Uttarkashi Earthquake Research Articles

Himalayan hazard study on the basis of stress and strain state of 1991 Uttarkashi earthquake using Coulomb stress transfer model

Uttarkashi region of Himalaya was struck by a destructive earthquake (Mw = 6.8) on 19 October 1991 with its epicentre located at 30.78°N latitude and 78.77°E longitude.Here, we use Coulomb stress model for the study of this major event. Coulomb 3.1 application is used to generate the earthquake model and develop the stress change maps of the region. The 1991 Uttarkashi earthquake and succeeding earthquakes of Mb > 3.5 are used for the study. One of the motives of this work is to correlate the main shock and the successive minor earthquakes that have occurred in the region. Further, we have studied the 1999 Chamoli earthquake using this model and attempted to relate its occurrence with the Uttarkashi earthquake based on propagation of strain energy.The study of Uttarkashi 1991 earthquake by this method supports the main shock and its controlling effect on the later earthquakes’ observations in the nearby region with the view that many zones of such shock can occur in the future. This model is also used to predict the direction of propagation of strain energy, thereby locating the region that will be affected by the future shocks.

Ground response analysis of Kanpur soil along Indo-Gangetic Plains

The present work deals with 1D and 2D ground response analysis and liquefaction analysis of alluvial soil deposits from Kanpur region along Indo-Gangetic plains. Standard penetration tests and seismic down hole tests have been conducted at four locations namely IITK, Nankari village, Mandhana and Bithoor at 1.5m interval up to a depth of 30m below the ground surface to find the variation of penetration blows and the shear wave velocity along the depth. From the selected sites undisturbed as well as representative soil samples have been collected for detailed soil classification. The soil profiles from four sites have been considered for 1D and 2D ground response analysis by applying the free field motions of three Himalayan earthquakes namely Chamba earthquake (Mw—5.1), Chamoli earthquake (Mw—6.4) and Uttarkashi earthquake (Mw—6.5). An average value of Peak Ground Acceleration (PGA) obtained from 1D and 2D analysis is considered for liquefaction analysis and post-liquefaction settlement. The excess pore water pressure ratio is greater than 0.8 at a depth of 24m from ground surface for IITK, Nankari village, Bithoor sites. More than 50% of post liquefaction settlement is contributed by layers from 21–30m for all sites. In general, the soil deposits in Kanpur region have silty sand and sand deposits and are prone to liquefaction hazards due to drastic decrease of cyclic resistance ratio (CRR) at four chosen sites in Kanpur.

Estimation of Strong Ground Motion from a Great Earthquake Mw 8.5 in Central Seismic Gap Region, Himalaya (India) Using Empirical Green’s Function Technique

In the present study ground motions for a Mw 8.5 scenario earthquake are estimated at 13 sites in Kumaun-Garhwal region using the empirical Green’s function technique. The recordings of 1991 Uttarkashi earthquake of Mw 6.8 at these sites are used as an element earthquake. A heterogeneous source model consisting of two asperities is considered for simulating the ground motions. The entire central seismic gap (CSG) can expect acceleration in excess of 100 cm/s2 with NW portion in excess of 400 cm/s2 and SE between 100 and 200 cm/s2. The central portion can expect peak ground acceleration (PGA) between 200 and 400 cm/s2. It has been observed from simulation of strong ground motion that sites located near the rupture initiation point can expect accelerations in excess of 1g. In the present analysis, Bhatwari and Uttarkashi can expect ground accelerations in excess of 1g. The estimates of the PGA are compared with earlier studies in the same region using different methodologies and it was found that the results are comparable. This has put constrains on the expected PGAs in this region. The obtained PGA values can be used in identifying the vulnerable areas in the central Himalaya, thereby facilitating the planning, design and construction of new structures and strengthening of the existing structures in the region.

