Orogenic Collapse and Stress Adjustments Revealed by an Intense Seismic Swarm Following the 2015 Gorkha Earthquake in Nepal

Lok Bijaya Adhikari,Lily Li,Nicolas Wendling-Vazquez,Marine Laporte,Sophie Lambotte,Frédéric Girault,Laurent Bollinger,Kristel Chanard,Jérôme Vergne,Nabina Bishwokarma,Chintan Timsina,Frédéric Perrier,Mukunda Bhattarai,Bharat P Koirala

doi:10.3389/feart.2021.659937

Abstract

The April 25, 2015 Mw 7.9 Gorkha earthquake in Nepal was characterized by a peak slip of several meters and persisting aftershocks. We report here that, in addition, a dense seismic swarm initiated abruptly in August 2017 at the western edge of the afterslip region, below the high Himalchuli-Manaslu range culminating at 8156 m, a region seismically inactive during the past 35 years. Over 6500 events were recorded by the Nepal National Seismological Network with local magnitude ranging between 1.8 and 3.7 until November 2017. This swarm was reactivated between April and July 2018, with about 10 times less events than in 2017, and in 2019 with only sporadic events. The relocation of swarm earthquakes using proximal temporary stations ascertains a shallow depth of hypocenters between the surface and 20 km depth in the High Himalayan Crystalline slab. This swarm reveals an intriguing localized interplay between orogenic collapse and stress adjustments, involving possibly CO2-rich fluid migration, more likely post-seismic slip and seasonal enhancements.

Highlights

Crustal processes are important to understand in the Himalayas, where the convergence between India and Eurasia plates is able to produce in the near future a megaquake expected to affect Nepal and Northern India, one of the most-densely populated area on Earth (e.g., Bilham, 2019)
We found no candidate for a dynamic triggering by long-period surface waves, a phenomenon described further west in KumaonGarhwal Himalaya, with local microearthquake activity triggered by the surface waves of the 2007 Mw 8.5 earthquake (Mendoza et al, 2016)
The earthquake ruptured a 140 × 50 km2 locked fault patch to the South-East which was responsible for the subsidence of the high Himalayan range (e.g., Avouac et al, 2015; Grandin et al, 2015; Figure 4A)

Summary

Introduction

Crustal processes are important to understand in the Himalayas, where the convergence between India and Eurasia plates is able to produce in the near future a megaquake expected to affect Nepal and Northern India, one of the most-densely populated area on Earth (e.g., Bilham, 2019). The front of the high Himalayan range is located above the downdip-end of locked fault segments (e.g., Avouac et al, 2001; Bollinger et al, 2004; Ader et al, 2012; Lindsey et al, 2018; Ingleby et al, 2020) of the shallow dipping Main Himalayan Thrust (MHT) fault system (Nabelek et al, 2009). These segments break during large and great earthquakes, leading instantly to the coseismic subsidence of the high mountain range. The strain and stress rates within the hanging wall of the Earthquake Swarm in Nepal Himalaya

Methods

Results

Discussion

Conclusion