Topographic Loads Modified by Fluvial Incision Impact Fault Activity in the Longmenshan Thrust Belt, Eastern Margin of the Tibetan Plateau

Abstract

Whether external or internal forces of the Earth control the behaviors of upper‐crustal faults in a fold‐and‐thrust belt has been debated for decades. The Longmenshan thrust belt (LTB) along the eastern margin of the Tibetan Plateau may provide insights into such a debate, as the central segment of the LTB has relatively uniform shortening strains yet various fluvial incision capability along the strike. This tectonic setting enables a better assessment of the effects of external forces on fault activity. We analyzed the variations of topography, fluvial incision intensity, coseismic slips, and coseismic landslides along the central LTB. The Longmen subsegment in the northern half has higher elevation and lower fluvial incision intensity than the Hongkou subsegment in the south. We calculated the topographic stresses on the faults ruptured during the 2008 Wenchuan earthquake and found topographic introduced normal stress increase may explain the coseismic slip partitioning onto two subfaults along the Longmen subsegment. Our results indicate that fluvial incision may have produced the along‐strike variations of the topography, which may further produce the different rupture behavior. In addition, the mean hillslope angle along the central LTB appeared to be at the critical condition prior to the 2008 Wenchuan earthquake and reduced significantly by the earthquake, indicating that geomorphic indices may vary with different stages in an earthquake cycle. Therefore, scrutinizing the mean hillslope angle and other geomorphic indices may help identify potential seismic hazards in an active fault system.

Three‐Dimensional Model of the Lithospheric Structure Under the Eastern Tibetan Plateau: Implications for the Active Tectonics and Seismic Hazards

“…Isostatic rebound induced by rapid erosion‐related unloading is a key component in geodynamics that drives the ongoing mountain building processes along the eastern TP. Long‐term unloading related to rapid surface erosion may induce isostatic rebound at the boundary between the lower crust and upper mantle, thereby driving vertical uplift at the surface and maintaining the high relief of the LMS thrust belt (Fu et al, ; Tan et al, ). Recently, a study on erosion and the associated isostatic response to the 2008 Wenchuan earthquake suggests that the isostatic rebound contributes significantly to the formation of high peaks in the LMS (Z. Ren, Zhang, et al, ).…”

Source model for the Mw 6.0 earthquake in Jiashi, China on 19 January 2020 from Sentinel-1A InSAR data

“…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 Fig.…”

Source model for the Mw 6.0 earthquake in Jiashi, China on 19 January 2020 from Sentinel-1A InSAR data

“…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 at with lower dip angles (Styron and Hetland 2015;Yue et al 2017;Tan et al 2018). Thus, most earthquakes tend to nucleate on a at with a lower dip angle, and then expand to a deeper or shallower region (Hubbard et al 2016).…”