Structural controls on coseismic rupture revealed by the 2020Mw 6.0 Jiashi earthquake (Kepingtag belt, SW Tian Shan, China)

Abstract

Summary The Kepingtag (Kalpin) fold-and-thrust belt of the southern Chinese Tian Shan is characterized by active shortening and intense seismic activity. Geological cross-sections and seismic reflection profiles suggest thin-skinned, northward-dipping thrust sheets detached in an Upper Cambrian décollement. The January 19 2020 Mw 6.0 Jiashi earthquake provides an opportunity to investigate how coseismic deformation is accommodated in this structural setting. Coseismic surface deformation resolved with Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) is centered on the back limb of the frontal Kepingtag anticline. Elastic dislocation modelling suggests that the causative fault is located at ∼7 km depth and dips ∼7○ northward, consistent with the inferred position of the décollement. Our calibrated relocation of the mainshock hypocenter is consistent with eastward, unilateral rupture of this fault. The narrow slip pattern (length ∼37 km but width only ∼9 km) implies that there is a strong structural or lithological control on the rupture extent, with up-dip slip propagation possibly halted by an abrupt change in dip angle where the Kepingtag thrust is inferred to branch off the décollement. A depth discrepancy between mainshock slip constrained by InSAR and teleseismic waveform modelling (∼7 km) and well-relocated aftershocks (∼10–20 km) may suggest that faults within sediments above the décollement exhibit velocity-strengthening friction.

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

“…By constraining fault strike to east–west (260°–280°) and rake to nearly thrust (60°–130°), we obtained a focal mechanism in line with the regional tectonics and principal strain (dark gray focal mechanisms in Figure 5a). The optimal solution is a strike/dip/rake of 275°/22°/128° and a centroid depth of ∼8 km, this optimal mechanism is basically compatible with the previously published InSAR slip models (P. He, Wen, et al., 2021; S. Wang et al., 2022; Y. Yao et al., 2020; P. Yu et al., 2020), except a dip of 22° is a bit steeper than some of the InSAR models (e.g., S. Wang et al., 2022; P. Yu et al., 2020; Table S1 in Supporting Information S1). However, of those solutions with mechanisms consistent with the regional principal strain, the minimum misfit solution had the poorest P wave fits for stations to the north‐east (e.g., YAK; Figure 5d) despite having the lowest residuals overall.…”

“…Yao et al, 2020;P. Yu et al, 2020), except a dip of 22° is a bit steeper than some of the InSAR models (e.g., S. Wang et al, 2022;P. Yu et al, 2020;Table S1 in Supporting Information S1).…”

“…The SM‐derived slip model is mostly consistent with the InSAR‐derived model. The main differences are as follows: (a) the ratio of width to length is ∼10:20 km, half that of the InSAR‐derived model, and (b) the average slip is double that of the InSAR‐derived slip model (S. Wang et al., 2022).…”

“…This event has been studied by either combining Interferometric Synthetic Aperture Radar (InSAR) co‐seismic interferograms and geological data to model the slip distribution (P. He, Wen, et al., 2021; Y. Yao et al., 2020; P. Yu et al., 2020) or by integrating aftershock relocations to explore the relationship between the mainshock and aftershocks (Y. He, Wang, et al., 2021; Q. Yao et al., 2021; S. Wang et al., 2022). Most of these previous studies revealed a vertical separation between InSAR‐derived slip models and the aftershocks, with aftershocks almost totally rooted in the basement (>7 km), contrasting with the InSAR‐derived model depth of 7 ± 1 km (i.e., within the shallow décollement; Table S1 in Supporting Information S1; Y.…”

“…By integrating InSAR and teleseismic data, S. Wang et al. (2022) suggested that the vertical separation between InSAR‐derived models and relocated aftershocks reflects a real separation between the mainshock and its triggered aftershocks. This conclusion was based on the consistent centroid depth between InSAR‐derived models and teleseismic body‐wave (BW) modeling results.…”