Neotectonic and Isoseismal Atlas of North India (Geological Survey of India)

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An earthquake is generally caused by the release of very large amount of elastic energy by the sudden rupture of rocks at depths of several kilometers below the ground surface. Such ruptures generally tend to take place along preexisting weak planes within the interior of the earth. It is actually a long drawn process of strain accumulation along these preexisting weak planes, the culmination of which manifests itself in the form of release of vast amounts of destructive energy.

It is in this context the neotectonic studies acquire its importance, as the tectonic discontinuities having evidence of movements in geologically recent times, are supposed to be the features along which a future earthquake would eventuate. This along with the macroseismic study, which helps in the estimation of the maximum intensity and understanding the pattern of the distribution of the seismic intensity, eventually leading to identification ofthe causative fault, are the most important components for the Seismic Hazard Assessment of an area.

The present compilation, I believe, is going to be a very useful document for all those professionals and students alike who are involved in the earthquake studies. The first part of the document provides an insight into the neotectonic status of the various faults I thrusts, which is a rare collection of field data collected over a period of several decades. The second part is another rich compendium of isoseismal maps, mostly prepared by the field geologists of the GSI. It not only takes stock of the ground truth regarding the intensity of ground shaking consequent to different earthquakes but also provides an overview of the pattern of energy release by almost all the damaging seismic events, which have struck the northern part of India and the contiguous areas in the neighboring countries for the last 125 years.

I hope that this compilation will be of immense utility for the pre-disaster mitigation part of the disaster management studies, which are going full throttle after getting kick-started in the last decade of the last millennium- aptly designated as the International Decade of Natural Disaster Mitigation (IDNDM) - and accelerated subsequent to the 2001 Bhuj Earthquake.

Earthquake is single natural hazard, which owing to its gargantuan intrinsic energy and having a tendency of striking with little or almost no indications of its arrival, has always been capable of causing unimaginable destruction of the civilization. This combined with unabated growth in human population with concomitant development of civil structures has enhanced the vulnerability to the seismic hazard and thus the risk to the society to unfathomable levels.

The temporal as well as the spatial unpredictability of the damaging seismic events has always been the bane for the earthquake scientists. Whereas the temporal prediction is still a far cry, the spatial prediction, logically, to some extent can be taken care of by identifying the tectonic surfaces, which have a history of movements in the recent past.

This Atlas, which deals with the causative and effective aspects of the earthquakes in two parts, namely, the Neotectonic Manifestations and Isoseismal Maps, can certainly be utilized as a handy tool in the quest for preparing a realistic seismic hazard scenario of an area.

In order to make the data on Neotectonic Manifestations, contained in Part I, more useful and relevant to the seismic hazard assessment, the younger Neotectonic episodes (from late Pleistocene to Recent times) have been given precedence over the older ones. It incorporates mainly those neotectonic manifestations, which have either been recorded on the basis of direct evidences or interpreted on the basis of sound geological methods.

Part-II of the Atlas comprises isoseismal maps and crisp write-ups of the damaging earthquakes of the North India right from 1885 to 2006. Significantly, the macroseismic investigations of most of the earthquakes described in this Atlas have been carried out by the GSI. In my opinion, this fact imparts to this document an added advantage of having an extensive field data collected by scores of experienced field officers, rendering the eventual outcome as more authentic.

This document dealing with two different but inter-related subjects has been presented in two parts. Whereas, the first one describes various neotectonic evidences found and / or mapped in the field, the second part is devoted to the isoseismal maps of the earthquakes, which have affected North India. Here, the term North India, in general has been used to include areas north of Tropic of Cancer.

The term neotectonics, as originally proposed by V.A. Obruchev, intended to mean 'the earth science devoted to the movements of the earth's crust that have taken place during the Late Tertiary (Neogene) and Quaternary periods'. This definition had been proposed to encompass the special stage in geological history connected with the evolution of present topography and structure of continents and ocean floors. Some investigators have narrowed its concept and limited it to the Quaternary movements only. The term 'neotectonics' is used in a still restricted sense by seismologists and earthquake engineers for whom it is synonymous with the contemporary crustal adjustments, the continued manifestations of which could be monitored by precision geodetic surveys.

It is evident that there is a need for rationalizing the concept of neotectonics, as it is still not completely defined, particularly for the utilitarian purposes it has to be referred to. It may be pointed out that a very large time span of about 24 million years included in the first proposition may be of utility for understanding the global tectonic frame-work and the sculpture of the major orogenic episodes. But, there is a widespread need for shortening of the time frame and bringing the same as near to the contemporary times as possible so that this could be utilized for assessing the seismic hazard in a particular region.

