Abstract
The M7.8 April 25, 2015, Gorkha earthquake in Nepal was produced by a slip on the low-angle Main Himalayan Thrust, a decollement below the Himalaya that emerges at the surface in the south as the Himalayan Frontal Thrust (HFT). The analysis of the SAR interferograms led to the interpretations that the event was a blind thrust and did not produce surface ruptures associated with the seismogenic fault. We conducted a quick field survey along four active faults near the epicentral area around the Kathmandu Valley (the Jhiku Khola fault, Chitlang fault, Kulekhani fault, Malagiri fault and Kolphu Khola fault) from July 18–22, 2015. Those faults are located in the Lesser Himalaya on the hanging side of the HFT. Based on our field survey carried out in the area where most typical tectonic landforms are developed, we confirmed with local inhabitants the lack of any new surface ruptures along these faults. Our observations along the Jhiku Khola fault showed that the fault had some definite activities during the Holocene times. Though in the past it was recognized as a low-activity thrust fault, our present survey has revealed that it has been active with a predominantly right-lateral strike-slip with thrust component. A stream dissecting a talus surface shows approximately 7-m right-lateral offset, and a charcoal sample collected from the upper part of the talus deposit yielded an age of 870 ± 30 y.B.P, implying that the talus surface formed close to 870 y.B.P. Accordingly, a single or multiple events of the fault must have occurred during the last 900 years, and the slip rate we estimate roughly is around 8 mm/year. The fault may play a role to recent right-lateral strike-slip tectonic zone across the Himalayan range. Since none of the above faults showed any relationship corresponding to the April 25 Gorkha earthquake, it is possibility that a potential risk of occurrence of large earthquakes does exist close to the Kathmandu Valley due to movements of these active faults, and more future work such as paleoseismological survey is needed to assess the risk.
Highlights
It became apparent shortly after the M7.8 April 25, 2015, Gorkha earthquake of Nepal that it was a result of thrusting along the Main Himalayan Thrust (MHT) (USGS 2015)
In the field, equipped with the aerial photograph interpretation data, we focused on the study of the nature of tectonic landforms to find the precise location of the fault trace and the evidence of active faulting
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
Summary
It became apparent shortly after the M7.8 April 25, 2015, Gorkha earthquake of Nepal that it was a result of thrusting along the Main Himalayan Thrust (MHT) (USGS 2015). Khola fault (Khola = a stream in Nepali language), Chitlang fault, Kulekhani fault, Malagiri fault and Kolphu Khola fault) around the Kathmandu Valley in the epicentral area based on our brief visit to the area between July 18–22, 2015. Those faults are located in the Lesser Himalaya on the hanging side of the HFT. The results of the survey along the HFT and the Kathmandu southern fault have already been presented by Angster et al (2015)
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