Absolute gravity repeated measurements at Pixian station, about 35 km away from the epicenter of the 2008 Wenchuan Mw 7.9 earthquake, have been carried out over more than a decade. Four pre-earthquake measurements at the Pixian station show ~ 30 μGals increase from 2002 to 2008, but 13 measurements since 2008 shows that the trend of gravity increase ended after the Wenchuan earthquake. We analyzed the gravity effects from ground vertical motions using data from continuous GPS stations collocated with the Pixian absolute gravimetry station, and surficial and hydrological processes using local hydrological data. We found that these effects are much smaller than the observed gravity increase before the earthquakes, the continuously gravity increase before the 2008 Wenchuan earthquake cannot be explained by the groundwater level change and vertical deformation and related superficial reasons. All absolute gravity was measured by the FG-5 gravimeter. In this study, the high-precision gravity change revealed that the pre-earthquake gravity increase may be caused by strain and mass (fluid) transfer in broad seismogenic source regions. Further studies are needed to validate such pre-earthquake gravity changes, which however are difficult to be resolved from space-based gravity models.

Gravity Changes Before and After the 2008 M w 7.9 Wenchuan Earthquake at Pixian Absolute Gravity Station in More Than a Decade

We address the role of the concave and convex arcs (as observed from the subducting plate) on the deformation occurring along the Myanmar-Andaman-Sumatra margin. We categorize the pre- and post-seismic deformations of the lithosphere using earthquake database occurring either prior to 26th December 2004 M
 w 9.3 off-Sumatra mega-event or after the incidence. Analysis under pre-seismic domain shows that area near Sumatra records highest seismicity, which largely drops in the area past the North Andaman, and further increases towards north. Shallowest depth and minimum dip of the subducting lithosphere is recorded at the central segment where the arc transformed into concave shape. The annual moment energy release during earthquake decreases to more than two orders of magnitude past the North Andaman towards north under post-seismic deformation phase. Higher depths of continuity of events are presumably associated with more dipping Benioff zones in both the Indo-Myanmar and Andaman-Nicobar convex arcs. These observations obviously account for tectonic subdivision of the margin near concave shape arc around the central part. Absence of volcanism, presence of splay faults in the back-arc, sharp reduction in seismicity near central segment are interpreted to be caused by major tectonic impact of the NNE-ward converging buoyant Ninety-east Ridge against the Asian Plate. Shallowest dip, small elastic thickness, weak converging Indian lithosphere, and evidences of series of en-echelon blocks off the eastern side of the broken northern Ninetyeast Ridge might be incapable of generating great earthquake in this area.

Myanmar-Andaman-Sumatra subduction margin revisited: Insights of arc-specific deformations

The Andaman–Sumatra subduction margin is identified as a complex tectonic transition zone between the Myanmar and Java regions. The Late Cenozoic evolution of various basins, faults, plate slivers, and the Andaman back‐arc rift‐transform system altered the boundary. The sharp changes in free air gravity anomaly, north‐eastward swing of the Andaman Sea spreading ridge, increased dip of the subducting plate and the decrease in seismicity towards the north of the study area possibly account for increased interaction of the Ninetyeast Ridge (NER) against the Andaman Trench. Stress fields of the subducting plate were modified from semi‐radial compression (SRC) during pre‐seismic deformation to pure extension (PE), semi‐radial extension (SRE), and strike‐slip extension (SSE) during post‐seismic deformation phase in the central segment. The co‐seismic second energy burst, subsidence in the overriding plate, and subsequent reactivation of the Barren and Narcondum volcanoes also corroborate the transformation of the stress fields in the overriding plate into predominant extension. The dominant SRC stress in the subducting Indo‐Australian lithosphere in the south changed to pure compression (PC) prior to the 2004 MW 9.3 mega‐event. While the changes in stress field from SRC to pure strike‐slip (PSS) in the north account for stress relaxation after the occurrence of the mega‐event and the co‐seismic ~1,300‐km rupturing, the occurrences of uplift and subsidence in the northern part of the study area were possibly caused by inherited stress field from the Eastern Himalaya. The continued overall compression in the subducting Indo‐Australian Plate in the south, under pre‐ and post‐seismic deformation phases, is presumably correlated with the focal mechanism stress parameters of the 2004 mega‐event, whereas the strike‐slip‐dominated movement in the overriding plate remained almost unchanged. The stress perturbations in the central segment are interpreted to be the result of interaction between the NER and Andaman–Sumatra Trench.

Change of stress patterns during 2004 MW 9.3 off-Sumatra mega-event: Insights from ridge–trench interaction for plate margin deformation

An increase in the number of earthquakes and subsequent clustering in northwest India, particularly around the Delhi-National Capital Region (NCR) and adjacent NW Himalayan front, provides a good opportunity to understand the underpinning tectonic controls and the likelihood of any large earthquake in the future. The 2001 Mw 7.7 Bhuj, 2011 Mw 6.9 Sikkim and 2015 Mw 7.8 and 7.3Nepal earthquakes (and2004 Mw 9.2 Sumatra event) are important in this context. We analysed the seismicity around the Delhi-NCR and the adjoining Himalayan front, including event clustering and the spatio-temporal distribution of b-values, in the context of kinematics and the regional geodynamics. The overall moderate-to-low b-values, both in time and space, since 2016 provide information regarding an increase and subsequent stabilisation of the stress field in the study area. The analysis led to the identification of (i) a structurally guided stress field in the region between the Kachchh and the NW Himalaya that coincides with the direction of Indian plate convergence and (ii) frequent occurrences of earthquakes particularly in the Delhi, Kangra and Uttarkashi areas. We propose that faults in western Peninsular India, which pass through the margins of the Aravalli Range and the Marwar basin, the isostatically over-compensated Indo-Gangetic Plains beneath the Delhi and the Himalayan foothills of the under-plated Indian lithosphere, act as stress guides, concentrating and increasing stress in regions of lithospheric flexure. This enhanced stress may trigger a large earthquake.

/pdf/earthquake-shocks-around-delhi-ncr-and-the-adjoining-1036v6ws8k.pdf

Earthquake Shocks Around Delhi-NCR and the Adjoining Himalayan Front: A Seismotectonic Perspective

Active thrust induced realignment of recent near-surface stresses in the Darjeeling-Sikkim Himalayas: Reasons and implications

The occurrences of moderate to large magnitude earthquakes and associated subsurface geological processes were critically examined in the backdrop of Indian plate obliquity, stress obliquity, topography, and the late Tertiary regional tectonics for understanding the evolving dynamics and kinematics in the central part of the Himalayas. The higher topographic areas are likely associated with the zones of depressions, and the lower topographic areas are found around the ridges located in the frontal part of the orogen. A positive correlation between plate and stress obliquities is established for this diffuse plate boundary. We propose that the zone of sharp bending of the descending Indian lithosphere is the nodal area of major stress accumulation which is released occasionally in form of earthquakes. The lateral geometry of the Himalayas shows clusters of seismicity at an angle of ∼20∘ from the centre part of the arc. Such spatial distribution is interpreted in terms of compression across the arc and extension parallel to the arc. This biaxial deformation results in the development of dilational shear fractures, observed along the orogenic belt, at an angle of ∼20∘ from the principal compressive stress axis.

/pdf/earthquake-source-characteristics-along-the-arcuate-j30pcx5pfh.pdf

Earthquake source characteristics along the arcuate Himalayan belt: Geodynamic implications

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Earthquake source characteristics along the arcuate Himalayan belt: Geodynamic implications