**3.1. Triggered by the modern earthquakes of the Yingxiu-Beichuan active faults**

The MS 8.0 Wenchuan earthquake struck the Longmenshan area on 12 May 2008, the transition zone between the eastern margin of the Tibetan Plateau and the Sichuan Basin, China. Besides the huge casualties and property losses, a most complicated yet longest thrust-type co-seismic surface rupture zone was developed in the quake-hit area. The surface rupture extends over lengths of 270 km and 80 km along the NE-SW trending Yingxiu-Beichuan fault (hereafter YBF) and Guanxian-Anxian fault (hereafter GAF) (**Figure 2**), which are high-angle and low-angle thrusts in the Longmen Shan, respectively [36–38]. The largest surface vertical displacements were attained at 12 m in the northern segment in the Beichuan area [39, 40]. There also occurred several soft-sedimentary deformation structures in the YBF and the terraces of the Mingjiang River, including liquefaction, gravitational and hydroplastic deformations and other related deformations. There is a need to mention that most SSDSs were observed soon after the Wenchuan earthquake had disappeared or been destroyed, because these SSDSs were distributed in the farmlands, river terraces or lawn and sandy areas of town. Prof. Li Haibing, one of the authors of this paper, pays attention to the liquefaction of sand deformations during the investigation of the Wenchuan earthquake in Longmen Mountain and its adjacent area, and makes a preliminary study on the soft-sediment deformation shortly

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Various sand volcanos and liquefied mounds (**Figure 3**) have been observed in the Mingjiang terraces and adjacent areas of the Yingxiu-Beichuan Fault. They possessed different shape styles and internal sediment compositions. Unconsolidated saturated sedimentary sand layers are liquefied and flow upwards along vertical conduits and form mound-shape uplifts on the surface when the shaking occurred. The underground sands and gravels are brought by extrusion liquefied sand flow to the surface, and they form sand cones (**Figure 3D**) or gravel mounds (**Figure 3C** and **E**). Some of them are ejected out of the liquefied sands completely and form the craters (**Figure 3A**). Liquefied sand cones and gravel mounds usually occur in line along the Yingxiu-Beichuan faults (**Figure 3C** and **D**). The gravel mounds or cones are 1.5 m in diameter and 0.5 m high. If the thin gravel layer is covering the liquefaction layer, the upward sand flow entrainment gravels can form larger mounds. The largest one can reach 3.5 m in diameter and 1.5 m high. There are also small sand cones, which are parallel with the

Multiple liquefied sheet sands in a large area accompanied by ground fissures occurred in the Mingjiang terraces during the 2008 Wenchuan earthquake. The trending of ground fissures are parallel or perpendicular to the terrace margin, and the two groups of ground fissures constitute the netlike fissures. The underground liquefied sand flowed upwards and overflowed along the fissures to form the liquefied sheet sands (**Figure 3B**). The liquefied sheet

There were many linear collapse pits in the farmland (**Figure 3F** and **G**), and the orientation of arrangement was usually paralleled to the Yingxiu-Beichuan Fault. They resulted from the liquefied sand dunes. The underground liquefied sand upwelling towards the ground, due to the process of liquefaction, ceased; the density of local underground layers changed to smaller; the sediments changed looser, even the underground caves occurred; and the collapse pits were formed due to downwards suction. Some undeveloped collapse pits also have

sands also occurred from ring and radial cracks along the low sand dune.

after earthquake.

large mounds.

*3.1.1. Sand volcanos and liquefied mounds*

*3.1.2. Liquefied sheet sands and collapse pits*

the ring and ring and radial cracks (**Figure 3F**).

**Figure 2.** Simplified geologic and active tectonic map of the Longmen Shan and its adjacent area (adapted from 1:500,000 geologic maps, Ministry of Geology and Mineral Resources, 1991; [36, 37]) and observed SSDS sites during post-disaster investigation soon after the Wenchuan earthquake. F1, Maoxian-Wenchuan Fault; F2, Yingxiu-Beichuan Fault; F3, Anxian-Guanxian Fault.

and low-angle thrusts in the Longmen Shan, respectively [36–38]. The largest surface vertical displacements were attained at 12 m in the northern segment in the Beichuan area [39, 40]. There also occurred several soft-sedimentary deformation structures in the YBF and the terraces of the Mingjiang River, including liquefaction, gravitational and hydroplastic deformations and other related deformations. There is a need to mention that most SSDSs were observed soon after the Wenchuan earthquake had disappeared or been destroyed, because these SSDSs were distributed in the farmlands, river terraces or lawn and sandy areas of town. Prof. Li Haibing, one of the authors of this paper, pays attention to the liquefaction of sand deformations during the investigation of the Wenchuan earthquake in Longmen Mountain and its adjacent area, and makes a preliminary study on the soft-sediment deformation shortly after earthquake.