Intensity attenuation relation at Chamba–Garhwal area in north-west Himalaya with epicentral distance and magnitude

Seismic hazard assessment of any region depends on the attenuation relation which relates the seismological data with parameters of engineering interest. In the absence of sufficient strong motion data for northwest Himalayan region, isoseismal maps of previous earthquakes were the only source of information for computing the attenuation characteristics of that region. The northwest Himalayan region had experienced major and moderate earthquakes in the past and isoseismal maps had been developed by various authors for each of these earthquakes representing intensity pattern and decay of intensity in different directions. With this information, attenuation relations of intensity with distance can be obtained. The present study includes 10 moderate and major earthquakes (Ms ≥ 4.9) that had occurred during the last 100 years. These are primarily Kangra (1905), Chamba-1 (1945), Chamba-2 (1995), Kinnaur (1975), Dharamsala (1986), Uttar Kashi (1991), Garhwal-1 (1996), Garhwal-2 (1996), Sundarnagar (1997) and Chamoli (1999) earthquakes. Miezoseismal zones in majority of the isoseismal maps show two major directions, i.e., either the longest axis of these isoseismal maps which are aligned in NW–SE direction following the major longitudinal features or north-northwest to south-southeast direction, following the tectonic features which are transverse to the Himalayan trend. Isoseismal maps of Kangra (1905), Chamba (1945) and Uttar Kashi (1991) earthquakes are more symmetrical and elongated in south-east direction, but compressed in north-east and north-west directions, thus indicating fast attenuation characteristics in north-east and north-west directions reflecting lithological variation. On the other hand, Sundernagar (1997) and Kinnuar (1975) earthquakes show circular pattern of intensity distribution. Combining decay patterns from different magnitude earthquakes in different directions, two attenuation relationships have been developed using regression relationship (least square method).

Site Effects Estimation in Delhi from the Indian Strong Motion Instrumentation Network

The recent spurt in global earthquake activity has enhanced the consciousness about the increasing vulnerability of large population in the region close to Himalayas and has made significant long‐term social and economic impacts. The observation of strong motion and the investigation of the destruction from these earthquakes provide the disciplines of seismology and earthquake engineering with informative and valuable data, experiences, and lessons and raise a number of important scientific challenges. Himalaya is among the most seismically active regions in the world and has experienced several earthquake disasters during historical times. In the last 50 years, the population of India has doubled and resulted in the very rapid growth of settlements, especially in urban areas. At present, about 50 million people in India, living in the Himalayan region and the adjoining plains, are at risk from earthquakes. During the last 200 years, the Indian peninsula has experienced several great earthquakes with a magnitude above 8.0, namely, the Kutch, Gujarat earthquake (1819); the Assam earthquake (1897); the Kangra earthquake (1905); the Bihar–Nepal earthquake (1934); and the Assam earthquake (1950). Most of the parts of the Himalayas comprising North India and Northeastern India are mapped as either seismic zone IV or V in the seismic zonation map of India (on a scale of II–IV). Two of the recent significant earthquakes in North India are the Uttarkashi earthquake of 1991 ( M w 6.8) and the Chamoli earthquake of 1999 ( M w 6.5). Despite being one of the most seismically active regions of the world, the strong‐motion data set in India is, in general, very sparse. For this reason, the seismic hazard in India, until now, has been estimated using either the intensity data or attenuation relations derived for other regions (e.g., Khattri et al. , 1984; Krishna, 1992). Therefore, in the absence of adequate …

Natural Disasters and Risk Assessment in Uttarakhand with special reference to Uttarkashi Earthquake.

