Well‐constrained fault slip rates are important for understanding strain partitioning within a fault system and the associated seismic hazard. The Haiyuan fault is a significant active strike‐slip fault in the northeast margin of the Tibetan Plateau with ongoing controversy over its late Pleistocene slip rate. Previous work by Lasserre et al. (1999) suggested a slip rate of 12 ± 4 mm/year, which is higher than recent geodetically determined rates on adjacent fault sections. We reanalyze and reevaluate the slip rates benefiting from new high‐resolution airborne Light Detection and Ranging data at their two sites, located north of the Songshan village. Based on this data, we revise field‐mapped offset constraints. At the Majia Wan site, we document a sinistral displacement of 130 ± 10 m of the crest and 93 ± 15 m of the base of T1/T2 terrace riser, respectively. At the Xuanma Wan site, the offset of T4/T1′ terrace riser is updated to be 68 +3/−10 m. Combining new geochronology dates, we assess the abandonment age of T2 as 26.0 ± 4.5 ka and T1 as 9,445 ± 30 year. These data suggest a slip rate between 5.0 +1.5/−1.1 and 8.9 +0.5/−1.3 mm/year since ~ 26 ka, based on the upper terrace and lower terrace reconstruction, respectively. Our reevaluation supports that apparent slip rate discrepancies in northern Tibet possibly share a systematic bias due to the use of lower‐terrace reconstruction to interpret the age of offset.
27 April 2022 in Journal of Geophysical Research: Solid Earth
The Longmen Shan range, located on the eastern margin of the Tibetan Plateau, is characterized by steep topography and a shortening rate of <3 mm/yr. This peculiar configuration is a source of controversy and questions about the topographic evolution and dynamics of this orogenic plateau margin. Investigating the variations in surface denudation over different spatial and temporal scales is important for a better understanding of topographic evolution, but there is still a lack of erosion‐rate data averaged over millennial timescales along the frontal range of the Longmen Shan, especially in its southern part. We present 25 new catchment‐wide denudation rates derived from 10Be concentrations in river sediments across the southern Longmen Shan. Our results show that average denudation rates increase from 0.50 mm/yr inside the southern Longmen Shan. The denudation rates correlate with slope gradient, relief, channel steepness, and specific stream power but exhibit a fair degree of scattering at high values. In combination with previous 10Be and low‐temperature thermochronology analyses, we found the denudation rates over kyr‐ to Myr‐timescales to be roughly consistent across the southern Longmen Shan, suggesting this part of the range is close to an exhumational steady state. In terms of spatial distribution, high rates of exhumation and denudation are localized in the hanging walls of major thrust faults, highlighting the role of tectonic structures in regulating the pattern of denudation and topography across the Longmen Shan. Our results favor the “brittle crustal shortening” model in which rock uplift pattern across the Longmen Shan is largely controlled by upper crustal shortening. Along‐strike variations in the distribution of denudation rates and topography can probably be attributed to segmentation of subsurface fault structures.
The 2021 Mw 7.4 Maduo (Madoi) earthquake that struck the northern Tibetan Plateau resulted in widespread coseismic deformation features, such as surface ruptures and soil liquefaction. By utilizing the unmanned aerial vehicle (UAV) photogrammetry technology, we accurately recognize and map 39,286 liquefaction sites within a 1.5 km wide zone along the coseismic surface rupture. We then systematically analyze the coseismic liquefaction distribution characteristics and the possible influencing factors. The coseismic liquefaction density remains on a higher level within 250 m from the surface rupture and decreases in a power law with the increasing distance. The amplification of the seismic waves in the vicinity of the rupture zone enhances the liquefaction effects near it. More than 90% of coseismic liquefaction occurs in the peak ground acceleration (PGA) > 0.50 g, and the liquefaction density is significantly higher in the region with seismic intensity > VIII. Combined with the sedimentary distribution along-strike of the surface rupture, the mapped liquefaction sites indicate that the differences in the sedimentary environments could cause more intense liquefaction on the western side of the epicenter, where loose Quaternary deposits are widely spread. The stronger coseismic liquefaction sites correspond to the Eling Lake section, the Yellow River floodplain, and the Heihe River floodplain, where the soil is mostly saturated with loose fine-grained sand and the groundwater level is high. Our results show that the massive liquefaction caused by the strong ground shaking during the Maduo (Madoi) earthquake was distributed as the specific local sedimentary environment and the groundwater level changed.