D. Shanker

Thrust-wedge mechanics and coeval development of normal and reverse faults in the Himalayas

Abstract: The coeval development of the South Tibetan Detachment (a regional-scale normal fault in Tibet) and the Main Central Thrust together with the observed dominance of thrusting in the Himalayas, is modelled using stress simulation analysis. 2D non-linear elastic and homogeneous wedge models, representing cross-sections of the Himalayas and Tibet are used. Simulated stresses for a set of boundary conditions in which the stress magnitudes are sufficient to cause failure along the wedge base (lower boundary) and reverse faulting at its toe (updip end of the base), invariably lead to the simultaneous development of intra-wedge normal faults. Further, a decrease in shear strength of the wedge base relative to its interior favours the development of normal faults and/or reduction in the magnitude of thrusting stresses within the wedge. These results suggest that the presence of a relatively strong Main Himalayan Thrust, the plate boundary fault below the Himalayas, would have favoured the occurrence of thrusting in the wedge. Moreover, a weak Main Himalayan Thrust below Tibet along with initiation of the Main Central Thrust can explain coeval development of the South Tibetan Detachment.

On the validity of time-predictable model for earthquake generation in north-east India

North-east India is seismically very active and has experienced many widelydistributed shallow, large earthquakes. Earthquake generation model for the region was studied using seismicity data [(1906–1984) prepared by National Geophysical Data Centre (NGDC), Boulder Colorado, USA]. For establishing statistical relations surface wave magnitudes (Ms≥5·5) have been considered. In the region four seismogenic sources have been identified which show the occurrences of atleast three earthquakes of magnitude 5·5≤Ms≤7·5 giving two repeat times. It is observed that the time interval between the two consecutive main shock depends on the preceding main shock magnitude (Mp) and not on the following main shock magnitude (Mf) revealing the validity of time predictable model for the region. Linear relation between logarithm of repeat time (T) and preceding main shock magnitude (Mp) is established in the form of logT=cMp+a. The values ofc anda are estimated to be 0–36 and 1–23, respectively. The relation may be used for seismic hazard evaluation in the region.

Earthquake Time Cluster in North-East India During February to April 1988

During the time interval of 3 February to 20 April 1988 four independent main shocks occurred in North-East (NE) India: 03/02/1988, M = 5.5; 06/20/1988, M = 5.8; 15/04/1988, M = 6.2 and 20/04/1988, M = 5.8. The seismicity rate within this 78 day interval is increased by a factor of 15 and 6 with respect to the mean, long term seismicity for M ≥ 5.5 and M ≥ 6.1, respectively. In terms of probability, it has been found out that the probability of observing by chance four events of M ≥ 5.5 or one events of M ≥ 6.1 in NE-India is equal to only 0.4243 and 0.3680, respectively. These results imply that the observed seismicity has a non-random time clustering. Similar earthquake time clusters were identified to have occurred in NE-India in 1930 and 1951. A triggering mechanism has been proposed to interpret the earthquake clustering: the first event of the earthquake sequence produces transient stress changes that cause an acceleration to the static stress loading, and then to seismic failure, to remote highly pre stressed regions.

Evaluation of Hydrogeology of the Lower Son Valley Based on Remote Sensing Data

Remote sensing, one of the most important reconnaissance and feature identifying tools generally applied for surface and groundwater investigation, was used for water resources mapping for the lower Son Valley in this study. The mapping was done with the help of Indian Remote Sensing (IRS) satellite imagery IRS-LISS- 1-B1 for January 29, 1991 obtained during the day transit time. The area under study comprises adjoining parts of Uttar Pradesh and Bihar states of India and extends over the seven districts, namely Bhojpur, Rohtas, Patna, Jahanabad, Aurangabad, Ballia and Chapra. Geology of the study area is quite complex, tectonically disturbed and shows four major cycles of depositions after erosions during last one billion years (since Cretaceous). Two lineaments mapped by GSI (Geological Survey of India) in western side of river Son in the Bhojpur district can also be identified by the satellite imagery. In the present study, apart from these lineaments, two new lineaments have been investigated, which run almost parallel to river Ganga in northwest parts of the area in Ballia district. The lineaments may play a very vital role in groundwater monitoring in the area. The detailed study of geomorphology, vegetation and geology of the Lower Son valley on the basis of photo-interpretation techniques for surface features, drainage pattern and density, and drainage texture has been carried out to determine alluvial type, permeability, tributary, etc. These studies and Darcy velocity analysis show that the prospect of water is very good for entire lower Son valley. The Bhojpur district is most prospective for groundwater exploitation.

Earthquake Risk Evaluation in Koyna Region of Maharastra, India

Gumbel’s extreme value theory has been applied to estimate the probability of occurrence and return period of the largest earthquakes in Koyna region India. The statistical model of Epstein and Lomnitz (1966) is discussed with reference to the Gumbel’s extreme value theory. The expected extremes based on 21-years (1964–1984) seismicity data of six-monthly extreme values of earthquakes for the region have been plotted. The mean line of expected extreme (LEE) is drawn to study the mean return periods of the largest possible earthquakes with their probability of occurrence. The yearly expected number, mean return period and earthquake risk have been estimated. The most probable medium size earthquakes that may occur in the region are predicted. The results presented in this study are only best long-term estimates subject to the limitation of the data.

Spectra of different fluxes over the arid and semiarid zones of northwest India during MONEX 1979

Fluxes of heat, momentum and the kinetic energy over the arid and semiarid regions of Indian Peninsula have been computed. Spectral analysis of these fluxes at 50 mb interval were performed at each level in the vertical up to 300 mb surface. The eddy latent heat transport is governed essentially by short- and long-range periodicities. A 5-day periodicity is generally present in the vertical mean eddy fluxes. Long-range periodic oscillations are more pronounced in the extreme north of the Indian subcontinent. Dominance of medium-range periodicity is observed in all the fluxes except latent heat.