Uttarkashi lies in the main Alpine Himalayan belt, one of the most earthquake prone regions of the world. Crustal instability in this belt is ascribed to the movement of the Indian plate towords the Eurasian plate at the rate of about 50mm per year. Besides several local faults, two prominent thrusts tending northwest to southeast, from the conspicuous tectonic features. The basic aim of the present work is taken just one more step to make cities safer before the next disaster strikes. Development of a mitigation plan in more economic and effective in a long run than providing an ad-hoc relief after the disaster. The earthquake caused strong ground shaking over a large area with worst effects suffered in Uttarkashi Bhatwari region. Damage was observed in unreinforced masonry buildings as well as RC frame structures. Good construction performed much better than poor quality construction. The need for RC roof and gable bands in masonry buildings was clearly underlined by the performance of buildings at the ITBP campus at Mahidanda. There was enormous loss due to landslides and collapse of retaining walls. The failure of Gawana bridge needs to be studied. This may trigger revision of the Indian code. The earth is continually evolving and undergoing changes. From our view point at the surface of the Earth, we observe and are affected by both surface processes and the external expression of the activity occurring deep within the earth. This activity may be very slow and take place over millions of years, as in the case of a major earthquake. Much of the large scale earth movement is concentrated along faults or breaks in the earth’s crust e.g. Himalayan belt of Uttarakhand when movement occurs suddenly along a fault; energy is released in the form of an earthquake. Earthquake consists of vertical and horizontal waves like motions of the ground. The horizontal motions cause heavy destructive forces being larger than the vertical waves say 5 to 10 times greater. They may further be along any direction. Aims And Objectives Of Present Work: The basic aim of the present work is taken just one more step to make cities safer before the next disaster strikes. Development of a mitigation plan in more economic and effective in a long run than providing an ad-hoc relief after the disaster. Table :1 Districtwise House types as per MSK Classifcation Name of District No. of Houses Ares of District & houses (Per km) Number of A,B,C type houses with percentage Area under MSK Intensity KM Number of houses in Intensity Area Numbe r a type houses Number B-type houses Number of C type houses Almora 282620 5385 (52.48) A 273530 (96.78%) B 6515 (2.31%) C 2575 (0.91%) VIII 800 VII 1300 VI 3285 41986 68228 177406 40636 66033 166861 968 1573 3974 382

Modeling of strong ground motions for 1991 Uttarkashi, 1999 Chamoli earthquakes, and a hypothetical great earthquake in Garhwal–Kumaun Himalaya

In this study, stochastic finite fault modeling is used to simulate Uttarkashi (1991) and Chamoli (1999) earthquakes using all available source, path, and site parameters available for the region. These two moderate earthquakes are recorded at number of stations of a strong motion network. The predicted peak ground accelerations at these stations are compared with the observed data and the ground motion parameters are constrained. The stress drop of Uttarkashi and Chamoli earthquakes is constrained at 77 and 65 bars, respectively, whereas the quality factor Q C is 112 $$ f^{0.97} $$ and 149 $$ f^{0.95} $$ for these two regions. The high-frequency attenuation parameter Kappa is in the range 0.04–0.05. The constrained ground motion parameters are then used to simulate Mw 8.5 earthquake in central seismic gap region of Himalaya. Two scenarios are considered with epicenter of future great earthquake at locations of Uttarkashi and Chamoli earthquakes using above constrained parameters. The most vulnerable towns are the towns of Dehradun and Almora where expected PGA is in excess of 600 cm/s2 at VS30 520 m/s when the epicenter of the great earthquake is at the location of Uttarkashi (1991) earthquake. The towns of Shimla and Chandigarh can expect PGA close to 200 cm/s2. Whereas when the epicenter of the great earthquake is at the location of Chamoli (1999) earthquake, the towns of Dehradun and Almora can expect PGA of around 500 and 400 cm/s2, respectively, at VS30 620 m/s. The National Capital Region, Delhi can expect accelerations of around 80 cm/s2 in both the cases. The PGA contour maps obtained in this study can be used to assess the seismic hazard of the region and identify vulnerable areas in and around central Himalaya from a future great earthquake.

