Study reveals partitioned deformation across Pakistan’s eastern Sulaiman Range

Earthquake faulting in Pakistan’s eastern Sulaiman Range is divided into two thrust-dominated zones separated by a central belt of left-lateral strike-slip earthquakes, according to a regional study that combined 24 focal mechanism solutions, stress inversion and six years of local seismic records. The researchers found that the maximum horizontal compressive stress, SHmax, is broadly oriented northwest–southeast in all three zones. This pattern is consistent with ongoing oblique convergence between the Indian and Eurasian plates, but the different faulting styles indicate that regional deformation is partitioned among separate structures rather than accommodated by a single fault system.

The study focused on the eastern Sulaiman fold-and-thrust belt, an active part of the western Indo-Eurasian collision zone where crustal shortening and strain accumulation have produced major earthquakes. The western part of the range is better known because it experienced the magnitude 7.1 Quetta earthquake in 1997 and events of magnitude 6 or greater at Ziarat in 2008 and Harnai in 2021. The eastern segment has received less attention, even though historical records describe several magnitude 6 or larger earthquakes there. No instrumental event larger than magnitude 6.0 had been recorded in the eastern Sulaiman Range since 1976, according to the study, but that absence does not imply inactivity.

The authors analyzed standardized focal mechanism solutions from the Harvard Global Centroid Moment Tensor catalog covering 1981–2024. These solutions describe the style and orientation of fault movement during earthquakes and provide the principal inputs for stress-tensor inversion. The study divided the region qualitatively into eastern, central and western seismotectonic zones, a working framework based on the distribution of events, mapped structures and geological setting rather than a formal statistical classification. Six events fell in the eastern zone, nine in the central zone and eight in the western zone, with one normal-faulting event later excluded from stress inversion.

Thrust mechanisms were concentrated in the eastern and western zones. In the east, six earthquakes were predominantly reverse-faulting events associated with structures including the frontal Sulaiman fold-and-thrust belt and the north–south, eastward-verging Zindapir anticline. The selected fault planes generally had relatively low to moderate dips and a dominant north-northeast to south-southwest strike. Stress inversion gave an SHmax orientation of approximately 118 degrees with an uncertainty of 25 degrees for this zone. By comparison, eight thrust events in the west, spatially associated with the mapped Pir Koh and Karmari thrust fault systems, yielded an SHmax of about 132 degrees with an uncertainty of 24 degrees, broadly northwest–southeast to north-northwest–south-southeast.

The nine central events all recorded strike-slip movement, and their fault planes generally struck close to north–south. Their spatial distribution and orientation correspond broadly with the mapped Kingri fault system, previously identified as a left-lateral structure with evidence of Holocene activity. Inversion of these mechanisms produced an SHmax of approximately 142 degrees with an uncertainty of 12 degrees. Although the eastern, central and western estimates differ somewhat, the combined pattern shows a coherent first-order northwest–southeast compression. The study’s authors cautioned that focal mechanisms alone cannot establish the subsurface geometry, continuity or direct connection of an earthquake fault with a named mapped fault.

Stress inversion was performed with Win-Tensor software through an initial Right Dihedron screening step and a subsequent Rotational Optimization stage. The procedure tested the two nodal planes of each focal mechanism, progressively selected the plane with the lower misfit and iterated until the stress tensor stabilized. Events with angular misfit greater than 60 degrees were rejected. Solutions with uncertainty of 15 degrees were assigned the highest quality class, while progressively larger uncertainties up to 40 degrees remained eligible for interpretation. The method assumes a homogeneous stress field within each zone, an assumption that may be imperfect in a structurally complex fold-and-thrust belt. Nodal-plane ambiguity and the limited number of focal mechanisms further constrain the resolution of the stress estimates.

The study also examined earthquake frequency and magnitude in a homogenized catalog assembled from Pakistan’s Centre for Earthquake Studies and Pakistan Meteorological Department records for 2020–2025. Magnitude of completeness was calculated separately for each zone using the maximum-curvature technique and was estimated as 2.3 throughout. After applying that threshold, the catalog contained 300 earthquakes in the central and western zones and 105 in the eastern zone. Their Gutenberg–Richter b-values were 0.73 plus or minus 0.03 in the west, 1.05 plus or minus 0.08 in the central strike-slip zone and 0.81 plus or minus 0.05 in the east. The higher central value means that its catalog contains a greater proportion of smaller earthquakes relative to larger events than the thrust-dominated catalogs do.

