Tehran on the Brink of a Major Earthquake: Is the Mosha Fault the Key to Disaster?

According to the Society Desk of Saed News, quoting Rokna:
Dr. Mehdi Zare, professor of geology, warns in a note that although the probability of such an earthquake is low, its deadly scenario—with over one million casualties and hundreds of billions of dollars in damage—should not be ignored.

Dr. Mehdi Zare, full professor of geology, writes:

The February 6, 2023 earthquake in Turkey, with a magnitude of 7.8, ruptured about 220 kilometers of the Eastern Anatolian Fault in a single event, causing catastrophic damage. But is a similar rupture possible along the Mosha Fault in the Alborz Mountains?

The Eastern Anatolian Fault, with a slip rate of 5 to 10 millimeters per year, is longer and more active than the Mosha Fault. These characteristics increase the likelihood of earthquakes above magnitude 7.5. In contrast, the Mosha Fault has a lower slip rate (around 2 to 3 millimeters per year) and is more segmented, which reduces the probability of a full rupture.

The Alborz mountain range near Tehran is a highly seismically active region. The main concern about a major earthquake in this area stems from the dense population and the extreme vulnerability of Tehran’s urban infrastructure. Based on historical data and tectonic studies, the potential magnitude of an earthquake in this region could range from 7.5 to 7.8.

According to this data, the Mosha Fault (with the capacity to generate earthquakes between magnitude 7.5 and 7.8) and the North Tehran Fault (with magnitudes between 7.2 and 7.5) are considered the most dangerous faults around the capital. In the worst-case scenario, a major earthquake on these faults could result in 500,000 to over one million deaths and cause economic losses of $150 to $250 billion.

Despite its low slip rate, stress accumulation along fault systems in the Alborz over the long term leads to large but infrequent earthquakes. The Mosha Fault, as a major active system in the Alborz, has significant seismic potential. Its slip rate during the late Quaternary is estimated to be around 2 to 4 millimeters per year, and based on geomorphic studies—including the displacement of rivers and alluvial fans—about 2 to 3 millimeters per year.

The Mosha Fault is divided into three major segments, each with different seismic characteristics and slip rates:

• Western segment: Extends from Kalan village to north of Abyek. This section shows fewer geomorphic changes and weaker historical seismicity. However, based on geometric features, it can produce earthquakes with magnitudes between 6.5 and 7.5.

• Central segment: Runs from Kalan village to the southern slopes of Mount Damavand. This part displays more prominent geomorphic signs such as river and terrace displacement. It is believed that the 1830 Damavand-Shemiranat earthquake occurred in this segment.

• Eastern segment: Stretches from Damavand toward southern Firuzkuh and features a distinct left-lateral strike-slip mechanism.

In terms of location, the central and western segments are closer to Tehran. The entire Mosha Fault system spans about 200 kilometers across the southern Alborz. Based on various scenarios:

• Single-segment rupture (e.g., only the central part) could generate an earthquake with a magnitude of 7.0 to 7.3.

• Multi-segment rupture (e.g., central and eastern parts) could result in a magnitude 7.4 to 7.6 earthquake.

• Complete and simultaneous rupture of all segments, though very rare, could cause an earthquake of magnitude 7.8.

However, the Mosha Fault's complex geometry—including bends and rupture angles—may prevent a full rupture from propagating. Additionally, variations in slip rates across different fault segments make simultaneous and continuous rupture of the entire system highly unlikely.

Finally, it is important to note that the Mosha Fault connects to the North Tehran Fault at Kalan village in Lavasanat. This connection adds further complexity to potential seismic scenarios in the region.