Seismicity of region around dams in North West India

The collision of Indian and Eurasian continents caused large scale deformation and high seismicityof vast areas of both continents in the geological history. The North-West portion of the Himalayan arc which is lyingunder the rupture zones of Kangra earthquake of 1905, Uttarkashi earthquake of 1991 and Chamoli earthquake in 1999,has experienced many earthquakes of magnitude 6 and above. The region of North-West India between 30.0º - 35.0ºNorth and 73.0º - 79.0º East is, therefore, under intense investigations by various scientists since the origin of theHimalayas. India Meteorological Department had opened thirteen seismic observatories in early sixties for monitoringof earthquake activities in and around Bhakra, Pong, Pandoh dams in Punjab / Himachal Pradesh and Salal dam inJ&K on specific demand of the dam authorities. These observatories have recorded the earthquakes occurred in thisregion having magnitude even less than 2. The data collected for the last two decades is very useful for the scientiststo investigate seismicity and tectonics of the Himalayas. The present study could locate the regions which areseismically most active and also the region of seismic gap. Thus present study confirms association of seismic activityin the region with two major fault systems called Main Boundary Thrust (MBT) and Main Central Thrust (MCT).Comparative seismic activity within 100 km from each dam, reveal that most active region was around Pong followed byPandoh, Bhakra and Salal dams. The temporal variation of b-values for the whole period also shows that low b-valueanomalies are usually followed by large earthquakes of M > 5.5. No definite conclusions could be drawn with regard tothe relationship between the observed seismic activity around the dam sites with the corresponding water levelfluctuations in the reservoirs.

Stochastic ground-motion simulation of two Himalayan earthquakes: seismic hazard assessment perspective

The earthquakes in Uttarkashi (October 20, 1991, M w 6.8) and Chamoli (March 8, 1999, M w 6.4) are among the recent well-documented earthquakes that occurred in the Garhwal region of India and that caused extensive damage as well as loss of life. Using strong-motion data of these two earthquakes, we estimate their source, path, and site parameters. The quality factor (Q β ) as a function of frequency is derived as Q β (f) = 140f 1.018. The site amplification functions are evaluated using the horizontal-to-vertical spectral ratio technique. The ground motions of the Uttarkashi and Chamoli earthquakes are simulated using the stochastic method of Boore (Bull Seismol Soc Am 73:1865–1894, 1983). The estimated source, path, and site parameters are used as input for the simulation. The simulated time histories are generated for a few stations and compared with the observed data. The simulated response spectra at 5% damping are in fair agreement with the observed response spectra for most of the stations over a wide range of frequencies. Residual trends closely match the observed and simulated response spectra. The synthetic data are in rough agreement with the ground-motion attenuation equation available for the Himalayas (Sharma, Bull Seismol Soc Am 98:1063–1069, 1998).

Use of Acceleration Spectra for Determining the Frequency-Dependent Attenuation Coefficient and Source Parameters

High-frequency strong-motion data of two recent major earthquakes of the Garhwal Himalayan region of India have been analyzed to determine the Qβ ( f ) for this region. Data from a local array of 24 stations, two main shocks, and several aftershocks were used to determine Qβ ( f ) and source parameters. An estimate of root mean square (rms) error is given. The data set in the present work includes the Uttarkashi earthquake of 20 October 1991 ( M s 7.0) and the Chamoli earthquake of 28 March 1999 ( M s 6.6) and its aftershocks. The source region of these earthquakes falls in the central seismic gap region of Himalayan frontal arc. We find the frequency-dependent Q -relationship for this region to be Qβ ( f ) = 112 f 0.97. The validity of the obtained Qβ ( f ) relationship is checked by comparing the source spectra at various stations with the theoretical spectra and found to be consistent. Low value of coefficient ( 0.8) in the Qβ ( f ) relationship suggest that the region is seismically and tectonically active, characterized by a large number of heterogeneities. The present inversion of strong-motion data gives the corner frequencies for the Uttarkashi and the Chamoli earthquake as 0.09 and 0.20 Hz, respectively, which correspond to the stress-drop values of 31 bars and 98 bars, respectively.

