Segmentation of seismicity along the Himalayan Arc due to structural heterogeneities in the under-thrusting Indian plate and overriding Himalayan wedge

The seismicity along the Himalayan arc varies significantly with some well defined patterns of segmentations. We show that the earthquakes along the Himalayan Seismic Belt (HSB) are influenced by the structural heterogeneities existing in the underthrusting Indian plate as well as by the presence of various north-south trending active rifts in the overriding wedge of the Himalaya. Model calculations of stress distribution show that stress change due to strain accumulation is more on moderately steeper fault, simulating the mid-crustal ramp under the southern Higher Himalaya, than on the sub-horizontal thrust. Thus it is surmised that the presence of the ramp which connects the shallow section of seismically active detachment to the aseismically slipping deeper section of the detachment, causes high seismicity in the HSB region. It implies that the seismicity variation along the HSB may also be linked to the presence or absence of the ramp. Further, subsurface ridges on the down-going Indian plate probably control the rupture extent of the great Himalayan earthquakes. Over these ridges the seismicity of the HSB is generally low and it may imply that ramp may be absent in those regions. Finally, we show that the approximately north-south extending active rifts of the Tibetan and Higher Himalaya, cause stress shadow near their southern extent in the HSB which inhibit the occurrence of small and moderate magnitude thrust earthquakes.