Origin of SSDs
Many studies have tried to deny nonseismic processes for the creation of SSDs such as slumps and rapid sedimentation to explain the SSDs formation (cf. Gajurel et al., 1998; Martín-Chiviet et al. 2011; Mugnier et al., 2011), because there are no distinct single criterion to identify seismites (cf. Jones and Omoto, 2000; Montenat et al., 2007; Fortuin and Dabrio, 2008; Wallace and Eyles, 2015). Wide extent of SSDs horizons is also one of important factors for seismite identification.
Major deformation structures discovered here were (1) massive sand, (2) massive coarse silt with numerous mud and sand clasts, (3) stretched sand and silt lenses, (4) fine silt with cracks and wedges and (5) convolute structure. These structures as well as other minor structures imply that liquefaction, fluidization and brittle and ductile deformation occurred when SSDs were generated.
These are lines of strong evidence for liquefaction of the massive sand. The penetration of mud clasts in the massive sand of Example 1 from above without deformation of base of the sand, and the injection of sand into the underlying silt in Example 2, indicate the liquefaction of these sand. The ordinary massive sand associated with liquefaction due to strong tremor shows upward injections in many cases. In this case, the strong compressional stress exerted in the overlying silt interval due to sliding (discussed below) may have retarded the upward injection of liquefied sand or evidence of the upward injection may have been erased by the sliding to be convoluted portion (the lower portion of the silt part in the Example 1) or to be horizontally stretched as thin sand layers (Examples 1 and 2).
The coarse massive silt with sand and mud clasts is similar to the homogenized bed of Seilacher (1984), the intraclast breccia layer of Agnon et al. (2006) and mud breccia unit of Sakaguchi et al. (2011), which were interpreted as having been attributed to seismic ground motion, having caused Kelvin-Helmholtz instability between soft sediment layers (cf. Heifetz et al., 2005). The coarse silt described in the Examples 1 – 3 is different from those previously described in the following points: (a) the sand and mud clasts are rounded in this case: those in the previous description are angular in shape. Some of them are stretched to be sand and silt lenses, and (b) the massive coarse silt with sand and mud clasts is graded upward into the fine silt with cracks and wedges: the previously described SSDs layers are then covered with undeformed sediments deposited after deformation (cf. Seilacher, 1984; Agnon et al. 2006; Sakaguchi et al., 2011). Therefore the Kelvin-Helmholtz instability model cannot be simply applied for this case.
The generation of coarse silt with sand and mud clasts may have been explained by brecciation of slightly less dense but cohesive silt and sand and mixing of the brecciated clasts (sand and mud clasts) with less cohesive silt (silt slurry) created by seismic tremor. The rounded sand and mud clasts and the stretched sand and silt within the coarse silt portion imply that they were moved and sheared within silt slurry; fluidization occurred below the fine silt.
The overlying fine silt contains cracks and wedges filled with black mud derived from above. These can be recognized as open cracks and wedges. Some cracks and wedges were observed in a sandy interval (Fig. 4). Montenat et al. (2007) summarized the characteristics of thixotropic wedges that are developed in coarse-grained layers and are similar to the cracks and wedges in this case. Such wedges are attributed to the limited and periodical occurrence of liquefaction and subsequent sediment collapse (Montenat et al., 2007). However, Montenat et al. (2007) suggested that the cracks and wedges should be discriminated from those of cryogenic origin in order to specify the seismic origin. In this case, the Kathmandu Valley was located in subtropical and temperate climate zones and the presence of ice wedges is not realistic (cf. Gayer et al. 2006).
There was no agent causing strong tensional stress to create cracks and wedges as well as liquefaction of sand, fluidization of unconsolidated silt other than earthquake tremors, because the sediments above and below the SSD intervals consist of fluvial stream or marsh deposits, which are characteristic of almost flat environments. The setting of a fluvial-dominated delta system allows us to exclude the effects of large waves (storms) from consideration. The correlation of SSDs intervals between Locs. 9 and 9-1 revealed that many of them were correlated between locations. Those which were not found at Loc. 9-1 because of outcrop loss and erosion by fluvial channel, also have exposure-wide continuation of intervals. All the SSDs here can, therefore, be recognized as seismites (see discussions in Mugnier et al. (2011)).
