Active transverse faulting within underthrust Indian crust beneath the Sikkim Himalaya

Abstract

Deep focus earthquakes within the underthrust Indian lower crust beneath the Himalaya occur in very specific regions and have distinct source characteristics. The study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust Indian Plate, and its influence on the active deformation of the overlying Himalayan wedge. One of the most significant regions of these deep focus earthquakes is beneath the Sikkim and Bhutan Himalaya. We study the source characteristics of the 2011 September 18 (M w 6.9) deep focus Sikkim main shock and its major aftershocks using global, regional and local waveform data. We determined the focal mechanism of the main shock using moment tensor inversion of global P and SH waveforms, and ascertained the earthquake fault plane using rupture directivity from regional P-wave spectra. The main shock originated at 53 ± 4 km depth and ruptured at least 20 km thickness of the underthrust Indian lower crust. Faulting occurred on a near vertical dextral strike-slip fault oriented NW-SE (strike 127 • , dip 81 • and rake 167 •), oblique to the local strike of the Himalayan arc. The rupture initiated from the SE end of the fault and propagated to the northwest. The main shock was followed by 20 small-to-moderate aftershocks (m b > 3.0), which we relocated using phase arrival times. We computed the focal mechanisms of the larger ones (m b ≥ 3.5) using local waveform inversion. We find that all aftershocks originated SE of the main shock, between depths of 12 and 50 km, and have dominantly strike-slip mechanisms. Our results, combined with the source mechanisms of earthquakes from previous studies, reveals that the entire underthrust Indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the Sikkim and Bhutan Himalaya. These active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating 'microplates' in northeast India observed from GPS geodesy.

Three‐Dimensional Crustal Architecture Beneath the Sikkim Himalaya and Its Relationship to Active Deformation

“…These structures are marked by dashed lines within an ellipse (Figure c) and are observed to intersect the Moho and the midcrustal discontinuity. These low‐velocity zones may be active transverse structures within the underthrust Indian crust, similar to the ones which caused dextral strike‐slip earthquakes beneath the Sikkim Himalaya (Paul et al, ). A couple of strong negative arrivals are observed in the CCP, between stations NAMC and RABN at a depth of ∼20 km and between stations RABN and GTOK at depth range 30–40 km.…”

“…To the west, the central Himalaya deforms by arc‐normal crustal shortening (Avouac, ; Avouac & Tapponnier, ), while to the east, northeast India undergoes oblique convergence across the Eastern Himalaya and E‐W convergence across the Indo‐Burma subduction zone (Kumar et al, , Li et al, , ; Lei & Zhao, ; Mitra et al, ; Ni et al, ), resulting in widespread deformation south of the Himalaya. The whole thickness of the Sikkim Himalayan crust is seismogenic, displaying two distinct regimes of tectonic deformation: (a) crustal shortening within the Himalayan wedge and (b) strike‐slip motion within the underthrust Indian crust (Paul et al, ). Available 1‐D shear wave velocity models from previous seismological studies in the Sikkim Himalaya have highlighted first‐order variation in crustal structure (Acton et al, ; Singh et al, ).…”

“…This was interpreted as molasse sediment from the Siwaliks and Lesser Himalaya or as trapped metamorphic fluids lubricating the underthrusting of the Indian Plate. Paul et al () studied the source mechanisms of the 2011 Sikkim earthquake and its aftershocks, which originated in the underthrust Indian crust, demonstrating the presence of active transverse strike‐slip faults beneath the Sikkim Himalaya. The 2006 Phodong earthquake, which originated within the Himalayan wedge, had a thrust fault mechanism highlighting the different deformation regimes in the underthrust Indian crust and the overriding Himalayan wedge.…”

The 2015 April 25 Gorkha (Nepal) earthquake and its aftershocks: implications for lateral heterogeneity on the Main Himalayan Thrust

“…The details of this methodology has been described in Nabelek (1984); McCaffrey & Nabelek (1987); Berberian et al (1999);Paul et al (2015); Kumar et al (2015) and we will briefly outline the main steps. The algorithm estimates the strike and dip of the nodal plane(s), rake of the slip vector(s), centroid depth, and the variation of moment release with time (the STF).…”

“…The uncertainties on the modelled parameters are estimated through a number of tests described in details in our previous studies Paul et al 2015). We estimate the uncertainty in depth of the main-shock line source solution by fixing the depth of the earthquake to values bracketing the best-fitting solution and reinvert the waveform data for the other focal parameters.…”