On February 6, 2023, two devastating earthquakes rocked the Eastern Anatolian Fault system, unleashing seismic fury with moment magnitudes of 7.8 and 7.6. These events inflicted massive casualties and widespread destruction across regions of Turkey and Syria, bringing global attention to the tectonic complexities underlying the fault system. While the earthquake magnitudes alone were catastrophic, it is the peculiar mechanics of the rupture propagation, particularly during the M 7.8 event, that have become a focal point for geophysical research. This seismic rupture exhibited a striking bilateral nature, rupturing segments of the fault in opposite directions. However, the rupture velocities diverged dramatically: northeastwards, partial supershear rupture occurred, traveling faster than the shear wave speeds of the surrounding rock, while southwestwards, rupture proceeded at subshear speeds. The puzzle of these differing rupture speeds has long confounded seismologists, sparking queries into the underlying causes.
Recent advances in seismic tomographic imaging have now begun to unravel this mystery by illuminating the structural and stress heterogeneities along the fault system. Tomography, which leverages the travel times of seismic waves to resolve subsurface velocity variations, reveals critical variations in the Earth’s crust that appear to govern these rupture behaviors. In particular, researchers have identified a low-velocity anomaly beneath the southwestern Amanos–Pazarcık segment of the fault. This anomaly is indicative of areas where seismic waves slow down, typically due to the presence of fluids or fractured rock. Accompanying this low-velocity zone is a fault-parallel fast velocity direction measured from anisotropic wave propagation, hinting at the infiltration of fluids aligned with the fault fabric. Such fluid presence has profound implications for fault mechanics, potentially facilitating fault creep — a slow, aseismic mode of deformation that reduces the overall stress build-up on the fault plane.
In stark contrast, the northeastern Erkenek segment of the rupture zone manifests as a high-velocity anomaly accompanied by a fast velocity direction oriented perpendicular to the fault. This suggests a structurally distinct regime with limited fluid percolation, promoting the mechanical integrity of the fault rocks and allowing stress to accumulate more effectively over time. This elevated normal stress state in Erkenek is hypothesized to prime the fault for dynamic rupture at supershear velocities, where rupture propagates faster than the seismic shear waves, a phenomenon associated with intense ground shaking and potentially more destructive impacts. The stark contrast between these two fault segments suggests that fault structure and fluid presence shape the spatial distribution of stress accumulation, thus influencing rupture characteristics.
The existence of supershear ruptures has long intrigued seismologists due to their implications for earthquake energy release and ground motion patterns. Supershear rupture speeds exceed the shear wave velocity within the crust, creating shockwave-like effects that can amplify seismic shaking beyond typical expectations. Determining why such high rupture speeds manifest in one segment of a fault but not another is essential for improving seismic hazard models and forecasting. The findings from the Eastern Anatolian Fault system highlight the crucial interplay between geological heterogeneity and fault mechanics, demonstrating that not all fault segments are created equal in terms of their seismic behavior.
Fluid infiltration into fault zones has emerged as a fundamental factor influencing earthquake dynamics. Fluids can decrease effective normal stress by increasing pore pressure within fault rocks, potentially weakening the fault and promoting slip. The low-velocity anomaly and fault-parallel anisotropy measured in the southwest suggest active fluid processes that lubricate the fault, encouraging stable slip or creeping behavior rather than sudden, high-speed rupture. This scenario corresponds with the observed slower, subshear rupture velocities in the Amanos–Pazarcık segment. Conversely, in the northeast Erkenek segment, the absence of such infiltrating fluids allows the fault to sustain higher normal stresses, which can suddenly release during an earthquake and drive supershear rupture fronts.
Furthermore, the tomographic data not only reveal velocity anomalies but also provide insight into the directional properties of seismic wave propagation, known as anisotropy. Anisotropy measurements here reveal that fast velocity directions align differently relative to the fault strike in the two regions, reflecting structural variabilities in the fault zone fabric and stress regimes. In the southwest, fault-parallel fast directions imply aligned fractures or fluid pathways, facilitating easy slip. The fault-normal fast directions in the northeast indicate a more intact and less fractured fault zone, harboring stress that can intensify rupture propagation speeds.
