Variation of Vp/Vs ratios for granitic layers and weekly average number of earthquakes in NE India

Vp/Vs ratios for the granitic layers in Shillong Plateau and the partially overlapping Tezpur seismic area, have been calculated from Wadati diagrams drawn on the basis of seismic phase data. From Shillong plateau the average of 23 readings for some months of 1979 for Vp/Vs is found to be 1.71. For Tezpur area the average of 29 readings of Vp/Vs for 1991-92 is found to be 1.73, but for 1995-96, the average Vp/Vs from 49 readings is found to be 1.68. The overall average from 78 readings for this area is 1.70. The low Vp/Vs for 1995-97 seems to be precursory. For the granitic layer, taking Vp = 5.92 kms/s and Vp/Vs = 1.70. We get Vs=3.48 km/s for NE India. Values of Vp/Vs ratios and number of shocks per day are plotted against time and are shown to undergo sudden lowering before many M³4.2 earthquakes. Vp/Vs is generally lowered below 1.60 in such cases. In Shillong plateau the number of shocks per day for 1979 is found to be three times the number in the adjoining Tezpur area, for 1991-97.

Seismic Evidence of Pop-up Tectonics Beneath the Shillong Plateau Area of Northeast India

Our detailed analysis of high-quality arrival time data recorded by the local seismographic network using three-dimensional seismic tomography of the Shillong Plateau region using high-quality arrival times of the body wave phases recorded at a dense temporary seismic network. This technique is used to understand the heterogeneities of the crust and its implications for pop-up tectonics characterizing evaluation the of the Shillong Plateau. We investigated an area covering ~150 ×100 km2 that revealed seismicity to be confined in a depth range ≤ 60 km. High - velocity anomalies in the upper crust appear to be responsible for intense small to moderate seismic activity in the region. Crustal seismic velocities inferred from 3-D seismic tomography showed significant lateral heterogeneities beneath the lithosphere of the Shillong Plateau. High-velocity anomalies in the uppermost crust, interpreted as the Shillong Plateau act as a geometric asperity where interseismic strain may accumulate. Low-velocity anomalies in the lower crust probably play a major role to accommodating the stresses generated due to plate separation, culminating in future sources of great earthquakes. The geological faults are well imaged in the cross-sections and support the concept of Pop-up tectonics beneath the Shillong of NE India.

Drainage Basin Morphometry Analysis in Parts of South Shillong Plateau, Meghalaya: Implications for Landscape Development

The Shillong Plateau is characterized by multiple phases of deformation and number of prominent sets joints/fracture system. The Southern Shillong Plateau unlike the northern part is highly dissected by the scarp faces which are mostly erosional. The area is characterized by deep incising river networks which cuts across many lithological units. To understand the factors influencing the drainage characteristic of the area and landscape development, the present study has been done in context of morphometry, geomorphology and geology of the drainage basin. Fifteen sub basins of 4th order have been selected from the Um Sohrynkew River basin. Drainage morphometry, indices of active tectonism, geology and tectonics and landform features have been worked out for better evaluation of drainage characteristics. The study area forms part of the Meghalaya Precambrian province of upper Proterozoic age. The southern border of the Shillong Plateau is demarcated by Dauki (Also known as Dawki) fault which is a prominent structural lineament. It consists of at least four E-W trending normal faults with occasional reversal. The segment of the Dauki fault in the study area is believed to be active. The present study focuses on the drainage network and the landscape development of the study area where a very strong relationship has been observed between the lithology and structure together with the tectonic activity influencing the drainage pattern in the area.

Precise Locating of the Great 1897 Shillong Plateau Earthquake Using Teleseismic and Regional Seismic Phase Data

Pinched between the Eastern Himalaya and the Indo-Burman ranges, the Shillong Plateau represents a zone of distributed deformation with numerous visible and buried active faults. In 1897, a great (magnitude 8+) earthquake occurred in the area, and although a subsurface rupture plane has been proposed geodetically, its epicenter remained uncertain. We gathered original arrival time data of seismic waves from this early-instrumental era and combined them with modern, 3D velocity models to constrain the origin time and epicenter of this event, including uncertainties. Our results show that the earthquake has taken place in the northwest part of the plateau, at the junction of the short, surface-rupturing Chedrang fault and the buried Oldham fault (26.0°N, 90.7°E). This latter fault has been proposed earlier based on geodetic data and is long enough to host a great earthquake. Rupture has most likely propagated eastward. Stress change from the 1897 earthquake may have ultimately triggered the 1930 M 7.1 Dhubri earthquake, along a fault connecting the Shillong Plateau with the Himalaya.

Soft sediment deformation features in Dauki Fault region: Evidence of paleoearthquakes, Shillong Plateau, NE India

The northeastern region (NER) of India has a number of complex regional geological structures, out of which the Dauki fault (DF) is a prominent one. The E-W trending reverse DF, which is referred to go through the southern margin of Shillong Plateau (SP), have played major role in the regional deformation of the adjoining areas and was believed to be active during the Late Quaternary time. Previous paleoseismological studies conducted on the eastern and western part of the DF, Bangladesh, revealed that the fault ruptured in AD 849–920 and AD 1548 respectively. However there were no studies on the DF from southern side of the SP, India. For the first time, from Indian side, soft sediment deformation structures (SSDS) are reported from five trenches in and around the DF zone, SP. Close to the Dauki village, five trenches in the eastern part of the DF, SP, show presence of micro faulting, sand dykes, disturbed strata, and water escape structures. The detailed investigation of SSDS indicates that the origin for deformation is seismic trigger. The 14C AMS dating of deformation structures generated coseismically by earthquakes suggest three seismic events occurred between 130 and 920 year BP, 5415 to 9140 year BP, and at about 4285 year BP. This study confirms that DF is indeed active, at least, since the mid-Holocene. More trenching and dating of seismically induced deformation features are needed to accurately calculate the recurrence interval of major earthquakes that can strike the fast-expanding urban areas in India and Bangladesh.