📄 earth_science

Three-Dimensional P- and S-Wave Local Earthquake Tomography of the Eastern Alborz, Northern Iran

This study utilizes 3D P- and S-wave local earthquake tomography to reveal pronounced crustal heterogeneity and concealed fault structures in the eastern Alborz, demonstrating that active deformation is localized within mechanically heterogeneous domains and providing a refined geophysical framework for seismic hazard assessment in the Arabia–Eurasia collision zone.

Original authors: MohammadReza Sepahvand, Majid Nemati, Afsaneh Nasrabadi, Zahra Razazi

Published 2026-08-01

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: MohammadReza Sepahvand, Majid Nemati, Afsaneh Nasrabadi, Zahra Razazi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth's crust not as a solid, unbroken rock shell, but as a giant, jumbled jigsaw puzzle made of different materials. Some pieces are hard and dense, like a block of granite; others are cracked, hot, or soaked with fluids, making them softer and slower to transmit sound. This is the world of seismology, the science of listening to the Earth. When an earthquake happens, it sends out ripples of energy—like a stone dropped in a pond, but traveling through rock. These ripples come in two main flavors: P-waves (the fast, "push-pull" waves that arrive first) and S-waves (the slower, "side-to-side" waves that arrive later). By measuring how long these waves take to travel from the earthquake's source to listening stations on the surface, scientists can work backward to figure out what the rock looks like deep underground. It's a bit like a doctor using an ultrasound to see inside a body, except the "patient" is a mountain range, the "ultrasound" is a natural earthquake, and the "doctors" are trying to map hidden faults that could cause future disasters. Understanding this hidden structure is crucial because it tells us where the ground is weak, where stress builds up, and exactly where the next big shake might happen.

Now, picture the Eastern Alborz mountains in northern Iran. This is a tectonic hotspot where the Arabian and Eurasian plates are crashing together, crumpling the crust like a rug being pushed against a wall. While scientists knew this area was active, the deep, three-dimensional map of its insides was fuzzy. Enter MohammadReza Sepahvand and his team, who decided to take a closer look. They acted like cosmic detectives, gathering data from 2,048 local earthquakes recorded between 2007 and 2008 by a network of 32 seismic stations. Using a sophisticated computer algorithm called LOTOS, they didn't just look at where the earthquakes happened; they simultaneously calculated the speed of the P- and S-waves traveling through the crust to build a 3D model of the underground velocity.

What they found was a crust that is far from uniform. Instead of a smooth, solid layer, the Eastern Alborz is a patchwork of "fast" and "slow" zones. The team discovered extensive low-velocity zones—areas where the waves travel sluggishly—that line up perfectly with the major active fault systems and clusters of earthquakes. Think of these slow zones as the "soft spots" in the crust, where the rock is fractured, hot, or filled with fluids, making it easier for the ground to break and slip. These soft spots are where the Earth is actively deforming.

Perhaps the most exciting discovery was finding two "ghost" structures in the southeastern part of the study area. The team spotted two long, continuous lines of low-velocity rock (which they named F1 and F2) that didn't match any known faults on the surface map. It's as if they found two hidden rivers flowing underground that no one knew existed. These suggest there are concealed fault-related structures buried beneath the transition between the mountains and the plains, waiting to be investigated.

The team also took a closer look at the Damavand volcanic center. Beneath the volcano, they found a persistent low-velocity anomaly accompanied by high ratios of P-wave to S-wave speeds. This combination suggests the rock there is physically different from the surrounding crust—likely hotter, more fractured, or filled with fluids, which fits the picture of an active volcanic system.

The researchers are careful to note that their map is most reliable in the upper and middle crust, where they had plenty of earthquake data to work with. As they go deeper or toward the edges of their study area, the picture gets a bit fuzzier, much like trying to see the bottom of a deep pool when the water gets murky. However, within the clear zones, the evidence is strong: the crust is highly segmented, and the active deformation is happening right where the rock is mechanically weak and heterogeneous. This study doesn't just confirm what we suspected; it reveals hidden fault lines and provides a sharper, more detailed geophysical framework for understanding how this dangerous, beautiful part of the world is moving.

