Informatics Publishing Limited

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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Rajeev Kumar Yadav ¹, Bhaskar Kundu ², Kalpna Gahalaut ¹, V. K. Gahalaut ³

Affiliations
1 CSIR-National Geophysical Research Institute, Hyderabad 500 007, IN
2 Department of Earth and Atmospheric Sciences, National Institute of Technology Rourkela, Rourkela 769 008, IN
3 National Centre for Seismology, Ministry of Earth Sciences, New Delhi 110 003, IN

Abstract

The 12 May 2015 earthquake of M_w 7.3 occurred in the Kodari region, Central Nepal, 17 days after the 25 April 2015 Gorkha earthquake (M_w 7.8) along the Himalayan plate boundary. Both the earthquakes were associated with predominantly thrust faulting on the Main Himalayan Thrust (MHT). This is the largest aftershock of the 2015 Gorkha earthquake which occurred approximately 150 km east of it. Our analysis suggests that the 2015 Gorkha earthquake significantly increased the Coulomb stress on the shallow unruptured and updip part of the MHT, further west of the 2015 rupture and also in the hypocentre region of 12 May 2015 M_w 7.3 aftershock. In the following 17 days period, Coulomb failure stress increased further by the relaxation of coseismic pore pressure on the eastern side of its coseismic rupture, where the 12 May 2015 aftershock had occurred.

Keywords

Coseismic Rupture, Delayed Triggering, Earthquakes, Failure Stress, Thrust Faulting.

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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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S. N. Bhattacharya ¹, V. K. Gahalaut ¹, Narendra Pandey ¹, Shankar Pal ¹, Rajeev Manhas ¹, G. Suresh ¹

Affiliations
1 National Center for Seismology, New Delhi 110 003, IN

Abstract

On 11 November 2018, all global seismic stations recorded unusual long-period wave packets without any discernible body waves. Our analyses show that the wave packets are monochromatic with period 15.6 sec and are fundamental-mode non-dispersive Rayleigh waves. Using the arrival times of wave packets, the source is approximately located at the northeastern edge of the recent earthquake swarm zone, off the east coast of Mayotte, a volcanic island between North Madagascar and East Africa. Synthetic seismograms for an isotropic source at 15 km depth with a transient oscillation of the same periodas that observed in the wave packets, are consistent with the recorded seismograms. We estimate an equivalent magnitude (M_w) of ~4.8 for this source and suggest that a volcanic fluid sphere of radius ~5 km might have generated such radial oscillations. Interestingly, similar monochromatic waves with lesser amplitude were also recorded on many occasions from the same source during 21 June 2018 to 10 September 2019. The period of waves increased from 15.2 sec in June to 15.6 sec in October– November 2018 and then decreased to 15.0 sec in September 2019, which implies an increase and then decrease in radius of the fluid sphere at the source.

Keywords

Monochromatic Wave Packets, Seismograms, Seismic Stations, Source Characterization.

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J. Malsawma ¹, Paul Lalnuntluanga ¹, Saitluanga Sailo ², V. Vanthangliana ², R. P. Tiwari ³, V. K. Gahalaut ⁴

Affiliations
1 Department of Geology, Mizoram University, Tanhril, Aizawl 796 004, IN
2 Pachhunga University College, Mizoram University, Aizawl 796 001, IN
3 Department of Geology, Mizoram University, Tanhril, Aizawl 796 004, India; Central University of Punjab, Bathinda 151 401, IN
4 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

Earthquakes in the Indo-Burmese arc occur due to interaction of India and Sunda plates along the IndoBurmese Wedge and Sagaing Fault. Majority of the moderate to major magnitude earthquakes in the Indo-Burmese Wedge occur within the Indian slab and very few of them occur on the plate interface. Earthquakes within the wedge are rare and the 22 June 2020 earthquake of magnitude 5.5 (Mw) on the India–Myanmar border in Mizoram, India, at shallow depth is probably one such earthquake. The earthquake caused moderate damage (maximum intensity VIII on MSK scale) in remote border villages (Vaphai and Chawngtui) with sparse population without any fatality. The earthquake did not seem to be related with the Mat Fault, which was transverse to the north–south trending wedge, as various estimates of mainshock and the region of maximum damage was ~20 km northeast of the surface trace of the Mat Fault. It appeared to be associated with almost north– south oriented Churachandpur Mao Fault (CMF) with dextral slip which mapped extensively and monitored geodetically in the neighbouring regions of Manipur and Nagaland to the north. Occurrence of this shallow depth earthquake may imply that some segment of the CMF might be seismically active, unlike in the north, where it appears to be predominantly aseismic. This implies that the seismic hazard along the CMF may vary along its length.

Keywords

Churachandpur Mao Fault, earthquakes, Indo-Burmese arc, tectonics.

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