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21.
Alasdair Skelton Lillemor Claesson Govinda Chakrapani Chandan Mahanta Joyanto Routh Magnus Mörth Param Khanna 《Pure and Applied Geophysics》2008,165(1):45-61
Transient hydrogeochemical anomalies were detected in a granite-hosted aquifer, which is located at a depth of 110 m, north
of the Shillong Plateau, Assam, India, where groundwater chemistry is mainly buffered by feldspar alteration to kaolinite.
Their onsets preceded moderate earthquakes on December 9, 2004 (MW = 5.3) and February 15, 2005 (MW = 5.0), respectively, 206 and 213 km from the aquifer. The ratios [Na+K]/Si, Na/K and [Na+K]/Ca, conductivity, alkalinity
and chloride concentration began increasing 3–5 weeks before the MW = 5.3 earthquake. By comparison with field, experimental and theoretical studies, we interpret a transient switchover between
source aquifers, which induced an influx of groundwater from a second aquifer, where groundwater chemistry was dominantly
buffered by the alteration of feldspar to smectite. This could have occurred in response to fracturing of a hydrological barrier.
The ratio Ba/Sr began decreasing 3–6 days before the MW = 5.0 earthquake. We interpret a transient switchover to anorthite dissolution caused by exposure of fresh plagioclase to
groundwater interaction. This could have been induced by microfracturing, locally within the main aquifer. By comparison with
experimental studies of feldspar dissolution, we interpret that hydrogeochemical recovery was facilitated by groundwater interaction
and clay mineralization, which could have been coupled with fracture sealing. The coincidence in timing of these two hydrogeochemical
events with the only two MW ≥ 5 earthquakes in the study area argues in favor of cause-and-effect seismic-hydrogeochemical coupling. However, reasons
for ambiguity include the lack of similar hydrogeochemical anomalies coupled with smaller seismic events near the monitoring
station, the >200 km length scale of inferred seismic-hydrogeochemical coupling, and the potential for far-field effects related
to the Great Sumatra–Andaman Islands Earthquake of December 26, 2004. 相似文献
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S. Biswas N. Durgaprasad Banashree Mitra R. K. Singh M. N. Vahia J. S. Yadav A. Dutta J. N. Goswami 《Astrophysics and Space Science》1988,149(2):357-367
We report the first results on the determination of the ionization states of oxygen ions in the anomalous cosmic rays (ACR) from the measurements of their flux in the cosmic-ray experiment in Spacelab-3 (SL-3) mission of NASA flown at 350 km altitude during 29 April–6 May, 1985. The detectors used were specially prepared CR-39 plastics of very high sensitivity for recording tracks of ions withZ>2. The measured orbit averaged flux of ACR oxygen is (2.9±1.3)×10–4 particles m–2sr–1s–1 (MeV N–1) at an energy of 23 MeV N–1. We made an independent estimate of the expected ACR oxygen flux at SL-3 orbit from interplanetary data and compared this with the measured flux to infer the ionization states of ACR oxygen ions. The flux and energy spectra of ACR oxygen at 1 AU outside the magnetosphere is obtained from the data of Voyager-2, during the same epoch as the SL-3 flight, and using the measured radial intensity gradient of 15%/AU for ACR oxygen between 1–17 AU. We calculate the geomagnetic transmission factors for ACR oxygen ions of charge states O+1, O+2, etc., from the known cut-off rigidities in the world grid and using the SL-3 trajectories for 116 orbits in the 6-day mission to obtain the expected flux at SL-3 for different charge states. When these flux values are compared with our measured flux, the averge ionization state of ACR oxygen ions in the energy interval of 20–26 MeV N–1 is obtained as O+1. 相似文献
24.
Runti Choudhury Chandan Mahanta Swati Verma Abhijit Mukherjee 《Hydrogeology Journal》2017,25(4):1153-1163
The spatial distribution of arsenic (As) concentrations along three classified hydrogeomorphological zones in the Brahmaputra River Valley in Assam (India) have been investigated: zone I, comprising the piedmont and alluvial fans; zone II, comprising the runoff areas; and zone III, comprising the discharge zones. Groundwater (150 samples) from shallow hand-pumped and public water supply wells (2–60 m in depth) was analysed for chemical composition to examine the geochemical processes controlling As mobilization. As concentrations up to 0.134 mg/L were recorded, with concentrations below the World Health Organization and the Bureau of Indian Standards drinking-water limits of 0.01 mg/L being found mainly in the proximal recharge areas. Eh and other redox indicators (i.e., dissolved oxygen, Fe, Mn and As) indicate that, except for samples taken in the recharge zone, groundwater is reducing and exhibits a systematic decrease in redox conditions along the runoff and discharge zones. Hydrogeochemical evaluation indicated that zone I, located along the proximal recharge areas, is characterized by low As concentration, while zones II and III are areas with high and moderate concentrations, respectively. Systematic changes in As concentrations along the three zones support the view that areas of active recharge with high hydraulic gradient are potential areas hosting low-As aquifers. 相似文献