Micro-granulometry of sedimentary dykes, Kathmandu basin, Nepal

Soft-sediment deformation structures have been analyzed at six sites of the Kathmandu valley. Microgranulometric study (this Supplement and Fig. 3B of Mugnier et al., Tectonophysics, 2011) reveals that silty levels (60 to 80% silt) favor the development of soft-sediment deformation structures, while sandy levels (60 to 80% sand) are passively deformed. Nonetheless well sorted sand levels (more than 80% sand) generate over-fluid pressure during compaction if located beneath a silty cap, leading to fluidization and dike development. 3-D geometry of seismites indicates a very strong horizontal shearing during their development. Using a physical approach based on soil liquefaction during horizontal acceleration, we show that the fluidization zone progressively grows down-section during the shaking, but does not exactly begin at the surface. The comparison of bed-thickness and strength/depth evolution indicates three cases: i) no soft-sediment deformation occurs for thin (few centimeters) silty beds; ii) the thickness of soft-sediment deformation above sandy beds is controlled by the lithological contrast; iii) the thickness of soft-sediment deformation depends on the shaking intensity for very thick silty beds. These 3 cases are evidenced in the Kathmandu basin. We use the 30 cm-thick soft-sediment deformation level formed during the 1833 earthquake as a reference: the 1833 earthquake rupture zone extended very close to Kathmandu, inducing there MMI IX-X damages. A 90 cm-thick sediment deformation has therefore to be induced by an event greater than MMI X. From a compilation of paleo and historic seismology studies, it is found that the great (M ~ 8.1) historical earthquakes are not characteristic of the greatest earthquakes of Himalaya; hence earthquakes greater than M ~ 8.6 occurred. Kathmandu is located above one of the asperities that laterally limits the extent of mega-earthquake ruptures and two successive catastrophic events already affected Kathmandu, in 1255 located to the west of this asperity and in ~ 1100 to the east.

研究对加德满都谷地（Kathmandu valley）6处点位的软沉积物变形构造（soft-sediment deformation structures）展开了分析。显微粒度分析（本补充材料及Mugnier等人2011年发表于《构造物理学》（Tectonophysics）的图3B）结果显示：粉砂层（粉砂含量60%~80%）更易发育软沉积物变形构造，而砂层（砂含量60%~80%）仅发生被动变形。但若分选良好的砂层（砂含量＞80%）被粉砂层覆盖且处于压实过程中，则会产生超孔隙压力，进而引发流石化作用与岩墙发育。震积岩（seismites）的三维几何形态表明，其形成过程中伴随强烈的水平剪切作用。本研究采用基于水平加速度作用下土壤液化（soil liquefaction）机制的物理分析方法，结果显示：振动过程中流石化带逐渐向下延伸，但并非始于地表。通过对比地层厚度与强度/深度演化关系，可归纳出三种情形：① 薄（数厘米厚）粉砂层不会发生软沉积物变形；② 砂层上方软沉积物变形的厚度受岩性差异控制；③ 极厚粉砂层的软沉积物变形厚度则取决于振动强度。上述三种情形在加德满都盆地均有佐证。本研究以1833年地震形成的30厘米厚软沉积物变形层作为参照：1833年地震的破裂带紧邻加德满都谷地，当地引发了麦加利地震烈度（MMI）IX~X级的破坏。据此推测，90厘米厚的沉积物变形需由烈度超过MMI X级的地震事件引发。综合古地震与历史地震学研究成果可知：震级约8.1的特大型历史地震并非喜马拉雅地区最强地震，因此该区域曾发生过震级大于8.6的大地震。加德满都位于限制巨型地震破裂横向范围的一个凹凸体（asperities）下方，且已有两次连续的灾难性地震影响过该区域：1255年的地震震中位于该凹凸体西侧，约1100年的地震则位于其东侧。