M5.1 Earthquake near Ardestan, Central Iran, and Potential Link to Southern Seismic Activity

1. Tectonic Context of Iran

Iran is one of the world's most seismically active regions, situated on the complex Alpine-Himalayan seismic belt. This seismicity is primarily driven by the oblique convergence of the Arabian Plate and the Eurasian Plate. The Arabian Plate is moving north-northeastward relative to Eurasia at a rate of approximately 22 to 25 mm/yr near the Persian Gulf. This large-scale compression is accommodated by a combination of thrusting (crustal shortening) and strike-slip faulting (side-to-side motion) across several major geological provinces:

  • Zagros Fold-and-Thrust Belt: Located in the southwest, accommodating most of the plate convergence through reverse faulting (thrusting) and some strike-slip movement (e.g., the Kazerun Fault and the Main Recent Fault).
  • Central Iran Block: A relatively rigid microplate where deformation is slower but is accommodated by widely distributed, often active strike-slip faults within a transpressional regime.
  • Makran Subduction Zone: In the southeast (Hormozgan/Sistan-Baluchestan) where the oceanic crust of the Gulf of Oman subducts beneath the Eurasian Plate.

2. Seismogenic Structures near Ardestan (Central Iran)

The M5.1 event near Ardestan falls within the Central Iran region. Earthquakes here are typically associated with prominent, long-lived, and predominantly strike-slip fault systems that partition the strain of the Arabia-Eurasia collision.

  • Key Faults in the Ardestan Region: The area around Ardestan is intersected by major active strike-slip features. One significant structure is the Southern Ardestan Fault (SAF), which often exhibits dextral (right-lateral) transpressional kinematics. Other important features may include segments of the larger Na'in-Baft Fault System or localized thrust faults resulting from regional crustal shortening. The M5.1 is likely a result of stress release on one of these known, or perhaps a previously unmapped, local NNW-SSE or N-S trending fault segments that accommodate the northwestward escape of the Central Iran block.
  • Focal Mechanism and Seismic Parameters: The precise focal mechanism (e.g., strike, dip, rake) is crucial for identifying the causative fault. Given the regional tectonics, a strike-slip solution (either dextral or sinistral depending on the fault's orientation) is the most probable for a moderate earthquake in this part of Central Iran. The earthquake likely occurred at a relatively shallow depth (<15 km), which is typical for strike-slip events and is a parameter that increases the potential for surface shaking intensity.

3. Hypothesis of Seismic Activity Migration from Hormozgan

The hypothesis suggests a stress-triggered migration of seismic activity from the high-stress region of Hormozgan Province (Southern Iran) northward toward the Central Iran Block, culminating in the Ardestan earthquake.

3.1. High-Stress Origin in Hormozgan

Hormozgan Province lies at the junction of the Zagros Fold-and-Thrust Belt and the Makran Subduction Zone, making it highly seismically active.

  • Primary Fault Systems: Major active faults in this area include thrust and reverse faults associated with the Zagros, as well as significant strike-slip faults such as the N-S trending Minab Fault System and the Zendan-Minab-Pishgaman Fault (ZMP) which are critical structures accommodating the transition between the two major tectonic regimes. The ZMP is known for both historical and recent large earthquakes (M>6.0).
  • Seismic History: Hormozgan has experienced numerous significant earthquakes (e.g., a series of M>5.5 events in 2021-2022), indicating a sustained, medium to high-risk pressure regime and ongoing strain accumulation, as noted in the prompt.

3.2. Proposed Migration Mechanism

The proposed migration involves a Coulomb stress transfer mechanism:

  1. Initial Stress Accumulation: High strain accumulation on major faults in Northern Hormozgan (e.g., the northern terminations of the Minab/ZMP system) brings those faults closer to failure.
  2. Earthquakes in Hormozgan: Recent or ongoing seismic sequences (micro-seismicity or moderate M events) in Northern Hormozgan act as a stress shadow on the segment that ruptured, but they simultaneously increase the Coulomb Failure Stress (CFS) on adjacent, more distant faults further north along the Central Iran Tectonic Domain.
  3. Stress Triggering and Migration: The change in stress (ΔCFS) from the southern activity may have incrementally pushed a critical, already highly-loaded fault segment near Ardestan over its failure threshold. This kind of triggered activity can appear as an apparent northward migration of seismicity along the N-S oriented tectonic grain that connects the Zagros/Makran front to the Central Iran Block. This northward path is often defined by major structures like the Karezak-Dehshir Fault Zone or other strike-slip lineaments that transfer the convergence component inland.

4. Outlook on Medium-Risk Pressure near Hormozgan

The occurrence of the Ardestan earthquake (Central Iran) does not necessarily signify a complete release of pressure in the south.

  • Persistent High Strain: The Arabian-Eurasian convergence rate ensures continued high strain accumulation in the southern Zagros and Hormozgan regions. The existing medium-risk pressure is a geological constant.
  • Unruptured Segments: Even with the northern event, significant unruptured segments of the Minab Fault System or other major structures near Bandar Abbas and Qeshm Island likely still hold substantial strain, maintaining the high seismic hazard.
  • Future Activity: Continued monitoring of the b-value (the slope of the magnitude-frequency distribution, indicating stress levels) and localized GPS strain accumulation in Hormozgan remains critical. The M5.1 in Ardestan serves as a reminder of the regional tectonic connectivity but does not preclude large, independent future earthquakes in the south.

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