Rupture History of the Himalayan Frontal Thrust near Bagmati River crossing in Central Nepal

Koji Okumura, Prakash Pokhrel, Hisao Kondo, Tomoru Yamanaka, & Soma N. Sapkota

Submitted September 11, 2022, SCEC Contribution #12400, 2022 SCEC Annual Meeting Poster #097

The 2015 Golkha earthquake raised concern about the future large earthquake in south of the 2015 source area. The Golkha earthquake ruptured 10 to 20 km deep portion of the plate boundary interface ~50 km away from the surface trace of the Himalayan Frontal Thrust (HFT). Therefore it is necessary to evaluate the earthquake potential of the shallowest part of the thrust in central Nepal. After 2018 and 2019 paleoseismology works in Butwal area, in 2020 we excavated two trenches on the Himalayan front in east and in west of Bagmati river to examine the terminations of past events and to know the rupture history. At the Dumachaur Khola site 4 km east of Bagmati river, 3 or 4 events in 3500 years are excavated. The penultimate event here is dated around 3000 year B.P.. At the Gopalkoti-Dandatol site, 0.7 km west of Bagmati river, 3 events between 7th and 14th century C.E. are recognized. 1934 ruptures do not appear and 1255 (or 12th century) event is the last event in both trenches. The exposures were excellent with the master thrust fault through layered fine sedements containing many charcoal pieces, but the interpretation of the even time-series is difficult. Findings from three trenches around Butwal and these two across Bamati River are as follows. (1) 1934 rupture did not reach Bagmati River in west. (2) In west of Butwal, the timing of the last surface rupture coincides with 1344 earthquake without any evidence of 1255 (or 13 century) and 1505 ruptures around Butwal. (3) The last event around Bagmati River crossing occured in 1255 or in 12th centrury. (4) The timing of the penultimate event in west of Bagmati River is about 1000 year B.P. while it is about 3000 year B.P. in east of Bagmati River. Though a lot of paleoseismological works has been carried out along the HFT in central Nepal and Central Seismic Gap, we still know little about penultimates event on most segments. . After the pandemic we need to resume and continue investigations.

Key Words
Himalayan Front, paleoseismology, surface rupture

Okumura, K., Pokhrel, P., Kondo, H., Yamanaka, T., & Sapkota, S. N. (2022, 09). Rupture History of the Himalayan Frontal Thrust near Bagmati River crossing in Central Nepal. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology