Uttarkashi Earthquake Research Articles

Modelling of active lineaments for predicting a possible earthquake scenario around Dehradun, Garhwal Himalaya, India

The Doon valley which consists of the Doon gravels and Siwalik rocks is surrounded by the Main Boundary Thrust (MBT) in the north, the Main Frontal Thrust (MFT) (Mohand Thrust and Bhimgoda Thrust) in the south, the Ganga Tear Fault in the east and the Yamuna Tear Fault in the west. Lineaments, identified from the aerial photographs, show that a few of them trend parallel to the Himalayan orogen, and the others are transverse to it. The effect of the neotectonic activity is clearly reflected in the Doon valley, implying that the Doon valley is tectonically unstable today, and that the possibility of earthquakes in this region cannot be ruled out. The ruptures along identified active lineaments are modelled three dimensionally. Peak accelerations are obtained by using a methodology, based on the semi-empirical method of Irikura, which was used by Midorikawa. The efficacy of this modelling technique for the Himalayan region is established by simulating peak ground accelerations for the Uttarkashi earthquake of 20 October 1991 and comparing it with recorded one. A seismic study of the region shows, that it is prone to moderate as well as great earthquakes. The technique of Midorikawa has been applied for the preparation of zonation maps in this seismically active region. Two different zonation maps showing, respectively, the effects of moderate as well as great earthquakes in this region are presented in this paper. The zonation map for moderate earthquakes shows that the region can experience peak ground accelerations around 310 cm/s 2. This value is close to the peak ground acceleration of 304 cm/s 2 recorded for the Uttarkashi earthquake ( M s = 7.1) at Uttarkashi station, which is about 60 km north of Dehradun. The prepared map shows that among all identified active lineaments, rupture along four lineaments can cause major damage in the region. Among these four, three lineaments are transverse to the major tectonic zones, viz., the MFT and MBT in the region, and one extends along the MFT lying south of Dehradun. The zonation map for great earthquakes ( M ≥ 8) has been prepared by modelling a rupture in the Central Gap region of the Himalaya. Seven different possibilities of rupture nucleation within the rupture plane are modelled and a zonation map containing three different zones is prepared. The prepared zonation map for this worst possible scenario shows that cities like Tehri, Uttarkashi, Srinagar, Deoprayag, Pauri and Dehradun fall within zone 1 and can experience ground accelerations bigger than 600 cm/s 2. The region like Hardwar and Rishikesh fall within zone 2 and can experience ground accelerations around 300 cm/s 2. The zonation map shows that Tehri can experience accelerations up to 0.84g and this value is comparable with that obtained by other researchers. This study confirms that for very long periods, the destruction in the region will be dominantly affected by ruptures having longer lengths and least effected by shorter ruptures. Therefore, special consideration about the longer ruptures should be taken while considering effect of earthquake hazards in this region.

Precursor seismicity, foreshocks and aftershocks of the Uttarkashi earthquake of October 20, 1991 at Garhwal Himalaya

The Uttarkashi earthquake ( m b = 6.6) of October 20, 1991 occurred in a complicated geological/tectonic setting at the Main Central Thrust (MCT) zone of the Himalaya. The main shock was preceded by a precursory earthquake activity, which continued for about eight years, and then there was a quiescent period for about three years. Three foreshocks, M > 4.0, occurred 48 h before the main shock, two aftershocks, M > 4.0, occurred within 12 h of the main shock which was then followed by another two aftershocks, M > 4.0, after about seven days, on October 27, 1991. The largest aftershock, M 5.2, occurred within 12 h. The aftershocks, magnitude less than 4.0, continued for about two months, and it decayed at a rate t −0.87. About 1000 earthquakes, magnitude less than 1.0 and above, were recorded by a closely spaced five-station temporary microearthquake (MEQ) network. Out of these, 124 aftershocks which occurred inside the network are located by Homogeneous Station Method. Spatial variation of the aftershocks with time was observed. Fault-plane solutions of the main shock and aftershocks indicate dominantly thrust faulting. Based on the hypocentral data, depth estimate of the plane of detachment is made, and a fault-interaction model is presented. The model shows that the main shock occurred at the juncture of MCT zone, plane of detachment and the basement thrust. The north-dipping Uttarkashi fault, a parallel fault at the MCT zone, has been activated by the tectonic stress generated due to relative movement of the Tibetan plate over the Indian plate.

