Lithospheric flexural isostasy background of the 2017 Ms7.0 Jiuzhaigou earthquake and its implications for material extrusion in the northeastern Bayan Har block

Abstract

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. A program was designed to calculate the Moho flexure induced by topography loading, as well as the Bouguer gravity anomalies caused by that Moho flexure. The Te values were determined by minimizing the RMS differences between the observed and calculated Bouguer gravity anomalies. The results suggested that Te varies significantly from eastern Tibet to the Sichuan Basin. The eastern Bayan Har block and northern Sichuan-Yunnan block have low Te values (0 < Te < 20 km), indicating an easily deformable lithosphere. The Sichuan Basin has high Te values (40 km < Te < 100 km) acts as a rigid block that resists the eastward extrusion of plateau materials. The moderate Te values (30 km < Te < 40 km) found under the Longmen Shan fault belt indicate that the lithosphere of the Sichuan Basin plays an important role in supporting the Longmen Shan topography. The extremely low Te (Te < 10 km) observed in the seismogenic zone of the Ms7.0 Jiuzhaigou earthquake indicated that the topography is compensated locally, which is significantly different from the Longmen Shan topography. A banded region of relatively low Te values (< 40 km) stretching from the northeastern Bayan Har block to the southwestern Ordos block sketches out an escape channel for plateau materials, and challenges the existing of crustal flow on the north side of the Sichuan Basin.