Bhaskar Kundu

Oblique convergence and slip partitioning in the NW Himalaya: Implications from GPS measurements

We report GPS measurements of crustal deformation across the Kashmir Himalaya. We combined these results with the published results of GPS measurements from the Karakoram fault system and suggest that in the Kashmir Himalaya, the motion between the southern Tibet and India plate is oblique with respect to the structural trend. We estimated this almost north-south oblique motion to be 17 ± 2 mm/yr, which is partitioned between dextral motion of 5 ± 2 mm/yr on the Karakoram fault system and oblique motion of 13.6 ± 1 mm/yr with an azimuth of N198°E in the northwest-southeast trending Kashmir Himalayan frontal arc. Thus, the partitioning of the India-Southern Tibet oblique motion is partial in the Kashmir Himalayan frontal arc. However, in the neighboring Nepal Himalaya, there is no partitioning; the entire India-Southern Tibet motion of 19–20 mm/yr is arc normal and is accommodated entirely in the Himalayan frontal arc. The convergence rate in the Kashmir frontal Himalaya is about 25% less than that in the Nepal Himalayan region. However, here the Karakoram fault system accommodates about 20% of the southern Tibet and Indian plate convergence and marks the northern extent of the NW Himalayan arc sliver. The Kaurik Chango rift, a north-south oriented seismically active cross-wedge transtensional fault appears to divide the sliver in two parts causing varying translatory motion on the Karakoram fault on either side of the Kaurik Chango rift.

The 4 January 2016 Manipur earthquake in the Indo-Burmese wedge, an intra-slab event

ABSTRACT Earthquakes in the Indo-Burmese wedge occur due to India-Sunda plate motion. These earthquakes generally occur at depth between 25 and 150 km and define an eastward gently dipping seismicity trend surface that coincides with the Indian slab. Although this feature mimics the subduction zone, the relative motion of Indian plate predominantly towards north, earthquake focal mechanisms suggest that these earthquakes are of intra-slab type which occur on steep plane within the Indian plate. The relative motion between the India and Sunda plates is accommodated at the Churachandpur-Mao fault (CMF) and Sagaing Fault. The 4 January 2016 Manipur earthquake (M 6.7) is one such earthquake which occurred 20 km west of the CMF at ∼60 km depth. Fortunately, this earthquake occurred in a very sparse population region with very traditional wooden frame houses and hence, the damage caused by the earthquake in the source region was very minimal. However, in the neighbouring Imphal valley, it caused some damage to the buildings and loss of eight lives. The damage in Imphal valley due to this and historical earthquakes in the region emphasizes the role of local site effect in the Imphal valley.

Influence of anthropogenic groundwater unloading in Indo‐Gangetic plains on the 25 April 2015 Mw 7.8 Gorkha, Nepal earthquake

Groundwater usage in the Indo-Gangetic plains exceeds replenishment of aquifers, leading to substantial reduction in the mass. Such anthropogenic crustal unloading may promote long-term fault slip or may modulate seismic activity in the adjoining Himalayan region. Our simulation using Gravity Recovery and Climate Experiment data and hydrological models of such a process indicates that the thrust earthquakes on the Main Himalayan Thrust (MHT), including the recent 25 April 2015 Mw 7.8 Gorkha, Nepal earthquake, are probably influenced by the anthropogenic groundwater unloading process in the Gangetic plains. The groundwater withdrawal leading to crustal unloading in the Gangetic plains causes a significant component of horizontal compression which adds to the secular interseimic compression at the seismogenic depth (5–20 km) on the MHT beneath the Himalayan arc and at hypocentral depth of the 2015 Gorkha, Nepal earthquake. This effect enhances the Coulomb stress on the locked zone of MHT.

Dynamics of post-slab breakoff in convergent plate margins: a “jellyfish” Model

The Earths subduction zones are characterized by a complex architecture of the subducting slabs, including necking, tearing, detachment from the surface plate, breakoff or ultimate slab loss, in contrast to the idealized models as popularly envisaged. Here we address the fate of the subducted slab after its breakoff by employing a simple two dimensional numerical modeling. Our results agree well with the tectonic scenario and geometric evolution of slab breakoff and reveal that during the sinking of broken slab fragments, the shape of the low viscosity slab (1-1000 times more viscous than the surrounding mantle) evolves into an inverted plume, similar to the “jellyfish” model proposed in some recent studies on deep subduction slabs. Our results confirm that the penetration criteria of the jellyfish into the lower mantle are primarily governed by the viscosity contrast between the lower to upper mantle. An evaluation of the various parameters suggest that the broken slab fragments may achieve a diverse type of morphology, correlating well with a similar diversity observed from high resolution seismic tomographic studies.

Tectonic tremor on Vancouver Island, Cascadia, modulated by the body and surface waves of the Mw 8.6 and 8.2, 2012 East Indian Ocean earthquakes

Author(s): Kundu, B; Ghosh, A; Mendoza, M; Burgmann, R; Gahalaut, VK; Saikia, D | Abstract: ©2016. American Geophysical Union. All Rights Reserved. The 2012 East Indian Ocean earthquake (Mw 8.6), so far the largest intraoceanic plate strike-slip event ever recorded, modulated tectonic tremors in the Cascadia subduction zone. The rate of tremor activity near Vancouver Island increased by about 1.5 times from its background level during the passage of seismic waves of this earthquake. In most cases of dynamic modulation, large-amplitude and long-period surface waves stimulate tremors. However, in this case even the small stress change caused by body waves generated by the 2012 earthquake modulated tremor activity. The tremor modulation continued during the passage of the surface waves, subsequent to which the tremor activity returned to background rates. Similar tremor modulation is observed during the passage of the teleseismic waves from the Mw 8.2 event, which occurs about 2 h later near the Mw 8.6 event. We show that dynamic stresses from back-to-back large teleseismic events can strongly influence tremor sources.

Seasonal modulation of deep slow-slip and earthquakes on the Main Himalayan Thrust

The interaction between seasonally-induced non-tectonic and tectonic deformation along the Himalayan plate boundary remains debated. Here, we propose that tectonic deformation along this plate boundary can be significantly influenced by the deformation induced by the non-tectonic hydrological loading cycles. We explore seasonal mass oscillations by continental water storage in Southeast Asia and Himalayan arc region using continuous Global Positioning System measurements and satellite data from the Gravity Recovery and Climate Experiment. We suggest that the substantially higher transient displacements above the base of the seismogenic zone indicate a role of changes in aseismic slip rate on the deep megathrust that may be controlled by seasonal hydrological loading. We invoke modulation of aseismic slip on the megathrust down-dip of the seismogenic zone due to a fault resonance process induced by the seasonal stress changes. This process modulates mid-crustal ramp associated micro-seismicity and influences the timing of Central Himalayan earthquakes.The interaction between seasonally-induced non-tectonic and tectonic deformation along the Himalayan plate boundary is still debated. Here, the authors propose that seasonal hydrological loading can influence tectonic deformation along this plate boundary using continuous GPS measurements and satellite data.