector and Sonichar for coal mining in Tchirozerine sector, Figure 1 ). The structural work on the mining sites of Somaïr, Cominak and surrounding city of Arlit [6] , described the fractures with variable orientations: the N140˚ to N170˚, the N100˚ to N120˚, the N40˚ to N50˚, the N60˚, and the N70˚ to N80˚, all caused by the use of dynamites during the uranium mining. According to this author [6] , these fractures constitute the geotechnical risks and serious accidents related to the falling of the blocks during the uranium mining.

The second environmental origin would correspond to the vibrations of supersonic airplanes during take-off and landing. It can be noted that Agadez City has two airports: Airport of Agadez and the American Battalion 201 Airport ( Figure 6 ). The projection of coordinates of all studied sites on the Google Earth map has shown that the most affected buildings by cracking events in Agadez are surrounding the airports areas.

The third origin of building cracking in Agadez region can also be the non-respect of geotechnical standards of civil engineers for the building construction. Indeed, the field observations carried out in the case of this study showed that some buildings are built along the slope of streams, sometimes with incorrect cement dosage and or without reinforced concrete.