Jhiku Khola fault
The Jhiku Khola fault was recognized by Yagi et al. (2000), who found the strike-parallel bulge, depression and range-facing scarp on the colluvial fan surfaces along the eastern margin of the Panchkhal basin which lies to the east of the Kathmandu Valley. They recognized a NNW–SSE-trending and southwest-dipping thrust fault. Based on the combination of the 14C dating of the fan deposit and vertical displacement caused by the fault, they also concluded that the fault had a relatively slow rate of vertical slip with an average of about 0.5 mm/year. Our findings were that the fault extends further north up to Kodari Village situated at the right bank of Indravati River following the left bank of Chu Khola (Fig. 2a, b). The fault is at least ~17 km long, and the surface trace is well characterized by the strike-parallel bulge, strike-parallel depression, range-facing fault scarp and five stream offsets. The existence of right-lateral stream offsets suggests that it is a right-lateral strike-slip fault with thrust component. Since the southern extension of the fault connects with the northern end of the Sun Kosi–Rosi Khola fault system (Nakata 1982), which is also considered to have a right-lateral strike-slip component, these two faults must represent a continuous single fault system. Below, we described the characteristics of the fault based on the tectonic landforms and geologic features.
Along the left bank of the Jhiku Khola, two levels of terraces, one at higher (Tj1) and another at lower (Tj2) level, and the colluvial fan surface (Fs) are observed, and the both terraces are covered with the Fs deposit yielding 16 ka of age (Yagi et al. 2000). At Jaretar and Tinghare, the strike-parallel bulge and depression are observed on the lower part of the Fs surface, indicating that the fault movement has occurred during the Holocene. We made the detailed DSM image (Fig. 3a) here using the SfM-MVS software processing more than 200 sheets of photographs taken by the camera on the tip of the 10-m-high pole (Fig. 3b). This image provided us with clear tectonic feature of the bulge and the depression. Here, we measured approximately 7.5 m vertical displacement due to the fault slip on Fs surface (Fig. 2c). The error of the measurement is 0.2 m.
There is the north-trending bulge and depression at Tinghare. We considered two parallel faults in this area: one fault below the bulge as a thrust fault, and the other fault along the boundary between the bulge and the depression. The thrust fault below the bulge appears in the outcrop (Fig. 4a, b). The fault with N–S trending and 45°E dip offsets the Tj2 terrace deposit and the Fs deposit. The long-term fault movement has played a role to uplift the bulge. As the length of the bulge is 500 m and the bulge is not observed toward the north, the thrust fault is regarded as a secondary fault. However, we could not find the geological evidence relating to the other fault along the boundary between the bulge and the depression; we regarded the fault as a main fault (Fig. 4c), because the trace is recognized as a continuous depression on the Fs terrace toward the north.
The right-lateral offsets of the stream were observed at north of Shikharpur, Bhatpolebesi, Jaishithok, Mandan and Narvadapati along Cha Khola (Fig. 2b). All the lateral offsets except north of Shikharpur are located on the left bank of Cha Khola and deflected toward the upper stream of Chu Khola, implying that the pattern of these offsets is uphill flowing and provides the sure evidence of active faulting. Along Chu Khola, we classified two levels of the terraces of higher one (Tc1) and lower one (Tc2), and Talus surfaces (Ts), and alluvial plain. On the Tc2 terrace riser at Jaishithok, we observed the fault plane with N10°W trending and 80°E dip in the strongly weathered phyllite belonging to the Lesser Himalayan rocks (Fig. 5) and 2.2-m-wide fault breccia along the plane. At Narvadapati, a NNW-trending west-facing fault scarp has developed on the Ts surface, and the stream dissecting the surface shows right-lateral offset of approximately 7 m (Fig. 6a). There is an outcrop of the fault plane with N30°W trending and 80°W dip in the phyllite rocks at the stream-offset site (Fig. 6b). We also found an exposure of the upper Ts deposit that is composed of slightly reddish clay with granule, and 20-cm-thick inclined granule bed within the clay deposit (Fig. 6c). The exposure is located more than 3 m above the stream, which dissected the Ts surface. A charcoal sample in the clay bed 50 cm below the top surface was collected for 14C dating and yielded an age of 870 ± 30 y.B.P. (Beta-416339), implying that the Ts surface formed close to 870 y.B.P, and then, the stream initiated to dissect the Ts surface. Thus, either a single event or multiple events causing the 7-m offsets have occurred the last 870 years, and the slip rate we estimate roughly is around 8 mm/year. Since we could collect only one dating sample in Ts deposit and total stratigraphy of the Ts deposit was not known, this rate required to reexamine in the future. However, the slip rate of the previous study was only 0.5 mm/year (Yagi et al. 2000), and the slip rate could be faster than the previous one.
We surveyed more than 20 sites along the Jhiku Khola fault, but could not find any surface rupture as a result of the April 25 Gorkha earthquake; at the same time, there was no eyewitness account regarding the presence of any coseismic surface rupture.
Kulekhani and Chitlang faults
All the three NW–SE trending active faults, viz. Malagiri, Chitlang and Kulekhani faults, located on the southern side of the Chandragiri Mountain Range forming the southern boundary of the Kathmandu Valley, have their down-thrown side to the northeast (Nakata 1982; Yagi et al. 2000) (Fig. 7a). The fault lengths of the Malagiri fault, Chitlang fault and Kulekhani fault are 3.5, 7.3 and 10.2 km, respectively. The Chitlang fault runs parallel to the 3-km-long gap zone between two other faults. Yagi et al. (2000) showed them as normal faults. Based on the displacement on faulted fan surface and the 14C age of its deposit, we calculate the late Quaternary vertical slip rate of the Chitlang fault to be about 0.9 mm/year (Yagi et al. 2000). During the present survey, we surveyed three sites along the Kulekhani fault (Fig. 7b–e) and one site along the Chitlang fault (Fig. 7f), and no coseismic surface ruptures were seen anywhere along the faults as a result of the 2015 Gorkha earthquake.
Kolphu Khola fault
This ENE–WSW trending fault is located on the right bank of the Kolphu Khola, a tributary of the Trisuli River flowing through the area to the northwest of the Kathmandu Valley (Nakata 1982) (Fig. 8). North-facing fault scarps on mountain slopes and the fluvial terraces observed along the fault, suggesting that vertical displacement is down on the north (Nakata 1982). Nakata et al. (1984) have shown this fault as the active normal fault based on the observation of outcrop of the fault plane. According to our survey, this fault is traceable for a length of 15 km and found that there are two right-lateral stream offsets along the trace of this fault. The mode of the fault may be not only normal faulting, but also right-lateral component. Future investigations relating to fault movement are needed. We surveyed most prominent and recognizable tectonic landforms along this fault around Ratatar, but no serious damage of houses near the fault scarp and no surface deformation were found, suggesting that no coseismic surface rupture has occurred on this active fault during the 2015 earthquake.