Radon Precursory Signals for Some Earthquakes of Magnitude > 5 Occurred in N-W Himalaya: An Overview

The N-W Himalaya was rocked by a few major and many minor earthquakes. Two major earthquakes in Garhwal Himalaya: Uttarkashi earthquake of magnitude Ms= 7.0 (mb = 6.6) on October 20, 1991 in Bhagirthi valley and Chamoli earthquake of Ms= 6.5 (mb = 6.8) on March 29, 1999 in the Alaknanda valley and one in Himachal Himalaya: Chamba earthquake of magnitude 5.1 on March 24, 1995 in Chamba region, were recorded during the last decade and correlated with radon anomalies. The helium anomaly for Chamoli earthquake was also recorded and the Helium/Radon ratio model was tested on it. The precursory nature of radon and helium anomalies is a strong indicator in favor of geochemical precursors for earthquake prediction and a preliminary test for the Helium/Radon ratio model.

Estimation of the source parameters of the Himalaya earthquake of October 19, 1991, average effective shear wave attenuation parameter and local site effects from accelerograms

A simple and rapid methodology based on band-limited strong motion data to obtain estimates of double couple fault plane solution, average effective shear wave attenuation parameter ( Q seff) and locate and analyze sub-events of the earthquake process has been presented. This method will be particularly efficient in the study of smaller earthquakes that may have been recorded locally at a small number of seismological or strong motion observatories. The method has been applied to investigate the moderate magnitude Uttarkashi earthquake of October 19, 1991, that occurred in the central seismic gap of Himalaya and was recorded locally on a strong motion array. The spectra of the high-energy packets observed in the ‘SH waves’ (as approximated by the transverse components of accelerograms) is modeled in terms of Brune omega square model. Thereby we estimate parameters of the earthquake comprising of the fault plane solution (strike: 315°, dip: 19°, rake: 55°), seismic moment (1.7 × 10 26 dyne cm), stress drop (53 bar), fault slip (186 cm), source duration (8.2 s) and moment magnitude (6.7), which are found to be consistent with the corresponding values reported in published studies. The average value of Q seff in the region is estimated to be 731 ± 264, suggesting a sufficiently cool thermal regime. Thus, brittle behavior of crustal rocks with capacity for supporting earthquakes is expected to be a dominant process in the region. The effect of site amplifications on peak ground acceleration (PGA) has been investigated using a comparison of observed PGA with a representative regression for the region. Furthermore, two distinct phases of energy releases, corresponding to two sub-events of the earthquake have been identified on the ‘SH wave’ records. The sub-events, which represent asperities in the fault process, are separated in time by about 6 s and 20 km in space. The fault plane solution, seismic moment and stress drop of these asperities have also been obtained.

GENERATION OF SYNTHETIC ACCELEROGRAMS USING ENGINEERING EARTHQUAKE SOURCE MODEL

The strong motion records available during an earthquake can be treated as the response of the earth as a structural system to unknown forces acting at unknown locations. Thus, if the part of the earth participating in ground motion is modeled as a known finite elastic medium, one can model the source location and forces generated during the earthquake as an inverse problem. Based on this analogy, a simple model for the earthquake source is proposed, by assuming the source to be a sequence of impulses acting at locations yet to be found. These impulses and their locations are found using the normal mode expansion along with a minimization of mean squared error. The medium is assumed to be finite, elastic, homogeneous, layered and horizontal with specified boundary conditions. Detailed results are obtained for the Uttarkashi earthquake of 20th October 1991, in India. The impulse locations are shown to be closely associated with the underlying fault mechanism. The proposed model is then used to simulate the acceleration time histories at a few recording stations. The earthquake source expressed in terms of a sequence of impulses acting at different locations is applied to a 2D finite elastic medium. The acceleration time histories found from this model agree well with with the accelerations recorded for the earthquake.

Evidence on triggered seismicity associated with the October 20, 1991 Uttarkashi earthquake (Garhwal Himalaya)

Two swarms of small earthquakes occurred on 22-24 October, 1995 and 13-18 September, 1996 about 15 km to the West-South-West (WSW) of the 1991 Uttarkashi earthquake (mb = 6.6) in Himalayas. The later swarm has migrated 5 km to the WSW of the former. Analyses of their seismicity rates, width of apertures and migration rates show that these swarms are triggered by a disturbance caused by the occurrence of the Uttarkashi earthquake thus triggered by this event. The disturbance, having slow propagation, rate represents evidence of creep of the earth material transferring stresses to the WSW direction. Occurrence of creep (stable-slip) motion is supported by the inferred south-west (SW) orientation of compressive stresses in Uttarkashi earthquake and presence of the north-west (NW) trending shear zones in the region. Ongoing convergence between India and Tibet would have provided the necessary tectonic forces transverse to the NW-SE trend of the Garhwal Himalayas indicating the future seismic activity of the region.

Helium/radon precursory signals of Chamoli Earthquake, India

The Bhagirthi and Alaknanda valleys of Garhwal Himalaya, were rocked, respectively, by two major earthquakes: the Uttarkashi earthquake of magnitude m b=6.5, M s=7.0 on October 20, 1991 and the Chamoli earthquake of m b=6.8, M s=6.5 on March 29, 1999, during this decade. Both these seismic events are associated with ongoing deformation along the main central thrust of the Himalayas. Helium and radon anomalies on March 24 and March 27, 1999, respectively, were recorded at Palampur which is about 393 km from the Chamoli earthquake epicentre. A He/Rn ratio anomaly was recorded on March 20, 9 days before the Chamoli earthquake. The precursory nature of radon and helium anomalies is a strong indicator of the physical basis of earthquake prediction and a preliminary test for the proposed conceptual He/Rn ratio model.

The Simulation of Ground Motions Using Envelope Summations

— The technique of Midorikawa (1993) has been modified to obtain a resultant envelope function at the observation point by placing the rupture causing an earthquake in a layered earth model. The method and its dependency on various modelling parameters are studied in detail. The complete study shows that the generated resultant envelope follows important strong motion characteristics such as directivity and attenuation effects. The simulated resultant envelope is further used for generating synthetic accelerograms by multiplying filtered white noise with the envelope of accelerogram at a particular observation point. Filters through which white noise passes include the effects of geometrical spreading, anelastic attenuation and near-site attenuation at high frequencies.¶Uttarkashi earthquake is among few Indian earthquakes for which strong motion data are available at thirteen different stations. Using the technique presented in this work, envelope function as well as complete acceleration time history during Uttarkashi earthquake has been simulated at these observation points. Comparison of peak acceleration, duration and acceleration response spectra confirms the utility and efficacy of the approach.

Earthquake source model using strong motion displacement as response of finite elastic media

The strong motion displacement records available during an earthquake can be treated as the response of the earth as the a structural system to unknown forces acting at unknown locations. Thus, if the part of the earth participating in ground motion is modelled as a known finite elastic medium, one can attempt to model the source location and forces generated during an earthquake as an inverse problem in structural dynamics. Based on this analogy, a simple model for the basic earthquake source is proposed. The unknown source is assumed to be a sequence of impulses acting at locations yet to be found. These unknown impulses and their locations are found using the normal mode expansion along with a minimization of mean square error. The medium is assumed to be finite, elastic, homogeneous, layered and horizontal with a specific set of boundary conditions. Detailed results are obtained for Uttarkashi earthquake. The impulse locations exhibit a linear structure closely associated with the causative fault. The results obtained are shown to be in good agreement with reported values. The proposed engineering model is then used to simulate the acceleration time histories at a few recording stations. The earthquake source in terms of a sequence of impulses acting at different locations is applied on a 2D finite elastic medium and acceleration time histories are found using finite element methods. The synthetic accelerations obtained are in close match with the recorded accelerations.