In the silty SSDs reported in previous studies, the brittle deformation predominates in the lower part of the SSDs interval, and ductile or fluidization (homogenization) in the upper part due to deformation of less cohesive silty sediments (Seilacher 1969, 1984). Only Example 4 from the interval C shows deformation similar to those reported in previous studies (e.g. Seilacher, 1969, 1984; Agnon et al. 2006) (Figs. 9, 12a). On the contrary, almost all other intervals, including the interval C at Loc. 9, shows deformation different from the ordinary pattern mentioned above: fluidization in the lower coarse silt and brittle deformation in the upper fine silt (Fig. 12b). Some of examples like Example 3 have alternation the fluidized part and brittle-deformation part (Figs. 8, 12b). Evidence of ductile deformation was also observed in the transition from the coarse to fine silt as in Example 2. These facts obviously suggest that the upper brittly-deformed fine silt was compacted more than the fluidized lower coarse silt before they were deformed. This peculiar consolidation pattern may be explained by seasonal lake-level changes: near surface sediments may have experienced consolidation due to drying of the surface during the dry season. Sakai et al. (2001) discovered evidence of high-frequency lake-level changes due possibly to annual wet and dry condition from the previous Gokarna (containing Tokha Formation) and Thimi formations.
The fluidized coarse silt with sheared sand and silt layers may have been responsible for minor sliding of the SSDs horizons, which introduced shear stress within the liquefied layer below the surface. In particular, the interval E obviously shows sliding of the beds. The Locs. 9 and 9-1 are close to the buried flexure (Fig. 3), which was active during the Pleistocene (Sakai and Gajurel, 2012) and its activity could be the trigger of slidings. Tilting of the surface was the only possible trigger for sliding, because the environment of sedimentation was almost flat. This result suggests that sliding of the plain area could happen in conjunction with future large earthquakes centered around the Kathmandu Valley. As the future studies, the distribution of seismites hosting evidence of sliding within the valley should be specified for creation of the hazard map indicating the possibility of sliding as well as for the risk management of the plain area.
Scale of earthquakes
Seismites in location 9 are mostly 0.4–0.8 m in thickness except for the uppermost one, which is 0.2 m thick. Some parts of beds must have been lost due to sliding (as suggested by the presence of fragmented mud); thus, the initial seismite thickness may have been greater than presently observed. If we simply apply the empirical relationship between the intensity of tremors and seismite thickness (Hibsch et al., 1997) as applied by Gajurel et al. (1998), most of the seismites may have been formed by events stronger than X on the MMI scale. However, large deformation structures associated with the expulsion of a sediment-water mixture, suggesting large-scale liquefaction, have been documented in other cases (e.g. Montenat et al., 2007; Fortuin and Dabrio, 2008; Rossetti et al. 2011; Santos et al., 2012), and such large structures were not observed in these examples. Most previously described seismites are associated with sand and coarse silt, but the major sediment type of these examples is fine and coarse silt, which shows deformation sturctures different from the previously described examples. Therefore, the intensity of tremors for the seismites described here should be carefully discussed on the basis of additional information such as their lateral extent.
The recurrence period of large earthquakes during the Tokha phase
For the estimation of the recurrence period of earthquakes based on the sediment records, the chronological control is crucial for obtaining the more reliable value (e.g. Agnon et al. 2006). The ages of the lower and upper limits of the main body of the Tokha Formation (20 ka and 17 ka) are used for the recurrence period estimation and are suitable for its estimation due to the following reasons: (1) the lower limit, dated as 20 ka (17 ka in the uncalibrated age) was obtained from a well-preserved single pine cone collected from the base of the main body of the Tokha Formation (i.e. the age was obtained from the analysis of the ideal sample) and (2) the upper limit of the formation is also constrained by the oldest 14C age of the Patan Formation (17 ka: Sakai et al. 2008). Absence of unconformities and paleosols in this formation, suggestive of a continuous lake-level rise and sediment accumulation in the Tokha phase, is another suitable condition for the recurrence period estimation from this formation. The thickness of sediments between two seismites in the studied sections is almost constant (Fig. 5), implying the nearly-constant time intervals between two large earthquakes.
In this study, at least twelve seismites were identified from the Tokha Formation. Dividing the time period of the Tokha Formation by the number of seismites provides a simple estimate of the recurrence period of earthquakes, which is about 0.25 ka. Because some part of the section is missing due to outcrop loss, the presence of other seismites is expected in the missing interval as the case of SSDs interval between J and K at Loc. 9-1 which was ignored in this paper: thus, the period of large-earthquakes may be shorter than this value.
Mugnier et al. (2011) compiled historical earthquake records and showed that strong earthquakes with an MMI value larger than X occurred three times in the last 800 years. The time interval between the two major earthquakes of AD 1408 and 1934 is approximately 0.52 ka, but the time interval between the AD 1255 and AD 1408 events is about 0.15 ka. The average time interval of these earthquake events is 0.34 ka, which is close to the recurrence period obtained in this study. The value obtained in this study, 0.25 ka, for earthquakes that have intensity and magnitude strong enough to cause surface ruptures and sediment deformation in the muddy sediments of the basin seems to be appropriate, although additional detection of seismites is required to obtain a more reliable value for the recurrence period.