Geometric control of the 1985 Wuqia Earthquake Rupture: insights from optical image correlation

SUMMARY The 1985 Mw 6.9 Wuqia earthquake, one of the strongest instrumentally recorded seismic events in the Pamir foreland thrust system, caused significant surface ruptures. The Pre-earthquake KH-9 and post-earthquake WorldView-3 and SPOT-6 satellite images are used to investigate the fault rupture and slip behaviour of this earthquake. We revealed a more detailed ∼22 km long displacement belt beyond the previously documented ∼15 km rupture, using optical image correlation with sophisticated error post-processing. Several new fractures in western segment are identified which are confirmed in the displacement map. A comprehensive analysis of the strike change, near-surface dip and cross-fault offsets shows a ∼1.6 km dextral strike-slip tearing fault resulted from the heterogeneous strain release. Based on the empirical scaling relationship, a downdip rupture width of 10.55 km is estimated using the observed rupture length and inverted slip. Combined with the previously published 3-D fault geometry based on seismic imaging, we suggest that the 1985 Wuqia earthquake ruptured only the upper ramp. This study provides precise constraints on surface rupture characteristics, and new insights into the complex rupture pattern of a thrust-type earthquake within the tectonically active Pamir foreland region.