That contrast does not by itself establish the cause or future earthquake behavior of the region. A higher b-value is generally associated with a larger proportion of low-magnitude events and can be compatible with greater structural heterogeneity, denser fracturing or lower differential stress, while lower values can be associated with relatively greater stress accumulation. The measured pattern is consistent with reports that strike-slip settings tend to have higher b-values than compressional thrust settings. The authors noted that b-value variations may also reflect compositional and structural properties, loading rate, tectonic regime, depth, acoustic emissions from rocks, stress conditions, thermal gradients, crack density and material heterogeneity. Their interpretation therefore links greater heterogeneity and distributed fracturing to the strike-slip domain without claiming that the b-values alone predict a major earthquake.

Cumulative event counts showed broadly similar growth in all three zones during 2020 and 2021, followed by faster accumulation in the central zone through 2022 and a clustered sequence in the western zone. The central zone then remained comparatively steady for about six months before accelerating again toward 2023, while the west experienced a year-long lull from 2023 to 2024. Magnitude 5.0 events occurred in the eastern zone in 2024 and the central zone in 2025 but did not produce a comparably large increase in total event counts. The authors suggested that coseismic stress release in the strike-slip zone may have unclamped thrust faults optimally oriented to the regional stress, although they did not establish a causal triggering relationship.

The spatial pattern is broadly compatible with a preliminary Riedel shear model associated with the regional left-lateral motion of the Chaman Fault zone. In this conceptual framework, the Kingri Fault could be compatible with a synthetic R-shear, while the right-lateral Harnai Fault could act as an antithetic R-prime shear. Such an arrangement could allow strike-slip and thrust deformation to operate together in a transpressional system. The authors emphasized that this remains a working hypothesis: similar fault orientations can arise from other strain-partitioning mechanisms, and the focal mechanism dataset cannot uniquely determine whether the named faults form a single Riedel system or resolve their subsurface connections. The occurrence of a single normal-faulting event in the east may also fit localized extension in a releasing or transtensional segment, but one event cannot confirm that mechanism, and alternative explanations cannot be ruled out.

The study proposes that the relatively rigid Katawaz block acts as a tectonic buttress that obstructs northward motion of the Indian Plate and deflects strain around the eastern Sulaiman belt. Frontal and lateral ramps in this model could accommodate shortening, uplift and displacement transfer along the block’s margins, while the Kingri and Harnai faults accommodate part of the lateral shear. This arrangement may help explain the belt’s curved geometry, en-echelon structural patterns and abrupt changes between thrusting and strike-slip faulting. The study used 24 focal mechanisms to estimate stress orientation, faulting style and spatial patterns, while the 2020–2025 catalog was used to compare seismicity and magnitude-frequency behavior. Detailed fault mapping, paleoseismic evidence and geodetic strain measurements would be required to test the proposed geometry and determine how robustly the limited focal mechanisms represent the region’s full present-day stress field.

Subject of Research: Seismotectonic stress, faulting styles and earthquake magnitude-frequency patterns in Pakistan’s eastern Sulaiman fold-and-thrust belt

Article Title: Seismotectonics and structural dynamics of the eastern Sulaiman fold-and-thrust belt, Pakistan

Article References: Tahir, M., Shaukat, A. Z., Tahir, S., Saif, B., & Iqbal, T. (2026). Seismotectonics and structural dynamics of the eastern Sulaiman fold-and-thrust belt, Pakistan. Discover Geoscience, 4(1), Article 410. https://doi.org/10.1007/s44288-026-00780-3

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00780-3

Keywords: eastern Sulaiman Range, Pakistan tectonics, focal mechanisms, stress inversion, thrust faulting, strike-slip faulting, strain partitioning, Katawaz block, Riedel shear, Gutenberg–Richter b-value, seismicity, Seismotectonics