Sujit Dasgupta

Deep structure and tectonics of the burmese arc: constraints from earthquake and gravity data

Abstract Active subduction of the Indian plate is currently occurring beneath the Burmese arc along an east dipping Benioff zone which extends to a depth of about 180 km. The overriding Burma plate has an appearance of an inland seismic slab that is deflected downwards in the vicinity of the Benioff zone. A crustal seismic zone some 60–80 km east of the Benioff zone correlates to backarc activity. A triangular aseismic wedge in the top part of the crust outlines the Central Belt molasse basin east of the Burmese foldbdt. Fault plane solutions show that the Burmese Benioff zone is characterized by shallow angle thrusting at its upper edge whereas down-tip tensional events dominate its lower edge. Most of the backarc seismicity is accounted for by the Sagaing transform or by the activity of the Shan scarp normal fault zone at the margin of the Asian plate. A gravity anomaly pair with amplitude of 175 mGal coincides with the 1100-km long Burmese arc lying in a north-south direction. The gravity anomalies along a profile in central Burma and in adjacent areas of the Bengal basin are interpreted in terms of plate subduction as well as near-surface mass anomalies. This suggests that sediments below the Central Belt may have an average thickness of the order of 10 km but may be as thick as 15 km at the subduction zone. The oceanic crust underlying deeper parts of the Bengal basin experiences phase transition at about 30 km depth in a Benioff zone environment east of the Burmese foldbelt. Several thrust planes are present within the folded and deformed Cretaceous-Tertiary sediments of the fold belt; these are often associated with ophiolites and basic to ultrabasic rocks. A low density zone, at least 60 km wide, underlies the andesitic volcanic axis in the overriding plate.

Active transverse features in the central portion of the Himalaya

Abstract Many transverse lineaments or faults across the Himalaya are already known. Interpretation of satellite images of the central portion of the Himalaya given here adds to that number and suggests their continuity from the Himalaya to its foredeep. Several such faults appear to be seismically active, Their shear motion, recognized from geologic data and focal mechanism solutions, compares well with a classical stress model when applied to northward progress of India under the Himalayan collision front.

Seismicity and plate deformation below the Andaman arc, northeastern Indian Ocean

Abstract The seismic activity originating below the Andaman arc-Sea region is generally discernible into fore- and back-arc seismic zones which are traceable for nearly 1500 km in a N-S direction at the junctures between the Indian, Burma and SE Asia plates. The fore-arc seismicity displays an east-dipping (40–55°) Benioff zone upto about 200 km focal depths. Details of the Benioff zone, in correspondence to the observed gravity field, are discussed in four N-S sectors, which suggest some significant variations in the configuration of the Benioff zone. The back-arc seismicity affects only the top 40–45 km of the lithosphere below the Andaman Sea, where the back-arc spreading ridge splits the volcanic arc. Stress distribution and faulting due to earthquakes below the Andaman-West Sunda arc are studied here using 68 focal mechanism solutions. Their most significant results are: low-angle thrust events occur along the upper edge of the descending Indian plate, downdip tensional events have steeply dipping ( ⩾ 60°) nodal planes, and normal faulting takes place in most parts of the Benioff zone along moderately dipping (30–45°) planes. Downdip compressional events (high-angle reverse fault, nodal plane dip > 60°) or reverse faulting along moderately dipping (30–45°) nodal planes also occur below the Andaman arc. The compressive earthquakes dominate the shallower level of the subducting slab, and the tensional stress observed locally in north part of the Andaman Sea may be an outcome of the weak coupling between the descending and overriding plates. Generally, a more or less complete sequence of faulting i.e., thrusting below the trench, normal faulting below the fore arc, and strike-slip motion along the inner edges of the fore arc characterize the Andaman-West Sunda arc. In the southern Andaman region, a rather oblique convergence between the Indian Ocean and the SE Asia plates is needed to explain the existence of a somewhat contorted Benioff zone, in which, compressional stress dominates in deeper lithosphere. Oceanward, the Ninetyeast Ridge also impinges on the subduction zone in this region. Left-lateral shear motion along the east margin of the Ninetyeast Ridge is further inferred by the results of focal mechanism solutions.

