Holocene Paleoearthquake Records of the Issyk-Ata Fault near the Densely Populated Chui Basin: Evidence from the Dzhal Area, Kyrgyz Range, Tien Shan

The Issyk-Ata fault is a key active structure in northern Kyrgyzstan and represents the principal seismogenic source affecting the densely populated Chui basin, including Bishkek, the capital city of the Kyrgyz Republic. In 2025, a sequence of moderate earthquakes with magnitudes exceeding Mw 4 occurred along the fault, providing clear evidence of ongoing deformation and renewed seismic activity. Owing to its proximity to major population centers, the Issyk-Ata fault has been consistently identified as the dominant contributor to regional seismic hazard. The northern Tien Shan is an actively deforming intracontinental region characterized by distributed crustal shortening associated with far-field convergence. Long-term geodetic measurements indicate north–south shortening rates of up to ~20 mm/yr, resulting in recurrent strong earthquakes along the northern Tien Shan margin. The Issyk-Ata fault extends approximately 120 km in an east–west direction and forms the northern boundary of a young and actively growing anticline separating the Kyrgyz Range foothills from the Chui basin. As the youngest major fault system in the region, it transects the southern part of Bishkek, where extensive urban development has largely obscured its surface geomorphic expression. To better constrain the seismic behavior of the Issyk-Ata fault, we integrate high-resolution remote sensing, detailed geomorphological and structural field investigations, and paleoseismological trenching, with a particular focus on the Dzhal area of the Kyrgyz Range. These combined datasets allow systematic mapping of surface ruptures, measurement of cumulative vertical displacements, and identification of fault segmentation. Chronological constraints derived from optically stimulated luminescence and radiocarbon dating reveal at least two surface-rupturing earthquakes during the Holocene. Empirical scaling relationships suggest that these paleoearthquakes reached moment magnitudes of approximately Mw 6.6–7.1. Geological and geomorphological analyses in the Dzhal area indicate a long-term fault slip rate of ~1.15 mm/yr, reflecting sustained Quaternary deformation. The fault exhibits pronounced along-strike variability in rupture style and displacement, with individual segments recording distinct seismic histories and patterns of activity.

These results demonstrate that the Issyk-Ata fault accommodates deformation through segmented rupture behavior typical of low-strain intraplate settings. The occurrence of large Holocene earthquakes, together with recent moderate seismicity in 2025 and the fault’s direct interaction with the urban area of Bishkek, underscores the need for refined, segment-based seismic hazard models. Improved understanding of seismogenic sources and Quaternary deformation along the Issyk-Ata fault is essential for advancing seismic hazard assessment and risk mitigation strategies in the northern Tien Shan.