Technical Summary: Three-Dimensional P- and S-Wave Local Earthquake Tomography of the Eastern Alborz, Northern Iran

Problem Statement
The eastern Alborz region in northern Iran is a tectonically active sector of the Arabia–Eurasia collision zone, characterized by significant crustal shortening, active faulting, and the presence of the Quaternary Damavand volcanic system. Despite its seismic hazard potential, the detailed three-dimensional crustal structure of this region remains poorly constrained. Previous investigations, including regional tomography and receiver-function studies, have successfully resolved first-order lithospheric features but lack the spatial resolution necessary to image fault-scale heterogeneity. Consequently, the three-dimensional geometry, continuity, and depth extent of crustal velocity anomalies associated with the region's closely spaced active fault systems are not well understood. This study addresses the need for high-resolution local imaging to better constrain crustal segmentation and the relationship between seismic velocity heterogeneity and active deformation.

Methodology
The study employs local earthquake tomography (LET) to simultaneously invert for seismic velocity structure ( $V_p$ , $V_s$ ) and earthquake hypocenters.

Data: The analysis utilizes P- and S-wave travel-time data recorded between 2007 and 2008 by a combination of temporary seismic networks (deployed by the Geological Survey of Iran) and permanent stations (Institute of Geophysics, University of Tehran). Following rigorous quality control, 2,048 local earthquakes recorded by 32 stations were selected.
Inversion Algorithm: The LOTOS-12 Release tomography package was used for joint inversion. An optimal one-dimensional reference model was established through iterative minimization of travel-time residuals, incorporating a velocity interface at 35 km depth.
Resolution and Parameters: Model stability and resolution were assessed using 60 synthetic checkerboard tests with $\pm10\%$ velocity perturbations. Optimal inversion parameters (damping, smoothing, and weighting) were selected to balance data fit, model smoothness, and anomaly recovery. Ray paths were calculated using a bending ray-tracing algorithm on an irregular grid, with the final model derived by averaging solutions from rotated grids to reduce orientation effects.
Scope: Interpretations are restricted to the well-resolved portions of the crust, primarily the upper and middle crust (depths of 5–25 km), where ray-path coverage is densest.

Key Results
The resulting $V_p$ , $V_s$ , and $V_p/V_s$ models reveal pronounced lateral heterogeneity and crustal segmentation throughout the eastern Alborz:

Velocity Anomalies and Faults: Extensive low-velocity anomalies in both $V_p$ and $V_s$ models form elongated NE–SW trends parallel to the regional tectonic grain. These anomalies spatially coincide with principal active fault systems (North Alborz, Astaneh, and North Semnan faults) and clusters of relocated seismicity.
Concealed Structures: The study identifies two previously unrecognized, coherent low-velocity lineaments (F1 and F2) in the southeastern part of the study area near the mountain–plain transition zone. These features persist through multiple depth slices but do not fully align with previously mapped surface faults, suggesting the presence of concealed fault-related structures.
Damavand Volcanic Center: A persistent low-velocity anomaly is imaged beneath the Damavand volcanic center, extending to at least 25 km depth. This anomaly is associated with relatively elevated $V_p/V_s$ values, indicating significant physical heterogeneity (potentially thermal, compositional, or fluid-related) distinct from the surrounding crust.
$V_p/V_s$ Distribution: Elevated $V_p/Vs$ ratios consistently correlate with low-velocity regions and high seismicity, while high-velocity domains generally exhibit lower ratios. This suggests variations in crustal physical properties, such as fracture density, lithology, and fluid content.

Significance and Claims
The authors assert that this study provides the first high-resolution three-dimensional images of the crustal $V_p$ , $V_s$ , and $V_p/V_s$ structure in the eastern Alborz derived from local earthquake data. The primary contributions are:

Improved Resolution: The study resolves smaller-scale structural features and fault-scale heterogeneity that were not detectable in previous regional models.
Structural Constraints: The results offer new constraints on crustal segmentation and the localization of active deformation within mechanically heterogeneous crustal domains.
Discovery of Hidden Structures: The identification of the F1 and F2 lineaments suggests the existence of concealed fault-related structures that warrant further geological and geophysical investigation.
Geophysical Framework: The models establish an improved geophysical framework for understanding active tectonics and assessing seismic hazard within the northern sector of the Arabia–Eurasia collision zone.

The paper concludes that deformation in the eastern Alborz is preferentially localized within laterally heterogeneous crustal domains associated with major active fault systems, and that the observed velocity variations provide a basis for future multidisciplinary investigations into the region's seismotectonic evolution.

More like this