Seismotectonic domains of northeastern India and adjacent areas

Abstract Eastern Himalaya, Meghalaya plateau, upper Assam valley, northern part of the Indo-Burmese arc and the Mishmi block constitute major tectonostratigraphic domains in northeastern India. Many lineaments of faults, both parallel and oblique to the Himalayan trend, are already known. Interpretation of satellite images combined with surface geological studies suggest that many such oblique lineaments transgress the boundary of individual tectonic domains and some continue from the Tethyan Himalaya to the foredeep or cut across both Himalayan and Burmese arcs. The entire area is highly seismic; seismicity pattern, focal mechanism solutions, geological set up and fault/lineament fabric when studied together clearly defines several seismotectonic domains. In east Nepal-Sikkim, the northward push of India is accommodated through conjugate shear failure wherein seismic strike-slip movement occurs mostly along NE faults. Further east NW/WNW Kopili-Bomdila faults are associated with many large earthquakes and lateral motion along them allows a bulk southeastward movement of this segment of Himalaya towards the Burmese arc. The Mishmi block, structurally oblique to both the Himalayan and Burmese arcs, also indicates a net southeast tectonic transportation. The upper Assam valley is aseismic and arguably does not represent an area of seismic gap. Seismicity in both Meghalaya plateau and Sylhet plains is unrelated to movements along the Dauki fault.

Role of transverse tectonics in the Himalayan collision: Further evidences from two contemporary earthquakes

Two contemporary earthquakes originating in the central Himalayan arc and its foredeep (Sikkim earthquake of 18.09.2011, Mw 6.9, h: 10–60 (?) km and Bihar-Nepal earthquake of 20.08.1988, Mw 6.8, h: 57 km) are commonly associated with transverse lineaments/faults traversing the region. Such lineaments/faults form active seismic blocks defining promontories for the advancing Indian Craton. These actually produce conjugate shear faulting pattern suggestive of pervasive crustal interplay deep inside the mountains. Focal mechanism solutions allow inferring that large part of the current convergence across the central Himalayan arc is accommodated by lateral slip. Similar slip also continues unabated in the densely populated foredeep for distances up to several tens of kilometers south of the Main Boundary Thrust (MBT).

Incipient status of dyke intrusion in top crust – evidences from the Al-Ays 2009 earthquake swarm, Harrat Lunayyir, SW Saudi Arabia

The 2009 earthquake-swarm in the Al-Ays volcanic zone in Harrat-Lunayyir in NW Saudi-Arabia is unique because of its intense character and focal-depth distribution at two depth bands (5–10 and 15–20 km) in upper crust without volcanic eruption. We investigate an anatomy of the dyke-intrusion model that supports the mechanism for the swarm itself with seismotectonics, pore pressure diffusion process and inference model. Inferred dyke-intrusion initially started at depth had a five-day peak period (15–20 May 2009) since inception of event-recordings, following which the activity diminished. The process of pore pressure perturbation and resultant “r–t plot” with modelled diffusivity (D = 0.01) relates the diffusion of pore pressure to seismic sequence in a fractured poro-elastic fluid-saturated medium. The spatio-temporal b-values show high b-values (>1.3) along the zone of dyke intrusion (length 10 km and height 5 km) at ∼20km depth. The main-shock and other prominent earthquakes originated on a moderate b-value zone (∼1.0). Temporal b-value analysis indicates an exceptionally low b-value (∼0.4) during the main-shock occurrence. The Al-Ays lava-field is inferred to underlie a seismic volume trending NW-SE bounded on both sides by two NW-SE trending fault systems, dipping 40–50° opposite to each other within a proposed nascent rift setting.