Helium/radon precursory anomalies of Chamoli earthquake, Garhwal Himalaya, India

In the Garhwal Himalaya, the Bhagirthi and Alaknanda valleys were rocked respectively by two major earthquakes; the Uttarkashi earthquake of magnitude m b=6.5, M s= 7.0 on 20 October, 1991 and the Chamoli earthquake of m b =6.8, M s=6.5 on 29 March 1999, during this decade only. Both these seismic events are associated with ongoing deformation along the main central thrust (MCT) of the Himalayas. The helium and radon anomalies on 24 and 27 March 1999, respectively, were recorded at Palampur which is about 393 km from the Chamoli earthquake epicentre. A helium/radon ratio anomaly was recorded on 20 March, 9 days before the Chamoli earthquake. The precursory nature of radon and helium anomalies is a strong indicator of the physical basis of earthquake prediction and a preliminary test for the proposed conceptual helium/radon ratio model.

Strong motion envelope modelling of the source of the Chamoli earthquake of March 28, 1999 in the Garhwal Himalaya, India

Garhwal Himalaya has been rocked by two major earthquakes in the span of just eight years, viz. Uttarkashi earthquake of 20th Oct, 1991 and Chamoli earthquake of 28th March, 1999. Chamoli earthquake of March 28, 1999 was recorded at 11 different stations of a strong motion array installed in the epicentral region. The maximum peak ground acceleration (353 cm/s2) was recorded at an accelerograph located at Gopeshwar. The data from eleven stations has been used for comparison with the simulated acceleration envelopes due to a model of the rupture responsible for this earthquake. For simulation of acceleration envelope the method of Midorikawa (1993) has been modified for its applicability to Himalayan region. This method has earlier been used by Joshi and Patel (1997) and Joshi (1999) for the studyof Uttarkashi earthquake of 20th Oct, 1991. The same method has been used for study of Chamoli earthquake. Layered earth crust has been introduced in place of homogeneous one in this method. The model of rupture is placed at a depth of 12 km below the Munsiari thrust for modelling Chamoli earthquake. Peak ground acceleration was calculated from simulated acceleration envelope using layered as well as homogeneous earth crust. For the rupture placed in a layered crust model peak ground acceleration of order 312 cm/s2 was simulated at Gopeshwar which is quite close to actually recorded value. The comparison of peak ground acceleration values in terms of root mean square error at eleven stations suggests that the root mean square error is reduced by inclusion of a layered earth crust in place of homogeneous earth crust.

Modelling of rupture planes for peak ground accelerations and its application to the isoseismal map of MMI scale in Indian region

The rupture plane for an earthquake has been modelledby using the semi empirical technique of Midorikawa(1993). This technique estimates ground accelerationby modelling the rupture process during an earthquake.Modifications in this technique have been made for itsapplication to the Indian region. This has been tested forthe Uttarkashi earthquake of 20th Oct, 1991, India, whichwas well recorded at thirteen stations of installedstrong motion array in this region. After testingseveral possible rupture models, a final model has beenselected and peak ground acceleration due to thismodel is simulated at thirteen different stations.Dependency of methodology on model parameters, e.g.dip and mode of rupture propagation have also beenstudied in detail.Using this technique synthetic isoseismal maps wereprepared by converting peak ground acceleration intoMMI scale. Dependency of rupture models on syntheticisoseismals has also been studied in detail. Usingthis method, peak ground acceleration for the Laturearthquake of Sept 30, 1993 has been obtained atvarious places within meisoseismal area. Synthetic andfield intensity was compared at various well-knownsites. Since the region was not covered by anyinstrumental array during Latur earthquake, thesimulated peak ground accelerations are expected toserve basis of design criteria in this region.