# Dynamic Characteristics of Post-Cyclic Saturated Loess
## 1 Introduction  
As a problematic soil, loess is extensively distributed in Central and Eastern Asia, especially in Northern China. The Loess Plateau mainly includes the provinces such as Gansu, Shaanxi, Henan, Qinghai, and Shanxi, as well as the Autonomous Region of Ningxia and Inner Mongolia. The thickness of the loess layer has obvious regional differences, which vary from several meters to more than 500 m, including loess plateau, loess beam, loess mount, loess terraces, and a variety of geomorphic units [1; 2]. The particle composition of the loess is mainly silt content, which is commonly 50-80% [1]. The clay content and sand content have obvious regional differences [3]. The loess region in China is mostly an arid area. The loess is distributed with a large number of pores due to its formation; that is, fromthe eolian sediment and erosional-depositional by rain-wash [4; 5]. A significant portion of these pores are visible to the eyes and consist of multiple particles lapped together, while the cement between the particles is only a small amount of soluble salt and mucilage, which makes these pores exist naturally in a sub-stable state [6; 7]. This state causes the loess to have engineering properties such as high compressibility, strong water sensitivity, and high seismic sensitivity. In Northern China, there are large-scale seismic belt zones including the Haiyuan seismic belt zone, the Fenhe-Weihe fault zone, and the Western Qinling fault zone [8]. The neotectonic movements were strong and the seismic activities were frequent [9]. More than 570 strong earthquakes from 78 B. C., including many huge earthquakes, such as the Huaxian earthquake in 1556, the Haiyuan earthquake in 1920, the Hongtong earthquake in 1303, and the Tianshui earthquake in 1654. More than 1.4 million deaths are blamed, most of which are caused by seismic geohazards including landslides and mud flows caused by loess liquefaction (Figures 1 and 2) [1; 10; 11; 12; 13]. Figure 1: Earthquake-induced geological disasters in the Loess Plateau: (**a**) the Sunjiagou landslide triggered by the Haiyuan earthquake in 1920, (**b**) satellite imagery of the Sunjiagou landslide and the surrounding areas from Google Earth, (**c**) the Yongguang landslide and mudflow triggered by the Minxian-Zhangxian earthquake in 2013, and (**d**) satellite imagery of the Yongguang landslide and slide path of the mudflow. Note the red line in (**a,c**), and the yellow line in (**b,d**) are the boundary of the landslides, the red dotted line in (**d**) is the slide path of the mudflow. Figure 2: Location of the sampling site. The natural loess has a sub-stable structure with trellis pores, which are easy to damage under heavy rainfall, seismic action, and human activities such as agricultural irrigation and slope excavation [6; 14; 15], leading to serious geological hazards in the loess region. However, the co-seismic effect and post-seismic effect of strong earthquakes are one of the key causes of structural variations, as well as the geological hazards of the loess. After the earthquake, the damaged structure of the loess will be re-consolidated and become relatively stable [16]. Due to the structural changes caused by the destruction-consolidation process, the mechanical behaviors of reconsolidated loess aftershocks are significantly different from the behaviors of the original loess in the natural state. Therefore, the loess is loosened and cracked under seismic action, and the structural changes occur by the earthquake and post-earthquake reconsolidation [15; 16; 17]. It contributes to the development of geological hazards in the meizoseismal areas, underlying a huge hidden danger to engineering construction [18; 19]. The relationship between the post-seismic effects of strong earthquakes and landslides has played an important role in earthquake-induced landslide hazards and prevention.