Uttarkashi earthquake : ed H. K. Gupta & G. D. Gupta, (Geological Society of India, Bangalore; Memoir 30), ISBN (paperback) 81 85867 11 9, price Rs250.00 (US$30.00), 1995, 233 pp

Uttarkashi earthquake : ed H. K. Gupta & G. D. Gupta, (Geological Society of India, Bangalore; Memoir 30), ISBN (paperback) 81 85867 11 9, price Rs250.00 (US$30.00), 1995, 233 pp

Rupture history and seismotectonics of the 1991 Uttarkashi, Himalaya earthquake

The 19 October 1991 Uttarkashi, India earthquake occurred in the main thrust zone of the Himalaya. With a moment magnitude of 6.8, this event is characteristic of the present-day motion on the thrust fault system. We examine this earthquake using different sets of data in order to understand better the faulting process of a major earthquake in the Himalayan region. Firstly, the modeling of the teleseismic records indicates that the mechanism is similar to the published CMT and indicates a shallow (between 10 and 15 km depth) low-angle thrust event. In the vicinity of the source, the earthquake was recorded by a network of accelerometers run by the University of Roorkee. Six three-component accelerometers were triggered within a radius of 60 km. Two of them were very close to the surface projection of the fault. Forward modeling of those records shows that the rupture propagated toward the west. This forward modeling gives us the possibility to confirm the epicenter location and to evaluate the timing of the accelerograms. The accelerogram records are inverted to obtain the distribution of slip on the fault plane. The results show a complex rupture process. The slip maxima (1.5 m) occurred 10 km west and 15 km southwest of the hypocenter. The slip source function obtained with near-field data is similar to the function obtained from teleseismic records and shows a low moment release at the beginning of the rupture and a maximum rate of moment release 4 seconds after. The relation between the slip distribution obtained by inversion, isoseimals, mapped faults and the aftershocks location is then discussed and we finally propose a seismotectonic interpretation of this earthquake. The Uttarkashi earthquake probably occurred along the detachment surface which coincides with the upper surface of the subducting Indian lithosphere. This detachment surface is gently dipping under the Lesser Himalaya and south of the Vaikrita thrust. The Vaikrita thrust marks the line separating the very shallow-dipping detachment (along which big earthquakes like the Uttarkashi earthquake could occur) from the steeper-dipping, aseismic basement thrust. This observation is important for correctly estimating the seismic hazard in the Uttarkashi region.

A study of aftershocks of 20 October 1991 Uttarkashi earthquake

The Uttarkashi earthquake of 20 October 1991, which caused widespread damage in the Galhwal Himalayan region, was followed by a prominent aftershock. activity extending over a period of about two months. The aftershock activity was monitored using temporary networks established after the mainshock and the permanent stations in operation in the region. About 142 aftershocks could be located accurately using the data of these stations. The b-value of the Gutenberg-Richter's relationship for the aftershock sequence works out to be 0.6. The temporal distribution of the aftershocks suggests a hyperbolic decay with a decay constant (p) of 1.17. Macroseismic observations derived from field surveys show good agreement with the instrumentally determined source parameters.

