- Open Access
Geomorphic features of active faults around the Kathmandu Valley, Nepal, and no evidence of surface rupture associated with the 2015 Gorkha earthquake along the faults
© Kumahara et al. 2016
- Received: 30 October 2015
- Accepted: 18 March 2016
- Published: 2 April 2016
The M7.8 April 25, 2015, Gorkha earthquake in Nepal was produced by a slip on the low-angle Main Himalayan Thrust, a décollement 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.
- 2015 Gorkha earthquake
- Active fault
- Surface rupture
- Tectonic landform
- Jhiku Khola fault
Our mapping of the tectonic landform and fault traces was based on both of the interpretation of aerial photographs at scales of 1:50,000, and the field survey along the fault. 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. We also interviewed local people to find out whether or not some unusual features have developed immediately after the main shock in nearby areas along the trace of the fault, such as surface ruptures and offsets.
Also, we made the detailed digital surface model (DSM) image for the Jhiku Khola fault using the Structure from Motion–Multi-view System (SfM-MVS) software ‘Agisoft Photoscan Professional’ processing photographs at the different height ranging 10 m high to the ground level taken by the camera on the tip of the 10-m-high pole. The camera was connected with a smart phone through Wi-Fi, and we could monitor the finder view of the camera and shatter by handing the smart phone. Goto (2015) developed this technique and confirmed that the accuracy of the DEM using by the technique is equivalent to that of the data measured by the total station.
Jhiku Khola fault
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
Kolphu Khola fault
We show geological and geomorphological evidences of the late Quaternary activity of the active faults around the Kathmandu Valley, and no faults played any parts in triggering this large earthquake. Angster et al. (2015) claimed that no surface rupture along the trace of the HFT, the pattern of InSAR interferograms, focal mechanism and aftershock distribution of the event indicated that it was a low-angle thrust event on the MHT and the southern tip of the rupture ended below the mountain over 30 km north of the Gangetic plain. Our results also supported this idea.
The existence of an active fault reflects a potential seismic hazard for a shallow-depth earthquake around the fault. For example, the 2005 Kashmir earthquake (Mw = 7.6) in Pakistan occurred on a previously mapped active fault (Nakata et al. 1991; Kumahara and Nakata 2006), and the fault produced an average event interval of ~2 k.y. for the 2005 earthquake type events (Kondo et al. 2008). In the present situation, it is difficult to evaluate the potential seismic hazard due to lack of paleoseismological data of those faults; however, it is needed to carry this study forward for assessments of the Kathmandu Valley.
The Jhiku Khola fault, which shows a right-lateral strike-slip movement with 8 mm/yr of the slip rate, may play an important role in the active tectonics of the Himalayan range. Nakata (1989) claimed that an en-echeloned active right-lateral strike-slip fault system from northwest Nepal to eastern Nepal cuts obliquely across the Himalayan range due to slip partitioning, such that the block lying to the southwestern side of the fault moves northwestward with respect to the block lying to the northeastern parts. It is possible that the Jhiku Khola fault constitutes one of the members of this fault system.
YK planed and did all the fieldwork and wrote all the text and drew all the figures. DC discussed the interpretation of tectonic landform with YK and BU and interviewed the local people regarding to the surface rupture. BU discussed the interpretation of tectonic landform with YK and DC. All authors read and approved the final manuscript.
This work was supported by Grant-in-Aid for Special Purposes for the 2015 Nepal Earthquake Disaster Emergency Survey Research Project by the Ministry of Education, Science, Sports and Culture, Japan to YK. We thank Bibek Giri and Upendra Bayal, who led us to support our fieldwork, and Stephen Angster, who provided us the basal map of Fig. 1. We are grateful to anonymous reviewers for providing valuable comments and suggestions, which helped us in improving the manuscript.
The authors declare that they have no competing interests.
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