## 4 Discussion  
Investigations into the spatial distribution of faults and fissures are crucial for understanding the mechanisms of the underlying fault growth, and, more commonly, brittle failure behaviors [37; 38]. Linkage and forward propagation models can be used for the interpretation of field observations. The process by which fault segments coalesce and how this coalescence results in changes to the fault architecture has been well documented in a  
Figure 14: Correlative diagram between vertical fissure throws and fissure lengths. The horizontal and vertical axes are plotted on a logarithmic scale (L, T), and the equation and correlation coefficients are displayed.  
Figure 13: Three field examples of the fissures with the vertical throw in the study area (**a–c**).  
variety of tectonic settings. Additionally, numerous studies and measurements have been carried out to verify the remote data.  
Earth fissures are often connected by deep faulting, and their structural and active characteristics are compatible with the underlying active faults [39]. Some of the surface observed fissures can be directly formed by strong seismic vibrations [40]. Furthermore, coseismic earth fissures caused by historical earthquakes may release deformation energy in the form of fault creep or new seismic vibrations and re-emerge at the surface [41]. Some research indicates that earth fissures can appear on the hanging wall of normal faults. Because of these earth fissures, the thickness of sediment differs on both sides of the fissures [39]. In particular, there is evidence to show the occurrence of the synsedimentary fissures in the Fenwei Graben System [42; 43]. In some studies, the tectonic development of monoclines has been investigated in terms of the hanging-wall growth of faults according to field observation and numerical modeling [44].  
Our model of Birjand fissures agrees with their propagation fault model. As a result, the oblique displacement was used to explain the presence of folds in the cover sequence where fissures exist (Figure 10). The findings of this study indicate that fissures are related to the fault-propagation folds in the NW of Birjand (Figure 15), where North Birjand, a fault system with an EW trend, is located (Figure 15). This fault, with reverse and left-lateral strike-slip movements, is considered one of the terminal segments of the Nehbandan fault with a similar sigmoidal trend.  
The spatial distribution of faults and fissures, as well as changes in fault throws, makes it possible to study linkage and forward propagation models in both vertical and horizontal directions in the NW of Birjand. These observations feature the process of the fault segments' coalescence and changes in the fault architecture. As mentioned, one of the models of fissure structures resulting from compression and lateral extension (Figure 3d) is more in line with the gentler slopes in the transit and transpressional zones [45].  
This geohazard feature (the fissures), which has the potential to impact the environment by causing significant damage to civil foundations such as roads, dams, buildings,  
Figure 15: Geological map and model of fissure structure in the study area. The model is related to the fault-propagation folds. NB. F. stands for North Birjand Fault.  
and underground pipes, needs to be monitored by interferometric synthetic aperture radar (InSAR) and global positioning system (GPS). Civil foundations must be constructed at a farther distance from the fissure area.