Strain Accommodation and Seismic Hazards of the Kalpin Fold-And-Thrust Belt, Southwestern Tian Shan Foreland, China: Insights From the 2020 Mw 6.0 Kalpin Earthquake

Strain accumulation in foreland zones is important for understanding mountain-building processes and seismic hazards. Material heterogeneity in this border zone, particularly the contrast between sedimentary cover and basement, affects strain accommodation and fault behavior. One manifestation of this effect is depth separation of Interferometric Synthetic Aperture Radar (InSAR)-derived slip models and corresponding aftershock clusters; hypotheses for this vertical separation remain controversial. In this study, we investigated strain accumulation in the Kalpin fold-and-thrust belt (KFTB) of southwestern Tian Shan, China, using the integration of InSAR measurements, teleseismic body-waves, near-field strong motion data (SM), and relocated aftershocks of the 2020 M-w 6.0 Kalpin earthquake. The SM modeling and analysis of post-seismic InSAR time series are performed for the first time for this earthquake. Our results confirm a vertical separation of the mainshock and aftershock cluster, with the former on a weak decollement and the latter on faults within the basement. InSAR time series analysis shows that post-seismic deformation was dominated by afterslip on a splay fault directly above the ruptured decollement. Static stress transfer of co-seismic rupture cannot explain these observations. We speculate that fluid flow and high pore pressure along pre-existing fault planes may have reduced the fault strength and been involved in the evolution of this aftershock cluster. We conclude that compressive strain in the KFTB is accommodated by a mixture of thin- and thick-skin faulting and seismic deformation across the entire crustal thickness. Specifically, we suggest that the observed shortening is partly accommodated by infrequent large earthquakes on the weak decollement.

Automated summary

This study investigates strain accumulation in the Kalpin fold-and-thrust belt (KFTB) of southwestern Tian Shan, China, using InSAR measurements, teleseismic body-waves, near-field strong motion data, and relocated aftershocks. The results show a vertical separation between the mainshock and aftershock cluster, with the former on a weak decollement and the latter on basement faults. Fluid flow and high pore pressure may have reduced fault strength and contributed to the evolution of the aftershock cluster. The compressive strain in the KFTB is accommodated by both thin- and thick-skin faulting and seismic deformation across the entire crustal thickness.