**5. Trigger mechanism**

26 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

The load balance of the unconsolidated sediments may change by landslide, rock fall or seismic waves. The load structures are formed by that load changing. These structures contain the load marks, pseudo-nodules and flame structures. Merely flame structures exist

Fig. 4. Ball and pillow structures observed in silty and sandy lacustrine deposits of Lake

seismic tremors (Visher & Cunningham, 1981; Dasgupta, 1998).

Flame structures are observed in sandy and silty deposits of Lake Van. These structures are formed by penetration of silty sediments to sandy deposits. These structures have different dimensions in lacustrine deposits of Lake Van (Fig. 5). They access up to 80 cm wide and 70 cm high. Generally flame structures comprise over pressure, but they can also be formed by

Water escape structures are formed by sudden movement of pore-water to the upper level of deposits. Dish and pillar structures are formed by that mechanism in lacustrine sediments

Dish and pillar structures are frequently observed in sandy and silty deposits of Lake Van. They consist of water movement in unconsolidated sediments due to sudden over pressure or seismic waves. The movement of pore water composes dish-like structures with folding of layers. These dish-like structures are seperated with vertical channels, called as pillars

**4.2 Load structures** 

Van.

**4.2.1 Flame structures** 

**4.3 Water escape structures** 

**4.3.1 Dish and pillar structures** 

of Lake Van.

(Fig. 6).

in lacustrine deposits of Lake Van.

The most known occurence of soft sediment deformation structures are overpressure of sediments (Lowe & LoPiccolo, 1974; Lowe, 1975), storm waves (Molina et al., 1998; Alfaro et al., 2002) and seismic shakings (Seilacher, 1969; Lowe, 1975; Sims, 1975; Rossetti, 1999; Vanneste et al., 1999; Jones & Omoto, 2000; Rodriguez-Pascua et al., 2000; Bowman et al., 2004). Deformation structures in lacustrine deposits of Lake Van were evaluated in the light of these trigger mechanism. There is not any evidence or data about overpressure of sediments or the effect of storm waves. Therefore, seismic shaking mechanism were investigated in detailed.

Seismic waves may form deformation structures (seismites) in unconsolidated sediments becuse of changing of pore water pressure, existence of impermeable layers in sequence and

Fig. 6. Dish and pillar structures occured by the movement of pore water in sediments.

Fig. 7. Deformation structures observed among the undeformed parallel layers at different levels.

heterogeneity of grain size. Whenever, deformation structures in Lake Van deposits were evaluated for seismic origin; (1) grain size of the deformed sediments stay in liquefaction range (Port and Harbour Research Institute of Japan, 1997), (2) deformation structures are frequently observed in different levels of sequence which dissociated with undeformed, parallel beds (Fig. 7), (3) shapes, dimensions, geometry, sedimentologic and geotechnic properties of deformation structures are very similar to presented in previous works (Sims, 1975; Rossetti, 1999; Vanneste et al., 1999; Jones & Omoto, 2000; Bowman et al., 2004), (4) the region is very active for earthquakes (M≥5) and (5) soft sediment deformation structures in lacustrine deposits of Lake Van provide all criteria for the called as seismite (Sims, 1975; Obermeier, 1998; Rossetti, 1999).