Seismic hazard estimation using modelling of earthquake strong ground motions: a brief analysis of 1991 Uttarkashi earthquake, Himalaya and prognostication for a great earthquake in the region : K. N. Khattri, G. Yu, J. G. Anderson, J. N. Brune & Y. Zeng, Current Science, 67(5), 1994, pp 343–353

Seismic hazard estimation using modelling of earthquake strong ground motions: a brief analysis of 1991 Uttarkashi earthquake, Himalaya and prognostication for a great earthquake in the region : K. N. Khattri, G. Yu, J. G. Anderson, J. N. Brune & Y. Zeng, Current Science, 67(5), 1994, pp 343–353

Strong ground motion from the Uttarkashi, Himalaya, India, earthquake: Comparison of observations with synthetics using the composite source model

Abstract The Uttarkashi earthquake of 19 October 1991 (MS = 7.0) occurred in the greater Himalayan region north of the main central thrust, at an estimated depth of 12 km. The fault plane solution indicates a low-angle thrust mechanism, striking northwest, consistent with the tectonic pattern of thrusting in the region. Aftershocks define a belt parallel to, and north of, the surface trace of the main central thrust, roughly 10-km wide and 30-km long. The mainshock is located at the northeast edge of this zone. The earthquake was recorded on 13 strong-motion accelerographs at distances ranging from 25 to 150 km from the epicenter. One station at Bhatwari (peak horizontal acceleration of 272 cm sec−2) is above the aftershock zone. The maximum peak horizontal acceleration was about 313 cm sec−2 at Uttarkashi, at an epicentral distance of 36 km. The amplitudes and frequency content of the strong ground motions are more or less consistent with expectations for an earthquake of this magnitude in California. Synthetics generated using the composite source model and synthetic Green's functions (Zeng et al., 1994a, b) are successful in producing acceleration, velocity, and displacement with a realistic appearance and the correct statistical properties of the two accelerograms recorded nearest the fault (Bhatwari and Uttarkashi). To produce these, we introduced trial-and-error modifications of the layered-medium velocity model within uncertainties. At more distant stations, we first used the velocity structure that worked for the two nearest stations. Differences emphasize the large potential role of unknown site and wave-propagation effects. For the station at Tehri, we explored different velocity models, and found one there that was also quite successful. We then used these two velocity models to predict strong ground motions at Bhatwari and Tehri, from a potential magnitude 8.5 earthquake filling part of the seismic gap along the Himalayan frontal faults. The synthetics show peak accelerations that are only somewhat larger than those in the Uttarkashi event, but much longer durations and increased amplitudes of response spectra at long periods.

Temporal variation of seismicity in a Himalayan tectonic block and the 1991 Uttarkashi earthquake, India

Spatial and temporal variation of the seismicity between 1970 and August 1991 in the northwestern Himalaya and adjoining areas of Tibet, immediately to the north, were investigated using a function of the CN algorithm. The aim of the exercise was to detect a pattern which, if known earlier, could have been used to predict the Uttarkashi earthquake of 19 October 1991 in the Garhwal Himalaya. It is observed that strong earthquakes occurring in a block, defined by longitudes 75° and 83°E and latitudes 30° and 38°N approximately, yield a distinctive pattern of temporal variation in the seismicity for the purpose. We conclude from the analysis that the seismicity in the northwestern Himalayan segment of the Alpide-Himalayan seismic belt may not be independent of seismicity in southwestern Tibet.

IRS Detection of surface effects of the Uttarkashi earthquake of 20 October 1991, Himalaya

IRS Detection of surface effects of the Uttarkashi earthquake of 20 October 1991, Himalaya

Analysis of strong motion records from Uttarkashi earthquake for assessment of code provisions for different seismic zones : S. K. Jain & S. Das, Earthquake Spectra, 9(4), 1993, pp 739–754

Analysis of strong motion records from Uttarkashi earthquake for assessment of code provisions for different seismic zones : S. K. Jain & S. Das, Earthquake Spectra, 9(4), 1993, pp 739–754

Radon recording of Uttarkashi earthquake

Spatial and temporal distribution of radon is recorded in both soil‐gas and groundwater using two different techniques, viz. track etch method and emanometry. Radon recording stations have been set up at one site in Amritsar and four sites in the Kangra valley (Himachal Pradesh) under the Himalayan seismicity project. The track‐etch method gives integrated measurement of radon over a week or a fortnight whereas emanometry is used for daily recording of radon activity in soil‐gas and groundwater.The Uttarkashi earthquake (mb = 6.5, MS = 7.0) occurred on October 20, 1991 (Oct. 19 U.T.) in the Garhwal Himalayas (30.78°N, 78.77°E) about 330 km from our recording stations in the Kangra valley and about 450 km from Amritsar, respectively. Radon anomalies were recorded at all sites in Kangra valley and Amritsar about a week before the Uttarkashi earthquake, which clearly establishes that radon changes can be effective for forecasting some earthquakes.

