Segmentation of the Himalayan megathrust around the Gorkha earthquake (25 April 2015) in Nepal

Abstract

We put the 25 April 2015 earthquake of Nepal (Mw 7.9) into its structural geological context in order to specify the role of the segmentation of the Himalayan megathrust. The rupture is mainly located NW of Kathmandu, at a depth of 13-15 km on a flat portion of the Main Himalayan Thrust (MHT) that dips towards the N-NE by 7 to 10°. The northern bound of the main rupture corresponds to the transition towards a steeper crustal ramp. This ramp, which is partly coupled during the interseismic period, is only locally affected by the earthquake. The southern bound of the rupture was near the leading edge of the Lesser Himalaya antiformal duplex and near the frontal footwall ramp of the upper Nawakot duplex. The rupture has been affected by transversal structures: on the western side, the Judi lineament separates the main rupture zone from the nucleation area; on the eastern side, the Gaurishankar lineament separates the 25 April 2015 rupture from the 12 May 2015 (Mw 7.2) rupture. The origin of these lineaments is very complex: they are probably linked to pre-Himalayan faults that extend into the Indian shield beneath the MHT. These inherited faults induce transverse warping of the upper lithosphere beneath the MHT, control the location of lateral ramps of the thrust system and concentrate the hanging wall deformation at the lateral edge of the ruptures.The MHT is therefore segmented by stable barriers that define at least five patches in Central Nepal. These barriers influence the extent of the earthquake ruptures. For the last two centuries: the 1833 (Mw 7.6) earthquake was rather similar in extent to the 2015 event but its rupture propagated south-westwards from an epicentre located NE of Kathmandu; the patch south of Kathmandu was probably affected by at least three earthquakes of Mw ≥ 7 that followed the 1833 event a few days later or 33 years (1866 event, Mw 7.2) later; the 1934 *Manuscript Click here to download Manuscript: Himalaya-Mugnier-JAES_final.docx Click here to view linked References earthquake (Mw 8.4) had an epicentre ~170 km east of Kathmandu, may have propagated as far as Kathmandu and jumped the Gaurishankar lineament.This combined structural approach and earthquake study allows us to propose that the MHT in the central/eastern Himalaya is segmented by stable barriers that define barrier-type earthquake families. However for each individual earthquake within a family, the rupture histories could be different. Furthermore, the greatest earthquakes could have broken the barriers and affected the patches of several families. The concept of a regular recurrence of characteristic earthquakes is therefore misleading to describe the succession of Himalayan earthquakes.

Himalayan earthquakes: a review of historical seismicity and early 21st century slip potential

“…If Baramula and Maru Pergam are omitted from consideration, the remaining sites have a spatial distribution of 80 km along-strike which would reduce the estimated magnitude to M w 7.8, possibly to as low as M w 7.6 if the rupture were steep, and the rupture area reduced in width, as in the recent 2005 Kashmir earthquake. Mugnier et al (2017) argued that this is more likely, and that great earthquakes may not occur on the décollement beneath Kashmir, and instead NW-SE shortening is accommodated on steeply dipping thrust faults. They base this conclusion on an analysis of the several surface offsets of the Riasi Thrust, the thrust fault underlying the southern margin of the Pir Pinjal.…”

Establishing primary surface rupture evidence and magnitude of the 1697 CE Sadiya earthquake at the Eastern Himalayan Frontal thrust, India

“…Based on structural considerations, gravity anomaly variations along the arc, and seismicity, it can be argued that major ridges and transverse faults cross-cutting the Himalayan arc govern the along-strike extent of rupture propagation 37 – 39 . Lateral variability may exist at smaller wavelengths in the form of lateral ramps, for example, and it is possible that while large earthquakes may obey these boundaries, but great earthquakes surpass these discontinuities, rupturing multiple adjacent segments of the fault 40 .…”

Cross-Scale Modeling of Mountain Building and the Seismic Cycle: From Alps to Himalaya

“…4.1a). The 2015 Gorkha earthquake has a similar location as the 1833 earthquake, with estimated magnitude M w 7.6-7.7, which also caused significant damage in Kathmandu (Bilham, 1995;Mugnier et al , 2017). The geometry of the MHT is relatively well known in their hypocentral region from various geological and geophysical campaigns (Lavé & Avouac, 2000;Nábělek et al , 2009;Ader et al , 2012).…”