Relationship between earthquake swarm,rifting history,magmatism and pore pressure diffusion — an example from South Andaman Sea,India

An extraordinarily strong and persistent earthquake swarm (Andaman swarm 2005) originated in the Andaman back-arc following the aftershock sequences of the 26 December 2004 Sumatra earthquake. The swarm (n = 651, mb_max= 5.9) came mainly in two phases: January 26–31 and Feb.–Aug. 2005, in an area of size 90 × 40 km², at the centre of which lies a broad bathymetric depression and high gravity zone. The swarm demonstrates a complex faulting series, initially the strike-slip motion followed by normal faulting in repetitive sequences, whose representative fault planes orient at high angle to the regional faults. The swarm character as well as the distribution of stress-axes and their correlation to tectonic features lends speculation for formation of a nascent rift segment in NW-SE direction at the doorstep of the Sewell Seamount. The swarm has given rise to 21 episodes of rifting activities of variable time extent within 26–31 January 2005. The r-t plots corresponding to the swarm data, modelled with variable hydraulic diffusivity (D) values 4, 6, 8 and 10 m²/s, suggest for excess pressure front from ascending magmatic fluid. This eventually heralded the rifting; causing pore pressure perturbations that propagated in accordance with known diffusion parabolic equations.     

Evaluation of seismic soil-liquefaction at Guwahati city

Great earthquakes in the past (e.g. 1869 Cachar earthquake, 1897 great Assam earthquake) have caused large scale damage and ground liquefaction in the Guwahati city. Moreover, seismologists are of opinion that a great earthquake might occur in the unruptured segment of the North-East Himalaya that is near to Guwahati city. In this paper, the liquefaction hazard due to these events have been simulated. The obtained results are in general agreement with the reported damages due to the past earthquakes. The central part of the city (i.e. Dispur, GS road), that has large thickness of soft soil deposit and shallow ground water table, is highly vulnerable to liquefaction.     

Natural zeolites enhance groundwater quality: evidences from Deccan basalts in India

The zeolite minerals characterized with hydrated aluminosilicates, negative ionic charge and 3D framework structure are well known for purifying the groundwater occurring in basaltic aquifer systems. However, the filtering mechanism at in situ field conditions is a complex process, which is rarely studied, and hence, it needs to be demonstrated. This paper explores the mechanism of hydrochemical processes and evolution of natural zeolites associated with basaltic rock to enhance groundwater quality. We present the hydrochemical findings and evolution processes derived from 46 groundwater samples (N_t = 46) belong to zeolitic (N_z = 25) and non-zeolitic (N_nz = 21) zones of a micro-watershed (4.4 km²) beset over basaltic terrain, Deccan Volcanic Province (DVP), India. The groundwater samples collected for one hydrological cycle (pre- and post-monsoons) are examined for major ion chemistry to determine the aqueous solution mechanism and ion-exchange process occurred in zeolitic and non-zeolitic zones. Further, the hydrochemical parameters are appraised by means of dominancy of ions, rock–water interactions, silicate weathering, chloro-alkaline indices, cation-exchange bivariate plots and the mechanism controlling groundwater chemistry. The results show that: 1) the purifying efficiency of zeolites for total ionic strength is observed as 63.85 and 68.58% during pre- and post-monsoons, respectively, 2) the significant reduction (36.51%) in total hardness attributed to the positive trend of chloro-alkaline indices depicting the ion-exchange phenomenon between Na⁺ and K⁺ (alkalies) and Ca²⁺ and Mg²⁺ (alkali-earth) elements in the zeolitic zone, 3) Gibbs plot shows the rock–water interaction as the predominant mechanism controlling groundwater chemistry in the zeolitic zone, and 4) the groundwater quality parameters from zeolitic zone are found within the permissible limit of WHO drinking water standards.