Basab Mukhopadhyay

Facies Clustering in Turbidite Successions: Case Study from Andaman Flysch Group, Andaman Islands, India

Abstract Bed thickness data of two turbidite sections viz., Corbyns Cove section, South Andaman and Kalipur section, North Andaman those belong to Oligocene Andaman Flysch Group, a forearc submarine fan system, were assessed for facies clustering employing Hurst statistics. Both the sections show Hurst phenomenon and reveal clustering in terms of thick and thin beds. Forcing behind event (bed) depositions in either of the studied sections was assessed statistically and inferred to be non-random and with cyclicities of irregular physical length. The inferred paleogeography through Hurst criteria though worked well for distal fan setting i.e., basin floor sheet sandstones of Corbyns Cove section, its unequivocal application in proximal fan deposits remains to be tested. The mismatch in paleogeographic interpretation between Hurst test result (lobe-interlobe) and field observation (channel-levee) for the inner fan deposit is explained through differential facies stacking between fans grow in sea-level lowstand and highstand. Lower bed amalgamation, poor sand to mud ratio and subordinately present thick event deposits may be the result of active growth of Andaman Flysch fan in sea level highstand and expressed in lower Hurst K value for inner fan channel-levee association (Kalipur section) compared to many of the channel-levee deposits of lowstand fan systems observed world over.

Statistical appraisal of bed thickness patterns in turbidite successions, Andaman Flysch Group, Andaman Islands, India

Abstract The bed thickness distribution of two turbidite successions in the Oligocene Andaman Flysch Group, India, which differ in terms of their paleogeography and sediment supply, has been assessed statistically. The purpose was to evaluate depositional cyclicity using the non-parametric Waldron test, and also to assess possible environmental control on the bed thickness–magnitude distribution. Both sections reveal a positively skewed distribution in bed thickness patterns. The Z value obtained through Waldrons test failed to reject the null hypothesis of randomness and thus discards a cyclic pattern. Juxtaposition of sediments of different proximalities in an elongated forearc fan complex is the possible cause for this absence of cyclicity. The studied sections, however, differ in their bed thickness–magnitude distribution. Slope turbidite successions, with sediment supply solely from the outer arc, reveals a scaling behavior while the outer fan succession, with mixing of sediments from different sources, show a negative exponential distribution. This highlights the possible control of the depositional system and sediment supply on the bed thickness–magnitude distribution of turbidite successions irrespective of the triggering mechanism. Unequivocal application of the power law distribution in turbidite successions demands reconsideration.

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.

Genesis of a New Slab Tear Fault in the Indo-Australian Plate, Offshore Northern Sumatra, Indian Ocean

Following the December 2004 and March 2005 major shallow foci inter-plate earthquakes in the north Sumatra region, a slab-tear fault located within the subducting Indian plate ruptured across the West Sunda Trench (WST) within the marginal intra-plate region. Trend, length and movement pattern of this New Tear Fault (NTF) segment is almost identical to another such slab-tear fault mapped previously by Hamilton (1979), located around 160 km south of NTF. Seismic activity along the NTF remained quasi-stable till the end of the year 2011, when an earthquake of magnitude 7.2 occurred on 10.01.2012 just at the tip of NTF, only around ∼100 km within the intra-plate domain west of WST. The NTF rupture propagated further towards SSW with the generation of two more large earthquakes on 11.04.2012. The foreshock (10.01.12; M7.2) — mainshock (11.04.12; M 8.6) — aftershock (11.04.12; M 8.2) sequence along with numerous smaller magnitude aftershocks unmistakably define the extension of NTF, a slab-tear fault that results tectonic segmentation of the convergent plate margin. Within the intra-plate domain most earthquakes display consistent left-lateral strike slip mechanism along NNE trending fault plane.

Earthquake forerunner as probable precursor — an example from north burma subduction zone

The Burmese Arc seismic activity is not uniform for its ∼ 1100 km length; only the Northern Burmese Arc (NBA) is intensely active. Six large earthquakes in the magnitude range 6.1–7.4 have originated from the NBA Benioff zone between 1954–2011, within an area of 200 × 300 km2 where the Indian plate subducts eastward to depths beyond 200 km below the Burma plate. An analysis on seismogenesis of this interplate region suggests that while the subducting lithosphere is characterized by profuse seismicity, seismicity in the overriding plate is rather few. Large earthquakes occurring in the overriding plate are associated with the backarc Shan-Sagaing Fault (SSF) further east. The forecasting performance of the Benioff zone earthquakes in NBA as forerunner is analysed here by: (i) spatial earthquake clustering, (ii) seismic cycles and their temporal quiescence and (iii) the characteristic temporal b-value changes. Three such clusters (C1–C3) are identified from NBA Benioff Zones I & II that are capable of generating earthquakes in the magnitude ranges of 7.38 to 7.93. Seismic cycles evidenced for the Zone I displayed distinct quiescence (Q1, Q2 and Q3) prior to the 6th August 1988 (M 6.6) earthquake. Similar cycles were used to forecast an earthquake (Dasgupta et al. 2010) to come from the Zone I (cluster C1); which, actually struck on 4 February 2011 (M 6.3). The preparatory activity for an event has already been set in the Zone II and we speculate its occurrence as a large event (M > 6.0) possibly within the year 2012, somewhere close to cluster C3. Temporal analysis of b-value indicates a rise before an ensuing large earthquake.