**2. Genetic mechanisms of soft-sediment deformation structures**

Soft-sediment deformation structures (SSDSs) are deformations that originated in unconsolidated sediments [16, 18, 19]. SSDSs can occur in different tectonic settings, e.g., passive continental margins, deep (trench) subduction zones and strike-slip tectonic transitions, and they can form in almost all sedimentary environments, preferably in shallow-marine, lagoonal, lacustrine and fluvial environments.

and also occur at the pre-existing fault or weak stress belt in the plate and can be triggered by

Paleo-seismic records are induced by earthquakes during geological history and reserved in strata. They include two categories: fault rocks and seismites. The fault rocks are composed of cataclastic rocks, mylonites, pseudotachylytes and fault gouge. They can occur and have been preserved at active fault zones and their adjacent areas in different geological times. The fault rocks are formed in different focal mechanisms of normal faults, thrust faults and strike slip faults [8–12, 5, 13]. Pseudotachylytes can be thought of as "an earthquake fossil". The fault rocks can be as the records of the active fault during different times. But the fault zone is often located in the long-term tectonic activity areas, and the later tectonic activities are often superimposed on the previous activities. Therefore, it is difficult to distinguish them for different

Seismites are the soft-sediment deformation structures (SSDSs) produced by strong earthquakes. SSDS is deformation that originates in unconsolidated sediments and usually occurs rapidly at or close to the surface during or shortly after deposition and before lithification takes place. During the deformation process, the original sediment particles are rearranged and migrated, the compositions are not changed and no new minerals would be produced. The triggers for SSDS are tectonism, glaciogenic, mass movement, collapse and some other physical and biological processes, and the earthquake trigger is one of important dynamic drives. Seismites had been observed from 1780s [14], and the liquefaction triggered by modern earthquakes were mainly studied. But the term 'seismites' was first proposed by A. Seilacher to interpret an effect of strong earthquakes on paleoslope for fault-graded beds in the Monterey Shales (Miocene), the north of Santa Barbara, California (USA) [15]. From the 1970s to 2000, several SSDSs triggered by earthquakes were identified and the formation mechanisms were primarily analysed [16–18]. In the last 20 years, they have been widely developed and classified [14, 19–21]. The special SSDSs (seismites) with inconsistence are preserved in the normal sedimentary strata, which provide opportunities to understand tectonic

In this chapter, we introduce the typical SSDSs observed in China, which are triggered by the recent to the Mesoproterozoic earthquake activities, and formed in different deposition environments and are composed of different sedimentary rocks. Especially, the structural styles, preserved positions, occurrence times, formation mechanisms and relationship with the activities of faults are discussed. The aim of the chapter is to reveal the high-frequency active events of the faults in different geological history and provide evidences for the paleotectonic and

Soft-sediment deformation structures (SSDSs) are deformations that originated in unconsolidated sediments [16, 18, 19]. SSDSs can occur in different tectonic settings, e.g., passive continental margins, deep (trench) subduction zones and strike-slip tectonic transitions,

**2. Genetic mechanisms of soft-sediment deformation structures**

impacting of meteorites [7].

104 Tectonics - Problems of Regional Settings

activity at different times.

paleogeographic reconstruction.

time and determine the activity of faults.

There are mainly four deformation mechanisms: (1) intergranular shear [22, 23]; (2) plastic or hydroplastics [24, 25]; (3) liquefaction [18, 24, 27] and (4) fluidization [24, 26]. The driving forces of deformation mechanisms include the tectonism, gravity acting on slopes, disequilibrium loading caused by topographical irregularities in the sediment-water interface, gravitational instabilities due to a reverse density gradient where denser sediments overlie less dense sediments, shear waves or other currents, and biological and chemical agents [14, 15, 18, 27–29]. The various morphology and deformation styles of the SSDSs can be formed with respect to sedimentation in different lithology, driving forces, sediment rheology and deformation mechanisms of the deformation [20, 27, 30–33].

Numerous schemes of classification of SSDSs have been proposed [14, 24, 26, 28, 29, 32]. The formation mechanisms of deformations induced in earthquake may be classified into five main categories, which include liquefaction, thixotropic deformation, hydroplastication, superposed gravity driving deformation and brittle deformation. And the secondary classification is also proposed according to genetic types, sediment compositions and deformation styles (**Figure 1**).

The strict criteria of SSDSs triggered by seismic events (seismites) have been discussed [14, 15, 18, 20, 27, 28, 34]. The commonly received criteria include (1) the deformation emerges in laterally continuous, vertically recurring layers, separated by undeformed layers; (2) the deformation occurs in marine, lacustrine or fluvial sediments; (3) deformed and undeformed beds have similar lithologies and facies features; (4) the deformation is related to a seismically or tectonically active area; (5) the deformation shows systematic increases in frequency or intensity toward a likely epicentral area. "Liquefied deformation" as a seismic record is associated with many modern and ancient seismogenic deposits and is related to surface-wave magnitudes Ms > 5 [20, 30, 32, 35].

**Figure 1.** Classification of SSDSs according to the genetic mechanism (modified from [14, 18, 20, 21]). The deformations in italics are triggered by more than two mechanisms.