Seismic cluster analysis for the Burmese-Andaman and West Sunda Arc: insight into subduction kinematics and seismic potentiality

The Burmese–Andaman Arc System (BAAS) and the West Sunda Arc (WSA) in NE Indian Ocean are well known for their high seismic hazard and tsunami potentiality. Seismicity is caused by eastward subduction of the Indian plate to intermediate focal depths below the BAAS, but the penetration depth goes even deeper to about 500 km below the WSA. The seismicity map and its correlation to crustal and mantle faults for this extensive plate margin are presented. This is achieved by using frequency–magnitude relationship to select larger (m b ≥ 5.0) and comparatively well-recorded events from the available earthquake catalogue that span for a period of little more than a century (1906–2008). Barely 14% of the events qualify the treatment, and the events so selected are subjected to cluster analysis using a statistical function ‘point density’. The clusters found for the arc demonstrate significant relationship to subduction geometry in their respective areas; 11 out of a total of 13 clusters commonly originate below the fore arc. Earthquakes within the individual clusters have linear fractal geometry consistent with the traces of seismogenic surfaces that actually produce them. Correlation of clusters to seismologic depth sections and the composite results derived from 518 CMT solutions of earthquakes establish a close spatial relationship between the shape and orientation of the clusters with stress axes and regional tectonics. This provides a three-dimensional perspective on the stress distribution within the respective clustered seismic zones. Seismic potentiality for five most conspicuous clusters is also inferred.

Seismotectonics at the terminal ends of the Himalayan Arc

The Himalayan arc has an arcuate E–W trending geometry with reversal of trend at the terminal ends – Nanga-Parbat (western) syntaxis and Namcha-Barwa (eastern) syntaxis. Both ends are characterized by an actively deformed uplifted dome with its flanks bounded by active shear zones/faults that cause the majority of the seismicity. Compiled map data and seismo-geological depth sections around these two syntaxial zones have brought out active crustal structure and seismotectonic setup. The Nanga-Parbat syntaxis exhibits upward bending and subsequent thickening of the Indian plate with the cluster of seismicity along the NNE–SSW trending Raikhot fault/Diamer shear in its western margin and a comparatively less active Rupal–Chichi shear zone of N–S trend with diffused seismicity towards the east. The 2005 Kashmir earthquake is spawned due to interaction of the Main Boundary thrust and the Muzaffarabad fault. The Namcha–Barwa syntaxis displays a fault-bounded upliftment and thickening of the Indian plate where Canyon thrust marks the boundary between the Indian and Eurasian plates. The occurrence of the 1950 Assam earthquake in the vicinity of the eastern syntaxis is attributed to a regional right lateral strike-slip motion on the causative fault plane. The seismicity in the syntaxes is primarily controlled by strike-slip faults/shear zones along the flanks of popup antiforms.

Aftershock Propagation Characteristics During the First Three Hours Following the 26 December 2004 Sumatra-Andaman Earthquake

Abstract Within three hours of the mainshock rupture of the 26 December 2004 Sumatra-Andaman earthquake, 45 aftershocks occurred that are distributed all along the mega-thrust fault plane and also along the West Andaman fault. Seven of these aftershocks struck sequentially and unilaterally from the mainshock in the south towards north within 2h 9m 50.76s indicating an overall rate of aftershock propagation to the tune of 167 meters/sec. Seismic moment calculated from fault parameters gives a value of 1.2 × 1030 dyne cm. Three separate fault segments are identified from distribution of aftershocks with propagation rates 330, 250 and 85 meters/sec in the southern, central and northern segments. These 7 unilaterally propagating shocks along the mega-thrust are probably not aftershocks of the mainshock rather these are sequentially triggered shocks each rupturing a small segment of the fault. Location of the mainshock and several aftershocks are guided by several lithospheric hinge faults identified previously.

Clustering of Earthquake Events in the Himalaya - Its Relevance to Regional Tectonic Set-up

Abstract The earthquake events of Himalaya of magnitude ≥5.0 from the time window 1905–2000 are statistically analysed. The inter-event time between earthquakes shows Hurst phenomena of temporal clustering which are spatially located in five distinct domains along the Himalayan fold-thrust belt. Out of these, two domains, one around Uttaranchal-Nepal border and the other around Nepal-Sikkim border reveal maximum number of temporal clusters and thus considered as seismically most potential zones of the Himalaya. Both these zones are located at the interface of the orthogonally disposed major tectonic discontinuities of the Peninsular Shield and Himalayan fold-thrust belt. Such zones are geologically most favourable locales for strain accumulation during later-tectonic movement. Statistical analysis points towards a probability of recurrence of seismic events in near future in these two zones. However, validity of such statistical results can be ascertained by detailed geological and geophysical modelling of the terrain.