Following the preparation of fault-zonation maps for the city of Tehran, the first revision of the Tehran surface faulting hazard regulations was issued in 2020. In these regulations, hazard level is defined on the basis of empirical relationships as a function of fault length, such that faults shorter than 10 km are assigned a low hazard level. By presenting examples of severe deformation associated with short faults within Tehran, most of which are less than 3 km in length, the present study highlights the potential surface-faulting hazard posed by some of these short faults. Field surveys, trench investigations, and aerial photograph analysis were employed to examine the characteristics of these faults.

According to internationally recognized codes and guidelines, faults are classified into three categories based on the nature of deformation within the fault zone: Well-defined, Distributed, and faults with uncertain deformation characteristics. Faults exhibiting Well-defined deformation commonly generate severe ground deformation and are therefore associated with higher levels of hazard.

Our findings indicate that zones of intense deformation along short faults are not controlled by faulting mechanism and may develop under a range of kinematic settings. The focal mechanisms of microearthquakes and small-magnitude earthquakes in the Alborz region, together with regional geological investigations, indicate that compression and strike-slip faulting are the dominant deformation styles among the active faults of the study area. Accordingly, short normal faults and short right-lateral strike-slip faults cannot be interpreted as segments of a major seismogenic fault; rather, they should be regarded as subsidiary faults associated with a principal fault. In the regulations developed for the enforcement of construction controls within Tehran’s fault zones, the probability of surface rupture hazard is defined solely as a function of fault length. Under this framework, even short faults with high potential for surface rupture are classified within the “very low” hazard category. In practice, such faults are excluded from the regulatory framework; consequently, all buildings and urban infrastructure located on or across them remain without any specific technical measures for hazard mitigation.

This study further demonstrates that, although fault rupture and displacement are generally correlated with fault length in earthquake-generating faults, subsidiary faults, particularly secondary faults, do not necessarily conform to this relationship because of their distinct kinematic characteristics. Despite their limited length and irrespective of conventional length-displacement scaling relationships, such faults may generate zones of intense deformation with displacements exceeding 1 m and, in some cases, reaching several meters. Therefore, given that the current regulations for Tehran define the probability of surface faulting hazard exclusively as a function of fault length, with hazard level decreasing as fault length diminishes, these provisions require reconsideration in future revisions.

In this study, ArcGIS software was used for the preparation of base maps, digitization of faults and alluvial deposits, georeferencing of aerial photographs, and generation of photomosaics. In addition, graphical interpretations, figures, and maps were produced using Surfer, while FaultKin was employed for the kinematic analysis of fault planes, fractures, and bedding surfaces.