#### *3.1.1. Sand volcanos and liquefied mounds*

**3. Typical cases of the SSDSs triggered by earthquakes during the** 

**3.1. Triggered by the modern earthquakes of the Yingxiu-Beichuan active faults**

The MS 8.0 Wenchuan earthquake struck the Longmenshan area on 12 May 2008, the transition zone between the eastern margin of the Tibetan Plateau and the Sichuan Basin, China. Besides the huge casualties and property losses, a most complicated yet longest thrust-type co-seismic surface rupture zone was developed in the quake-hit area. The surface rupture extends over lengths of 270 km and 80 km along the NE-SW trending Yingxiu-Beichuan fault (hereafter YBF) and Guanxian-Anxian fault (hereafter GAF) (**Figure 2**), which are high-angle

**Figure 2.** Simplified geologic and active tectonic map of the Longmen Shan and its adjacent area (adapted from 1:500,000 geologic maps, Ministry of Geology and Mineral Resources, 1991; [36, 37]) and observed SSDS sites during post-disaster investigation soon after the Wenchuan earthquake. F1, Maoxian-Wenchuan Fault; F2, Yingxiu-Beichuan Fault; F3,

**geological history**

106 Tectonics - Problems of Regional Settings

Anxian-Guanxian Fault.

Various sand volcanos and liquefied mounds (**Figure 3**) have been observed in the Mingjiang terraces and adjacent areas of the Yingxiu-Beichuan Fault. They possessed different shape styles and internal sediment compositions. Unconsolidated saturated sedimentary sand layers are liquefied and flow upwards along vertical conduits and form mound-shape uplifts on the surface when the shaking occurred. The underground sands and gravels are brought by extrusion liquefied sand flow to the surface, and they form sand cones (**Figure 3D**) or gravel mounds (**Figure 3C** and **E**). Some of them are ejected out of the liquefied sands completely and form the craters (**Figure 3A**). Liquefied sand cones and gravel mounds usually occur in line along the Yingxiu-Beichuan faults (**Figure 3C** and **D**). The gravel mounds or cones are 1.5 m in diameter and 0.5 m high. If the thin gravel layer is covering the liquefaction layer, the upward sand flow entrainment gravels can form larger mounds. The largest one can reach 3.5 m in diameter and 1.5 m high. There are also small sand cones, which are parallel with the large mounds.

#### *3.1.2. Liquefied sheet sands and collapse pits*

Multiple liquefied sheet sands in a large area accompanied by ground fissures occurred in the Mingjiang terraces during the 2008 Wenchuan earthquake. The trending of ground fissures are parallel or perpendicular to the terrace margin, and the two groups of ground fissures constitute the netlike fissures. The underground liquefied sand flowed upwards and overflowed along the fissures to form the liquefied sheet sands (**Figure 3B**). The liquefied sheet sands also occurred from ring and radial cracks along the low sand dune.

There were many linear collapse pits in the farmland (**Figure 3F** and **G**), and the orientation of arrangement was usually paralleled to the Yingxiu-Beichuan Fault. They resulted from the liquefied sand dunes. The underground liquefied sand upwelling towards the ground, due to the process of liquefaction, ceased; the density of local underground layers changed to smaller; the sediments changed looser, even the underground caves occurred; and the collapse pits were formed due to downwards suction. Some undeveloped collapse pits also have the ring and ring and radial cracks (**Figure 3F**).

*3.1.3. Formation mechanism of SSDSs' response to the Yingxiu-Beichuan activity*

driving force of deformation structures are increased largely.

continent collision between the India and Asian continent.

*3.2.1. Soft-sediment deformation structures at the southeast end of TFSSF*

**strike-slip fault (TFSSF)**

**3.2. Triggered by the early Jurassic earthquake activities of the Talas-Ferghana** 

Talas-Ferghana strike-slip fault has been active since the Mesozoic, but the initial time of strike-slip is still in dispute [42–44]. The Wuqia pull-apart basin of the Lower Jurassic was controlled by this huge fault (**Figure 5a**), with NW-trending, which is located at the southeast end of the significant Talas-Ferghana fault, SW Tianshan. Soft-sediment deformations were preserved in sandstone layers at top of the Lower Jurassic Kangsu Formation, and three earthquake-induced deformation sequences have been recognised within 10 m thickness of sandstones deposited in the lacustrine environment (**Figure 5b**). They are included as load

cast, ball-and-pillow, droplet, cusps, homogeneous layer, and liquefied unconformity.

