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 (Te) in the eastern Bayan Har block and its adjacent areas. Te values were determined by comparison of observed and flexural modelled Bouguer gravity anomalies. The results suggested that Te vary significantly from eastern Tibet to the Sichuan basin. The eastern Bayan Har block and northern Sichuan-Yunan block had low Te (0<Te<20km), indicated easily deformable lithosphere. The Sichuan basin had high Te (40km<Te<100km), and acted as a rigid block which resisted the eastward extrusion of plateau materials. Moderate Te (30km<Te<40km) under the Longmen Shan fault belt indicated that the lithosphere of the Sichuan basin plays an important role in supporting the Longmen Shan topography. The extremely low Te (Te<10km) in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake indicated that the topography is compensated locally, different from the Longmen Shan significantly. A banded relatively low Te (<40km) region stretched from northeastern Bayan Har block to southwestern Ordos block sketches out a plateau material escaping channel, questions the exiting of crustal flow in the north side of the Sichuan basin.