Lithospheric Flexural Isostasy Background of the 2017 Ms7.0 Jiuzhaigou Earthquake and Its Implications on Material Extrusion in the Northeastern Bayan Har Block

: We applied a 2-dimensional, non-spectral technique to investigate the spatial variations of the lithospheric effective elastic thickness ( T e ) in the eastern 20 Bayan Har block and its adjacent areas. T e values were determined by comparison of 21 observed and flexural modelled Bouguer gravity anomalies. The results suggested that 22 T e vary significantly from eastern Tibet to the Sichuan basin. The eastern Bayan Har 23 block and northern Sichuan-Yunan block had low T e (0< T e <20km), indicated easily 24 deformable lithosphere. The Sichuan basin had high T e (40km< T e <100km), and acted 25 as a rigid block which resisted the eastward extrusion of plateau materials. Moderate 26 T e (30km< T e <40km) under the Longmen Shan fault belt indicated that the lithosphere 27 of the Sichuan basin plays an important role in supporting the Longmen Shan 28 topography. The extremely low T e ( T e <10km) in the seismogenic zone of the Ms7.0 29 Jiuzhaigou earthquake indicated that the topography is compensated locally, different 30 from the Longmen Shan significantly. A banded relatively low T e (<40km) region 31 stretched from northeastern Bayan Har block to southwestern Ordos block sketches 32 out a plateau material escaping channel, questions the exiting of crustal flow in the 33 north side of the Sichuan basin. 34

In this paper, we estimate T e in the focal area of the Ms7.0 Jiuzhaigou earthquake, 80 including the eastern Bayan Har block, based on flexural isostasy analysis of gravity 81 and topography data (Watts, 2001)  The absolute differences between the inputted and recovered T e are lower than 2km.  rigid block, has high T e (40<T e <100km).

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The reliability of our T e structure can be evaluated by considering the crustal 178 isostatic status. Isostatic gravity anomalies are defined as differences between the observed Bouguer gravity anomalies and the effect of the compensation mass, and 180 will be nearly zero if a suitable isostatic model is applied. Fig.6(c) shows that flexural 181 isostatic gravity anomalies are generally subdued, and with amplitude lower than 182 20mGal in most of the studied region. For the Airy isostatic model, the topography is 183 compensated locally, equivalent to flexural isostatic model with a T e of 0km. Fig.6(d) 184 shows that Airy isostatic gravity anomalies are lower than -40mGal in most of the 185 studied region, which means that the Moho is deeper than predicted by Airy model.

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The most distinct feature of the Airy isostatic anomalies is the positive anomalies that

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Acknowledgements 301 We are grateful to the NCEI and ICGEM for providing free topography and gravity velocity data in Fig.7(b) and S.S. Liang for providing relocated aftershocks in Fig.1. 305 We thank the anonymous reviewers and the editor for their helpful comments to 306 improve our manuscript.

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Ethics approval and consent to participate 309 Not applicable.

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Competing interests 312 The authors declare that they have no competing interests.   The recovered Te structure (left) and the differences between the inputted and recovered Te (right). The results indicate that the Te structure can be recovered well in the center 200km×200km of the plate, and the absolute differences between the inputted and recovered Te are lower than 2km.

Figure 4
The topography from ETOPO1 in the studied area. Te was calculated in the white dashed box. S1~S4 are seismic surveying pro les. M1~M7 are magnetotelluric surveying pro les. S1:  The Bouguer gravity anomalies from EIGEN6C4 in the studied area. S1~S4 are seismic surveying pro les, and M1~M7 are magnetotelluric surveying pro les, the same in Fig.4.

Figure 6
The recovered Te over the eastern Bayan Har block and its adjacent areas (a), the calculated Bouguer anomalies (b), the exural isostatic anomalies (c), and the Airy isostatic anomalies (d).

Figure 7
The recovered Te structure and distribution of the high-conductivity zones on MT pro les (a). The red dashed lines show the same MT pro les in Fig.4. The red shadows on MT pro les indicate high-conductivity zones. The relative low Te (<40km) region from the northeastern Bayan Har block to southwestern Ordos block correlates well with the high-conductivity zones. The GPS velocity relative to Southeastern China (Dr. B. Zhao, personal communication) also directed to northeastern in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake (b). The pink arrows indicate possible channel of material extrusion.

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