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The tectonic wedge model describes a triangular-shaped structure, which has a duplex geometry and contains completely opposite thrusting vergences within a triangle zone (e.g.

On 23 March 2021, an Mw 5.2 earthquake struck Baicheng city at the southern foreland of the Tian Shan mountains and produced unexpected severe damages. Based on Interferometric Synthetic Aperture Radar observations, geological investigations, petroleum seismic-reflection profiles, and available seismological results, we found that (1) the earthquake rupture was sinistral-slip dominated and mostly concentrated at ≤2 km depth, suggesting an extremely shallow earthquake; (2) the earthquake produced a 4.1-km-long surface rupture zone, along which the lateral slip ranged from 0 to 135 cm and showed a significant short-wavelength variation; and (3) the causative fault is poorly developed (i.e., has low slip accumulation and slip rate) and cuts through a shallow-buried salt dome. These findings, along with those of previous studies, document some unique characteristics of moderate earthquakes at extremely shallow depths (≤3 km) relative to those at greater depths. First, shallow earthquakes occur on faults that are poorly developed and bracketed by rocks being able to behave as velocity weakening at shallow depth (including crystalline basement rocks, carbonites, volcanic rocks, or evaporites). Second, the surface rupture produced by shallow earthquakes has abnormally large length and slip, and possibly includes an abrupt slip variation. These characteristics highlight potential severe damages of the shallow moderate earthquake but enhance the challenge of identifying the earthquake causative fault (the earthquake source) prior to the event using traditional methods (e.g., geomorphic mapping, geophysical imaging surveys, and paleoseismic trenching).

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