## 3 Observed Damage Mechanisms in Eastern T\(\ddot{\text{u}}\)rkiye's Masonry Buildings
### Damage Modes Attributable to Poor-Quality Units and Mortar
#### 3.1.4 Poor-Quality Mortar  
Mortar functions as the essential binding matrix that ensures shear and tensile continuity for masonry units [4]. In several rural and semi-urban areas of eastern Turkjye, particularly inside the EAF zone, mortars are manufactured with little standardization or chemical regulation. Inferior-quality mortar is a primary factor contributing to the significant structural failures seen during moderate-to-severe seismic events. The following sections examine critical elements of inferior mortar quality, its conventional manufacturing techniques, and the manner in which it engenders a wide array of significant damage mechanisms under seismic stress.  
The inclusion of soil-based aggregates or \"clayey\" sands contributes organic materials and particles that impede cohesive bonding. During the 2011 Van earthquakes, the 2020 Elazig-Sivrice earthquake and 2023 Kahramanmaras earthquakes, field testers identified mortar layers containing substantial aggregates of silt or clay, whereby incomplete hydration of these aggregates resulted in microvoids capable of expansion under dynamic stress [41; 44].  
It should be noted that due to logistical constraints and the emergency nature of the field deployment, no Schmidt hammer, rebound testing, or core extraction was performed. Instead, material assessments were conducted visually, with qualitative evaluations based on crack morphology, separation behavior, and unit fragmentation patterns during field inspections. Field studies indicate that the compressive strengths of local mortars can fall below 2 MPa, far lower than the minimum 5 MPa often advised in contemporary construction regulations for load-bearing masonry. Furthermore, the tension and shear capabilities are significantly inadequate, and are parameters essential for withstanding lateral stresses during seismic events. In the 2023 Kahramanmaras earthquakes, whole masonry courses significantly lost their vertical load-bearing capability with the emergence of diagonal fractures in the mortar, resulting in the top parts of walls detaching and collapsing outward. Further, shear slip at horizontal mortar joints was consistently recorded, indicating that the mortar was insufficiently robust to convey in-plane pressures beyond modest seismic activity levels.  
A detrimental interaction arises between inferior mortar and low-quality masonry components, such partly burned bricks, rubble stone, or uneven blocks. The usual limitations in mortar strength and adhesion exacerbate the underlying flaws in the pieces, resulting in monolithic action just in name. In instances when mortar is inadequate to cover substantial voids, the frictional or cohesive adhesion is negligible. Consequently, any isolated fissures in the mortar, initiated by minor lateral displacement, rapidly propagate to the adjacent units. Reconnaissance from the regional earthquakes reveals that structures constructed with random-rubble stone and inferior mortar are susceptible to out-of-plane bulging and rapid corner separations. Furthermore, in structures using HCTs, insufficient or partial mortar coverage leads to premature failure of tile webs, causing them to separate from the mortar matrix under horizontal shear forces. Typical failure modes due to poor-quality mortars can be categorized as bed-joint sliding, diagonal cracks focusing on joints, and out-of-plane wall ejection. Field observations concerning this issue are shown in Figure 9.