Load casts and ball-and-pillow are the main types of deformation in the third layer of SSDSs in the Kangsu formation. The parental sand layer providing load casts to subside is 80-cmthick laminated siltstone and consists of limonite debris, feldspar, quartz and muscovite. The

Sand volcanos, liquefied mounds and collapse pits are recognised in the earliest earthquake records. And the liquefaction and ejection of fluid induced by the big earthquakes are key to fluid escape structures (sand boiling and sand volcano), which are composed of gas (often sulphur emanations), water, mud and sand, and under unstable fluidization environments [14, 30]. Nichols's experiments demonstrated the lower part material was fluidized and was blocked by the overlying non-fluidization layer, and when the grain size is more than 15% compared to the overlying layers, the biggest ejections can be produced [41]. But in huge modern earthquakes, sand volcanos and liquefied mounds are published little: first, because it is smaller than the earthquake rupture zone; second, it is easy to be destroyed by human activities and cannot be preserved. The liquefaction mounds and other SSDSs induced by the Wenchuan earthquake are extremely valuable geological records. Liquefied sand dunes, sand volcanos and liquefied gravel mounds reflect the process of the liquefaction, and the size and

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The Wenchuan earthquake produced the Yingxiu-Beichuan fault and Guanxian-Anxian faults with the NE-trending NW-dipping occurred surface rupture on the Longmenshan Fault Belts and with dextral-slip thrusting [36, 37]. The Longmenshan Fault Belts are the main margin fault of the West Sichuan foreland basin to the Songpan-Ganze terrane, and it is activated since the Triassic. From the distance of observed SSDSs to the faulted belts, range from 5 to 30 km, induced that the reactivated faults and triggered fault is the Yingxiu-Beichuan Faults (see **Figure 4**). These liquefied gravel mounds and sand volcanos are the records of events of the activity of the Longmenshan Fault Belts, and it is a response to the Indo-Asia collision and eastern extrusion of the Qinghai-Tibetan Plateau terrane. The activities of the Longmenshan Fault Belt is a result of the stress that originate from the Qinghai-Tibetan Plateau terrane converging to the north and escaping to the southeast, which were driven by the continent-

**Figure 3.** Sand volcano, liquefied dune, sand sheet, liquefied deformation of sand bed, triggered by May 12, 2008 Wenchuan earthquake (photographed by Li Haibing). (A) Sand volcano in the terraces of Mingjiang river; (B) liquefied sand sheets in the terraces of Mingjiang river; (C) a higher liquefied sand mounds in the Yingxiu-Beichuan fault zone; (D) liquefied sand dunes distributed in the lineal arrangement and parallel with the fault trend of the Yingxiu-Beichuan; (E) a liquefied sand and gravel mound, the gravels have been carried over the top of mound; (F) collapse sink resulted from liquefaction; (G) linear arrangement of the collapse sinks, diameter of collapse sinks are about 80–90 cm, the orientation is also parallel to the Yingxiu-Beichuan fault, showing the orange colour dot line.

#### *3.1.3. Formation mechanism of SSDSs' response to the Yingxiu-Beichuan activity*

Sand volcanos, liquefied mounds and collapse pits are recognised in the earliest earthquake records. And the liquefaction and ejection of fluid induced by the big earthquakes are key to fluid escape structures (sand boiling and sand volcano), which are composed of gas (often sulphur emanations), water, mud and sand, and under unstable fluidization environments [14, 30]. Nichols's experiments demonstrated the lower part material was fluidized and was blocked by the overlying non-fluidization layer, and when the grain size is more than 15% compared to the overlying layers, the biggest ejections can be produced [41]. But in huge modern earthquakes, sand volcanos and liquefied mounds are published little: first, because it is smaller than the earthquake rupture zone; second, it is easy to be destroyed by human activities and cannot be preserved. The liquefaction mounds and other SSDSs induced by the Wenchuan earthquake are extremely valuable geological records. Liquefied sand dunes, sand volcanos and liquefied gravel mounds reflect the process of the liquefaction, and the size and driving force of deformation structures are increased largely.

The Wenchuan earthquake produced the Yingxiu-Beichuan fault and Guanxian-Anxian faults with the NE-trending NW-dipping occurred surface rupture on the Longmenshan Fault Belts and with dextral-slip thrusting [36, 37]. The Longmenshan Fault Belts are the main margin fault of the West Sichuan foreland basin to the Songpan-Ganze terrane, and it is activated since the Triassic. From the distance of observed SSDSs to the faulted belts, range from 5 to 30 km, induced that the reactivated faults and triggered fault is the Yingxiu-Beichuan Faults (see **Figure 4**). These liquefied gravel mounds and sand volcanos are the records of events of the activity of the Longmenshan Fault Belts, and it is a response to the Indo-Asia collision and eastern extrusion of the Qinghai-Tibetan Plateau terrane. The activities of the Longmenshan Fault Belt is a result of the stress that originate from the Qinghai-Tibetan Plateau terrane converging to the north and escaping to the southeast, which were driven by the continentcontinent collision between the India and Asian continent.
