Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

V. K. Gahalaut ¹, R. Chander ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee 247667, IN

Abstract

The data presented suggest that uplift of the outer Himllaya in this region is continuing even today through sudden co-seismic elevation changes during large thrust earthquakes and secular aseismic uplift during intervals between such earthquakes. The observed co-seismic ground elevation changes during the Kangra earthquake are interpreted so as to simulate the cross-sectional shape of the buried active thrust fault responsible for this continuing episodie as well as secular uplift of the Outer Himalaya. This fault is assumed to be the surface of detachment between the Himalayan rocks above and the Indian shield rocks below. It is concluded that over most of its extent in Dehra Dun region, the detachment surface has gentle dip to the northeast but a few interspersed, northeast dipping steeper ramps are not ruled out. The depth of the detachment is estimated to be between 0 and 3 km beneath the SW limit and about 10 km beneath the NE limit of the Outer Himalaya in the Dehra Dun region. The two main levelling observations regarding the current inactive status of the Main Boundary Thrust (MBT) are, (1) the involvement of this thrust in the occurrence of the Kangra earthquake is definitely ruled out from the levelling data, and (2) the measured rate of secular uplift of the ground increases from NE to SW across the surface trace of the thrust suggesting that the Outer Himalaya is rising faster than the Lesser Himalaya to the NE across the MBT.

Keywords

Ground Elevation, Earthquake, Tectonism, Dehra Dun.

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V. K. Gahalaut ¹, R. Chander ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee - 247 667, IN

Abstract

We interpret here the ground elevation changes along the Saharanpur - Dehradun - Mussoorie line as brought out through the levelling surveys of 1905-07 and 1926-28. These data can be simulated with a model of slips, varying in amount between 3 ± 3 and 16 ± 8 cm, on the same subsurface detachment on which the great Kangra earthquake of 1905 may have occurred as argued elsewhere in the literature. Since ground elevation changes occurred in the Dehradun region during the Kangra earthquake also, we suggest that slips related to plate tectonics on the detachment lead to ground elevation changes partly aseismically during periods between great earthquakes and partly coseismically at the times of such earthquakes. The annual rates of slips obtainable from the above numerical results being much less than the estimated rate of 18 ± 7 mm/year for plate convergence accommodated at the Himalaya, we conclude that earthquake generating strains accumulated in the Dehradun region during the 21 years between the two levelling surveys.

Keywords

Geodetic Levelling, Dehradun Region, Outer Himalaya, Plate Tectonics

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P. Mahesh ¹, Amit Bansal ¹, B. Kundu ¹, J. K. Catherine ¹, V. K. Gahalaut ¹

Affiliations
1 National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad - 500 007, IN

Abstract

The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004 Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with predominant normal motion on approximately north-south to northeastsouthwest oriented plane is very similar to the 27 June 2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting 90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could trigger only a few aftershocks in its immediate vicinity.

Keywords

2004 Sumatra Andaman Earthquake, Andaman-Sumatra Subduction Zone, GPS Measurements, 90°E Ridge.

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Bhaskar Kundu ¹, V. K. Gahalaut ¹, J. K. Catherine ¹

Affiliations
1 National Geophysical Research Institute (CSIR), Uppal Road, Hyderabad - 500 007, IN

Abstract

We suggest that the spatial location of the 2011 Tohoku earthquake rupture and slip distribution on it was strongly influenced by the subduction of seamount chains. Subduction of seamounts across the Japan trench caused weak coupling on the plate interface which acted as barriers to the 2011 Tohoku earthquake rupture and thus delimited it.

Keywords

Tohoku Earthquake, Seamounts, Coupling, Japan Trench.

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