Structural control on along-strike variation in the seismicity of the northwest Himalaya

Abstract

An overview of seismicity along the northwest Himalaya exhibit along-strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting Indian Plate as well by rift and nappe structures in the overriding wedge of the Himalaya. The segmentation exists for thrust dominated large magnitude earthquakes (M > 6) seated on the active detachment beneath the Outer and Lesser Himalaya. Segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow Himalaya Seismic Belt (HSB) straddling northern Lesser Himalaya and southern Higher Himalaya. Numerical calculations of stress distribution favour that the degree of seismicity in the HSB is a good proxy to the presence of mid-crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath Higher Himalaya. Further, gap or diffused pattern in the concentrated seismicity in the HSB, in agreement with mapped high electrical conductive structure, suggests ramp structure may be absent where underthrusting Delhi-Hardwar Ridge interacts with the Himalaya arc. In the nappe dominated tectonics, the accommodation of the accumulating strains on the listric thrust faults, produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment. This may explain relatively less frequent occurrences of large earthquake in the Kangra–Chamba region compared to that in the Garhwal Himalaya. The low level seismicity in the Simla region may also be influenced by the active Kaurik Chango Rift in the Higher Himalaya, as the former casts a stress shadow on the latter.