5.1. Planar fault vs. listric fault
The deep seismic-reflection profiles showed that the Kalping fault is a listric structure with flat and ramp segments. The dip of the ramp near the surface is 30 ~ 70° (Qu et al. 2003). The dip of the flat segment is 2 ~ 3° at a depth of 9 km (Yang et al. 2008; Gao et al. 2013). Therefore, a listric fault geometry is used to invert for the slip model by SDM (Figure S4). The result showed that the RMS is 7 mm for ascending datasets and 6 mm for descending datasets, respectively. The data-model correlation is 93.45%.
The slip models from the planar fault (Figure3) and the listric fault (Figure S5) both demonstrate that the Jiashi earthquake is a shallow rupture and caused by a thrust fault with slight strike slips. Considering that the planar fault model has the minimum RMS and is more concise, we take the planar fault model as our preferred source model. The Kalping fault is the seismogenic fault since the position of the modeled fault is close to it.
The preferred model in this study implies the seismogenic fault is very flat with a dip angle 8.8°, which is consistent with the geometrical model of Zhou (2013) who proposed a low-dipping fault of 8.2° for the Kalping thrust system. Studies of geology and seismic reflection profile proposed that the Kalping fault has a listric geometry with typical characteristics of the thin-skinned tectonics (Qu et al. 2003). For thin-skinned tectonics, the sedimentary cover of the thrust sheets is detached from its crystalline basement along a décollement layer. The thickness of the detached cover sediments is typically on the order of 1 to 10 km (Pfiffner, 2017). Here, the coseismic slips of the Jiashi event are distributed in a low dip fault 5~7 km deep underground. The InSAR coseismic measurements further determined the exact geometry of the listric structure for Kalping fault in this study. It is suggested that the mainshock occurred on the flat segment of the listric fault, while the ramp segment did not rupture.
5.2. A shallow event
Results show that the maximum slip of the Jiashi earthquake occurs at the depth 6.3 km which is very shallow. It is contributed to the fault geometry and regional geological settings. The thin-skinned tectonics with shallow décollement structure is the main cause of shallow earthquakes. On the other hand, shallow earthquakes often occur in ancient craton regions, such as the southwest region of Western Australia (Langston 1987; Dawson et al. 2008) and Lake Ontario region in North America (Ma and Atkinson 2006; Kim et al. 2006), where a large number of shallow earthquakes with a depth of less than 5 km occurred. Due to the strong strength of the cold and hard ancient shield, the rock strength in all parts except near the surface exceeds the surrounding deviatoric stress. Driven by deviatoric stress, the fault rupture is difficult to expand to the deep, so it can only rupture at the shallow part to form shallow earthquakes (Langston 1987; Luo et al. 2011). The Tarim Basin, developed on the crystalline basement, has the property of stable craton, which is conducive to form shallow earthquakes in the area.
In orogenic belts, the normal stress due to the large mountain ranges makes the ramp with higher dip angles tend to have greater frictional resistance than the flat with lower dip angles (Styron and Hetland 2015; Yue et al. 2017; Tan et al. 2018). Thus, most earthquakes tend to nucleate on a flat with a lower dip angle, and then expand to a deeper or shallower region (Hubbard et al. 2016). In this context, earthquakes in orogenic belts mostly occur on low-angle detachment faults with a burial depth larger than 10 km, and only a few ruptures of strong earthquakes can expand to ramps near the surface. Therefore, shallow earthquakes larger than Mw 6.0 occurring at the transition zone from fault ramp to fault flat independently in orogenic belts are rare.
5.3. Regional seismic risk assessment
Being the front edge of the Kalpingtag thrust system, the slip rate of the Kalping fault is only 1 ~ 2 mm/a since late Cenozoic, which is just a quarter of the subduction rate of the Tarim Basin in this region (Yang et al. 2008). It indicates that the deformation pattern of the Kalpingtag thrust system is different from that of other orogenic belts. Deformation of most orogenic belts is dominated by front thrust faults and deep detachment faults, while internal faults within orogenic belts are weakly deformed such as the Himalayan orogeny belt (Stevens et al. 2015) and the New Guinea Highlands Fold-and-Thrust Belt (Koulali et al. 2015). However, the Kalping fault and the internal faults of the Kalpingtag nappe have been active since late Quaternary, and the tectonic activities have not completely migrated to the frontal fault of the nappe (Wu et al. 2014). This special deformation pattern makes the faults of Kalpingtag nappe have a low slip rate of 1 ~ 3 mm/a, but all faults can accumulate stress and trigger earthquakes. The coseismic Coulomb stress of this earthquake increased the stress of adjacent faults, but the magnitude is not large. For example, the Ozgertaou fault located on the north side of the Kalping fault has a stress disturbance of up to 0.7 bar (Figure 5).The faults in the Kalpingtag nappe are locked, accumulating elastic strain, but the detachment fault under the south Tian Shan is free to creep due to the weakening of friction under high-temperature conditions, driving the convergent deformation of the Kalping block (Li et al. 2015). Li et al. (2015) suggested that the locked area of the detachment fault extends to the north of Maidan fault based on the interseismic deformation of InSAR and GPS, while Yang et al. (2008) and Qiao et al. (2017) proposed that the locked point of the detachment fault is at the Totgumbaz–Alpaleh fault. The two views are slightly different, but it indicates that the rupture of the Jiashi earthquake is far from the locked point of the detachment fault. The Jiashi earthquake significantly increased the stress in the locked area of the detachment fault (Figure 5). The maximum stress disturbance is 10.1 bar, which is located at the deep margin of the rupture area. If the remaining locked area of the deep detachment fault is completely ruptured, the deformation width will exceed 50 km, and the released seismic moment is equivalent to an Mw 7.5 earthquake (Wells 1994).