龙门山地区类似2008年汶川大地震滑坡物质河流卸载时间的定量估算

Time response of river evacuation of great earthquake-induced landslide mass in the Longmen Shan region:a cast study of the 2008 Wenchuan great earthquake

定量研究地震滑坡物质河流卸载时间对理解地震与造山带地貌演化之间的关系有着十分重要的意义.本文以青藏高原东缘龙门山构造带内岷江流域为例,定量估算了2008年汶川大地震滑坡物质的河流卸载时间.研究结果表明,如果以位于龙门山构造带内的岷江河段现有搬运能力计算,并且岷江可以有效地搬运汶川地震滑坡物质,地震滑坡物质至少在3100yr内被岷江卸载出龙门山.而龙门山构造带中段类似2008年汶川Ms8.0级大地震的复发周期约为3000yr左右,暗示大地震所产生的滑坡物质量可以在大地震复发周期内基本上被侵蚀和剥蚀所平衡,并被卸载出龙门山构造带.因此,我们推断:除了周期性大地震造成的地表抬升的累积外,龙门山地区地震及其它地表过程所产生的剥蚀物质通过河流快速卸载驱动了地壳均衡反弹和深部物质上涌,形成了青藏高原东缘的高陡地形梯度带.相关的地球物理证据表明在青藏高原东缘可能存在由地表快速剥蚀(或侵蚀)所引发的地球深部地幔软流圈物质上涌.

Great earthquakes play an active role in geomorphic evolution of orogenic belts,and the time response of river evacuation of landslide mass induced by great earthquakes is a key aspect in the field of tectonic geomorphology.Here we quantitatively estimate the time response of unloading of the coseimsic landslide mass and sediments due to the 2008 Wenchuan great earthquake by the Minjiang River,flowing across the NE-trending Longmen Shan belt in easternmost Tibet.Assuming a steady state transport capacity of landslide mass the same as that of the modern Minjiang River,our results indicate that a full withdrawal of the landslide mass generated by the Wenchuan earthquake needs to take 3100 years.This timescale is similar to the recurrence interval of great seismic events(~3000 years) like the 2008 Wenchuan earthquake in central Longmen Shan.Such consistency suggests an approximate balance between production and erosion of coseismic landslide mass within the recurrence interval of great seismic events in this region.This process of mass unloading may also cause isostatic rebound of the crust beneath this region in addition to other surface processes.The consequent uplift,together with the tectonic shortening and uplift caused by repeated large seismic events,collectively result in the steepest topographic margin in the eastern margin of the Tibetan plateau.Recent geophysical imaging shows a potential of upwelling of asthenospheric materials beneath this region that we interpret as a result of rapid surface unloading induced by repeated great earthquakes.