Seismicity in Far Western Nepal reveals flats and ramps along the Main Himalayan Thrust
Abstract
Unravelling relations between lateral variations of mid-crustal seismicity and the geometry of the Main Himalayan Thrust system at depth is a key issue in seismotectonic studies of the Himalayan range. Indeed, these relations may reveal along-strike changes in the behaviour of the fault at depth related to fluids, the basal friction, the thermal structure or the local ramp-flat geometry and more generally of the stress build up along the fault. Some of these variations may control the may control the rupture extension of intermediate, large or great earthquakes and are therefore crucial to document for better assessing the regional seismic hazard. Far Western Nepal is particularly exposed to large and great earthquakes, the last of which dates back from 1505 CE. The region is also associated to striking lateral spatio-temporal variations of the mid-crustal seismicity monitored by the Regional Seismic Network of Surkhet-Birendranagar. This network was supplemented between 2014 and 2016 by 15 temporary stations deployed above the main seismic clusters giving new potential to regional studies. Absolute, relative locations and focal mechanisms were determined to gain insight on the fault behaviour at depth. We found that the microseismicity is clustered in three belts parallel to the front of the Himalayan range and confined between the Main Boundary Thrust and the Main Central Thrust. Finest locations reveal close relationships between seismic clusters and fault segments at depth among which two mid-crustal ramps and reactivated tectonic slivers. Interpreted fault planes of computed focal mechanisms are consistent with compressive stress and regional deformation inferred from geodetic studies. Our results support a geometry of the Main Himalayan Thrust involving several fault patches at depth separated by 3 to 6 km-high ramps and tear faults. This has an impact on the assessment of the geometry of the coseismic ruptures breaking partially or totally the locked fault zone in the future.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T43A..07L
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8004 Dynamics and mechanics of faulting;
- STRUCTURAL GEOLOGY;
- 8123 Dynamics: seismotectonics;
- TECTONOPHYSICS