The Uttarkashi earthquake of 20 October 1991: field observations

ABSTRACTThe epicentre of the destructive 20 October 1991 earthquake is in the north‐east of the Uttarkashi region of the Higher Himalaya. The earthquake was felt up to 250–350 km away from Poh and Keylong in the north to Delhi in the south and beyond Chandigarh in the west. Seismologists of the Seismotectonic Group of Wadia Institute of Himalayan Geology studied fissures, surface breaks, and the foreshock and aftershock activity caused by this event. Land fissures show normal dislocations of 0.06–1 m, run E‐W and NE‐SW in the epicentral region and could be followed for 30–40 km.

Seismotectonics of the 20 October 1991 Uttarkashi earthquake in Garhwal, Himalaya, North India

ABSTRACTThe 20 October 1991 Uttarkashi earthquake killed over a thousand people and caused extensive damage to property in the Garhwal Himalaya region. The body wave magnitude of the earthquake was 6.6, and the fault plane solution indicates reverse faulting. The hypocenrre was located at a focal depth of about 12 km between the Chail and Jutogh Thrusts, but movement propagated southward along the Jamak–Gangori Fault (JGF) and Dunda fault (DF) which are developed as blind faults related to the growing Uttarkashi antiform.

Analysis of Strong Motion Records from Uttarkashi Earthquake for Assessment of Code Provisions for Different Seismic Zones

Strong motion records have been obtained at 13 stations during the Uttarkashi earthquake of October 20, 1991 (magnitude 6.6). A study has been conducted on these time histories to assess the codal provisions in India. Emphasis of the study is on evaluating relative consistency of design provisions for different seismic zones in India. The average response spectra from this earthquake show concentration of significantly more energy in low period range and less energy in high period range. The magnitude of seismic design force for zones I, II, and III is consistent while it is too low for zone IV; no records were obtained in area with shaking intensity corresponding to zone V. It is seen that for buildings in zones I, II, and III, the present design provisions may be lowered either by relaxing the requirement of special ductile detailing, or by reducing the design force. On the other hand, design provisions for zone IV need to be revised upwards.

Identification of potential areas for the occurrence of strong earthquakes in Himalayan arc region

Morphostructural zoning (MSZ) scheme of the Himalayan arc region as obtained from a joint study of topographic, geological and tectonic maps as well as satellite imagery is analysed. Three types of morphostructures have been determined: territorial units (blocks of different ranks), linear zones limiting these blocks (lineaments) and intersections of the lineaments (knots). Comparison of MSZ scheme with the know seismicity indicates epicenters of strong earthquakes (M≥6·5) clustered around some of these knots. Pattern recognition method is used to determine seismically potential areas for the occurrence of recognition method is used to determine seismically potential, for the occurrence of strong earthquakes of magnitude ≥M 0. We have carried out two such studies for the Himalayan arc region, one forM 0=6·5 and the other forM 0=7·0. Out of a total number of 97 knots, 48 knots are found to be seismically potential for the occurrence of earthquake ofM≥6·5. The results of the study forM 0=6·5 were presented in the symposium on “Earthquake Prediction” held in Strasbourg, France, March 1991 (Gorshkovet al 1991). The epicenter of Uttarkashi earthquake of magnitude,M b=6·6 that occurred in the late hours of 19th October 1991 (UTC) lies in the vicinity of one such knot. The second study carried out subsequently shows that only 36, knots are potential for the occurrence of earthquakes ofM≥7·0, which include the knot, associated with theUttarkashi earthquake.