These structural and stress contrasts have vital implications for earthquake rupture models, which must account not only for regional tectonic forces but also for local geological conditions that modulate stress gradients and rupture dynamics. The observed rupture speed dichotomy in the 2023 Kahramanmaraş earthquake underscores the necessity of integrating seismic tomography with geodynamic modeling to capture the complex fault behavior. Such integrated approaches enable a refined understanding of how fault segments vary in their failure modes, and help predict the potential for supershear ruptures and their associated hazards.
The findings also stimulate new discussions on the role of stress loading versus fault zone structure in controlling the rupture process. Classical models often treat stress accumulation as a simple function of tectonic plate motion rates, but the Eastern Anatolian data demonstrate that mesoscale geological heterogeneities and fluid distributions can significantly alter this picture. Fault creep facilitated by fluid infiltration can reduce the stress loading rate on one segment, inhibiting extreme rupture speeds, whereas elevated normal stresses in less permeable zones create conditions ripe for energetic, supershear seismic ruptures.
Understanding the behavior of the Eastern Anatolian Fault is especially urgent given its position as a major tectonic boundary accommodating complex plate interactions between the Arabian, African, and Eurasian plates. The 2023 earthquakes exemplify the potential for large, damaging ruptures along this fault; however, the new insights into rupture speed variability suggest that earthquake hazard assessments depend critically on localized fault zone properties. Incorporating tomographic imaging and anisotropy analysis into routine seismic monitoring may greatly enhance the ability of scientists to anticipate which fault segments may rupture at high speeds and pose amplified shaking risks.
The advanced tomographic imaging techniques used in this study represent a leap forward in resolving subsurface fault zone properties. By deploying dense seismic networks and utilising waveform inversion methods, researchers can now map velocity variations and anisotropic structures with unprecedented resolution. This technological progress allows the identification of subtle geophysical signals of fluid presence and stress regime differences that were previously inaccessible, providing a direct link between subsurface fault properties and surface earthquake expressions.
Moreover, this research opens new avenues for exploring the interplay of fluids, stress, and rupture dynamics along other major fault systems worldwide. Comparing velocity anomalies and anisotropy patterns across different tectonic settings could clarify whether the behavior observed in the Eastern Anatolian Fault is a common mechanism influencing rupture speed heterogeneity globally. Such comparative studies may lead to generalized models of fault strength variability and more accurate seismic hazard predictions on a global scale.
In summary, the 2023 Mw 7.8 Kahramanmaraş earthquake rupture reveal a complex story of fault zone heterogeneity controlling rupture speeds. The contrast between a fluid-irrigated southwest segment with low-velocity and fault-parallel anisotropy facilitating slower rupture, and a northeast segment with high velocities and fault-normal anisotropy primed for high stress and supershear rupture, highlights the deep links between crustal structure and seismic behavior. These findings represent a paradigm shift in understanding earthquake rupture physics, emphasizing the necessity for high-resolution, multidisciplinary investigations of fault zones to unravel the intricate controls on earthquake dynamics and ultimately improve societal resilience against destructive earthquakes.
Subject of Research: Earthquake rupture dynamics and fault zone structure in the Eastern Anatolian Fault system during the 2023 Mw 7.8 Kahramanmaraş earthquake
Article Title: High normal stress promoted supershear rupture during the 2023 Mw 7.8 Kahramanmaraş earthquake
Article References:
Chen, J., Xu, M., Bai, Y. et al. High normal stress promoted supershear rupture during the 2023 Mw 7.8 Kahramanmaraş earthquake. Nat. Geosci. (2026). https://doi.org/10.1038/s41561-025-01893-z
DOI: https://doi.org/10.1038/s41561-025-01893-z
