The M w 6.9 Sikkim–Nepal earthquake of September 2011: a perspective for wrench faulting in the Himalayan thrust zone

Abstract

In the present study, we analyze the strike-slip-type earthquakes along the Himalayan arc that are transverse to the collision front, with a special emphasis on the Sikkim region. The study examines seismicity and focal mechanism data in conjunction with crustal structure derived from receiver functions and gravity gradients, supported by results from GPS data. We present a relation between the north-south depth distribution of the strike-slip earthquakes in Sikkim, with the northward dipping trend of the Indian crust beneath Eurasia deciphered from teleseismic receiver function analysis of data recorded by our seismic network. Results suggest that transverse faults caused by thrust partitioning along the Himalayan arc manifest as vertical strike-slip faults cutting across the crust of the descending Indian plate down to 60 km. Further, we argue in favor of a consistent rightlateral sense of slip among all the strike-slip-type earthquakes cutting across the Himalayan arc from Kumaon in the west through Sikkim and Bhutan Himalaya, to the Burmese arc and the Sagaing fault on the easternmost end. We infer a wrench fault tectonic model for the Indian plate described by a series of right-lateral strike-slip faults from west to east in the Himalaya, in conjunction with under-thrusting of the Indian plate beneath Eurasia.

Indian plate structural inheritance in the Himalayan foreland basin, Nepal

“…Deep‐seated lineaments parallel to the edges of the Delhi‐Haridwar, Faizabad, and Munger‐Saharsa ridges represent surfaces that extend as deep as the base of the Indian lithosphere (Godin & Harris, 2014), and in the case of several ridges, appear to show opposing senses of dip. Several mapped basement faults align with these lineaments, including the Great Boundary, Lucknow, Kishangang, and West/East Patna Faults (Godin & Harris, 2014; Rao et al., 2015; Valdiya, 1976). Some of these faults have been interpreted to be active based on observations of recent soft sediment deformation structures (e.g.…”

Source characteristics of the 18 September 2011 Sikkim earthquake and zoning

“…Kumar et al [2012] inferred eclogitization of the lower crust in the Sikkim and southern Tibet regions wherein jelly sandwich model could explain seismogenesis by assuming a strong lower crust and up-per mantle. Rao et al [2015] used receiver function analysis and suggested that transverse faults caused by thrust partitioning along the Himalayan arc manifest as vertical strike-slip faults cutting across the crust of the descending Indian plate down to 60 km. de la Torre et al [2007] suggested that in the eastern Nepal and Tibet region, strike-slip earthquakes at depths of 70-100 km and thrust earthquakes at shallower depths could be attributed to Indian plate convergence accommodated through shear along vertical fault planes that extend to Moho depths.…”

Seismicity and structure of the Indian subcontin

“…However, comprehensive aftershock studies using temporary networks are carried out for the recent damaging earthquakes since 1991. These significant earthquakes are the 1991 Uttarkashi (M w 6.3) and 1999 Chamoli (M w 6.3) earthquakes in the western Himalaya, the 1993 Latur (M w 6.3), the 1997 Jabalpur (M w 6.0) and the 2001 Bhuj (M w 7.7) earthquakes in peninsular India, the 2002 Andaman (M w 6.5) and the 2004 Sumatra (M w 9.3) earthquakes in the subducting Indian oceanic plate, the 2005 Kashmir (M w 7.6), the 2009 Bhutan (M w 6.3) and the 2011 Sikkim (M w 6.9) earthquakes in different segments of the Himalayas and the 2016 Manipur (M w 6.7) earthquake in NE India region (e,g, Kayal 2008Rao et al, 2015;Singh et al, 2017 and references therein). Seismotectonics of the main shocks and aftershock sequences of the above strong / large earthquakes are well studied, that shed fair…”