Saed News: A professor at the International Institute of Seismology, in an interview with *Khabar Online*, stated: “Regions with low locking, like the Zagros Mountains, experience frequent small earthquakes but have less potential for large ones.
Saed News:
According to a report from Khabar Online, the concept of "fault locking" plays a key role in predicting earthquakes in Iran, as different regions show varying behaviors toward earthquakes depending on the extent of this locking. Fault locking or coupling describes how the two sides of a fault are tightly stuck together. When the plates are tightly coupled, they get locked and cannot easily slip past each other. Over time, stress and energy build up due to the friction between the plates, increasing until the stress exceeds the friction that holds them, causing the plates to suddenly slip and release energy as an earthquake. Some of this displacement happens gradually as creeping, where the plates slide past each other with minimal resistance, slowly releasing stress through seismic creep. This method of energy release reduces the likelihood of major earthquakes.
In this regard, Mehdi Zareh, a professor at the International Institute of Seismology and Earthquake Engineering, explained to Khabar Online, "Seismologists use fault locking analysis to assess earthquake risks. Locked fault regions are more prone to major earthquakes because the stored energy can be suddenly released. Monitoring how and to what extent faults are locked helps predict the location and timing of stress release."
Regions like Central Iran and Alborz are at risk of larger but less frequent earthquakes.
Zareh further explains through an analogy: "Imagine a door that is stuck and suddenly opens (similar to fault locking and coupling resulting in an earthquake); compared to a well-oiled door that opens and closes easily (like fault release through seismic creep without an earthquake). Locking determines whether movement along faults leads to sudden earthquakes or gradual harmless shifts."
Tectonic Pressures in Iran:
This seismologist highlighted the tectonic conditions in Iran and the influence of the Arabian and Eurasian plates: "In Iran, the continuous collision between the Arabian and Eurasian tectonic plates results in varying compression and shortening of the crust in different regions. The Arabian plate moves toward Iran at a speed of 20-25 millimeters per year, with 10 to 15 millimeters of this being absorbed by shortening within Iran. The remainder is consumed in strike-slip faults in eastern Iran."
About Iran's Faults:
Zareh describes the fault situation across the country: "In Iran, the Zagros Mountains are the most active region, where 5 to 10 millimeters of shortening occurs annually, and the Alborz Mountains experience about 4 to 8 millimeters of shortening per year. Central Iran experiences 5 to 20 millimeters of deformation annually, primarily at its borders."
In the Alborz Mountains, thrust faults like the Masha Fault and the Northern Alborz Fault experience 3-5 millimeters of strike-slip displacement annually. In the Koppeh-Dagh region in northeastern Iran, 5-7 millimeters of shortening occurs annually. In the Makran subduction zone in southeastern Iran, the Arabian plate subducts beneath the Iranian plate at a speed of 20-30 millimeters per year. In eastern Iran, the Sistan region shows 1-2 millimeters of shortening, along with 10-15 millimeters of right-lateral movement along the Nehbandan Fault annually.
Earthquake Variability in Different Regions of Iran:
The relationship between tectonic shortening and stress release, both seismically (through earthquakes) and aseismically (through creep, bending, and folding), differs across Iran. Zareh elaborates on the differences between high and low locking faults: "High locking faults tend to lock, building up stress that leads to larger earthquakes, while faults with low locking slide more easily and experience smaller, more frequent earthquakes."
Behavior of Faults in Iran:
Zagros: Frequent but small earthquakes
Zareh describes the fault behavior in the Zagros Mountains: "In the Zagros, with 5-10 millimeters of shortening per year, seismic locking is between 10-30% (low to moderate). Most shortening in the Zagros occurs through folding and creep and bending of sedimentary layers (like Hormuz salt)."
He emphasizes: "Therefore, in Zagros, earthquakes are frequent but small to moderate, usually with magnitudes less than seven. However, there was a significant earthquake in Zagros on November 21, 2017, in the Sarpol-e Zahab region, with a magnitude of 7.4, though this is rare."
Alborz and Central Iran: Larger earthquakes
Zareh mentions that in the Alborz Mountains, the shortening rate is 4-8 millimeters per year, and the seismic locking is between 30-50% (moderate to high). The Alborz crust is known for larger earthquakes. In the Koppeh-Dagh region, shortening occurs at a rate of 5-7 millimeters per year, with seismic locking at 20-40%, and a combination of compressional and strike-slip faulting with some seismic creep.
He adds: "In Central Iran, shortening is 1-3 millimeters per year, and seismic locking is 50-70% (high). Compressional faulting leads to large, less frequent earthquakes, such as the 1978 Tabas earthquake with a magnitude of 7.4, where almost all the accumulated stress was released during the event."
Makran: Rare but catastrophic earthquakes
Zareh explains the special conditions in the Makran subduction zone: "In the Makran subduction zone, the shortening rate is less than 1 millimeter per year due to subduction. Seismic locking is less than 10% (very low), and seismic creep occurs due to young sediments overlying the subduction zone. This region experiences rare but catastrophic earthquakes, such as the 1945 Makran earthquake and tsunami with a magnitude of 8.1, and the 2013 Gachsaran earthquake."
Eastern Iran: Strike-slip faults
In eastern Iran, the Sistan fault zone experiences a shortening rate of 1-2 millimeters per year, and seismic locking is between 10-20%. Zareh notes that strike-slip movement, such as the Nehbandan fault system, is dominant in this region.
Seismic Hazard Map of Iran:
Finally, Zareh concludes with a summary of fault locking and earthquake risks across Iran: "Overall, the total shortening across Iran is around 10-15 millimeters per year, with 10-30% of it released through earthquakes and the remaining 70-90% through creep, bending, or subduction. Low locking in regions like Zagros is due to deformable sedimentary layers (such as salt, shale, and gypsum), which increases folding and plasticity."
He concludes: "Regions with low locking (like Zagros) experience frequent small earthquakes but have lower potential for large ones. Regions with high locking (like Central Iran and Alborz) are at greater risk for larger, less frequent earthquakes."