This has been necessitated because the deformation styles have undergone changes and modifications many a times after the major crustal adjustments, which took place in Miocene times. Ravi Shanker etitations et al. (1989), while evaluating the tectonic events in the Himalaya, pointed out that these gigantic mountain chains had been subjected to at least three orogenic movements after the collision of lndo-Burmese and Eurasian plates. The Second Himalayan Orogenic Movement (HOM-2) accompanied by intense plutonism, metamorphism and uplift around 20 ± 5 Ma continued upto 3.5 ± 1 Ma and was followed by HOM-3 with attendant uplift of Central Crystallines and development of Karewa basin in Kashmir. This was followed by another episode around 0.8-0.5 Ma when the Himalaya attained its present geomorphic form accompanied by folding of Siwalik and Karewa. This deformation style, though restricted to discrete tectonic domains in the form of epiorogenic adjustments, has continued to the contemporary times as neotectonics.

It is amply clear from the above that simply defining the Neogene tectonic adjustments as neotectonic movements would be too general a classification and would not serve the purpose of correlating the same with possible earthquake sources, because the deformation styles as well as their locales have repeatedly undergone changes in more recent geological times. Keeping these constraints in view, Narula et al. (1989) codified and classified the information on Neotectonic adjustments by restricting these to Quaternary periods and further subdividing the same into Older and Younger Neotectonic episodes. According to them the older episodes belong to a time-span between Neogene- Quaternary boundary and late Pleistocene periods, and the younger episodes include those tectonic adjustments, which involve the late Pleistocene and Holocene sediments. Such a classification though is more utilitarian for assessing the more recent tectonic adjustments, needs further codification for the younger neotectonic episode so that it could provide direct inputs for seismic hazard evaluation in a particular domain.

Under the International Program Project-II-2, the World Map of Major Active Faults was to be compiled for Global Seismic Hazard Assessment Programme (GSHAP). For this purpose, Trifonov and Machette (1993) had identified that the faults with historic, Holocene and late Pleistocene manifestation of activity were to be defined as active faults. This category of active faults is included in the younger neotectonic episodes of Narula et. al. (1989). Thus, if the categorized and codified information of the crustal adjustments is available, it could be utilized as direct input for various thematic analyses.

For the seismic hazard evaluation, the active fault identification has assumed importance because about 90% of the damaging historical earthquakes have occurred along the active faults. However, many active faults do not demonstrate seismic activity, which could be explained either by long return periods with slow slip rates or by contemporary adjustments related to aseismic creep. It may therefore be said that though, identification of active faults is an important input in seismic hazard evaluation the same has to be critically examined and monitored before putting it to use in specific local hazard evaluation.

The history of neotectonic/active faults studies in India goes back to more than seven decades, when implication of active faults on the design of major Hydroelectric Projects were understood and assessed. Since then many neotectonic manifestations have been recorded, geologically mapped in detail and interpretative sketches and models generated. In isolated cases, dating and monitoring of the continued adjustment have also been attempted. Most of such data have been generated as spin off benefits of the geotechnical investigations conducted by the Geological Survey of India in the Northwestern Himalaya. This information lay scattered in the departmental geotechnical and other reports submitted from time to time. There has been a long felt need for codification and documentation of this information so that the same could be utilized as an input for seismic and environmental hazard evaluation. As a preliminary attempt, Narula et. al. (1989) synthesized such information to generate a Neotectonic Map of the Himalayan Region, which also contained the codified information on the younger and older episodes of neotectonic activity. This map has been utilized as the base map for depicting the locales where direct evidences of neotectonic activity have been recorded (Fig. 1 ).

The Department of Science and Technology, Government ofIndia had initiated a multi-institutional project to study the seismicity of the Himalayan region with intensive studies in two areas-the Kangra region of Northwestern Himalaya and the Northeastern region of the country. Under this project one of the important work components identified was preparation of the Atlas of Neotectonic Manifestations as well as Isoseismal Maps. With-a view to achieve these objectives, compilation of the data as well as the maps generated on neotectonic manifestations at different localities was initiated in the year 1985-86, and a preliminary compilation of the same for Northwest Himalaya was submitted by Srivastava et al. (1986). The present effort, based initially on this work, has been supplemented largely with additional information, wherever available, and includes reprocessing of the entire material including the text and figures to present them in a common format in the present Atlas.

This Atlas incorporates mainly those neotectonic manifestations, which have either been recorded on the basis of direct evidences or interpreted on the basis of sound geological methods. Hence, those based on other indirect evidences e.g. anomalous denudation patterns through geomorphological methods, which indicate the crustal adjustments in the form of uplift or subsidence, have not been included in this compilation.

In this Atlas the Younger Neotectonic episodes (from late Pleistocene to Recent times) have been given more prominence because of their direct relevance to seismic hazard evaluation. It may also be pointed out that in the absence of well-established database on the chronology and chronostratigraphy of the sediments, the categorization of active faults with respect to the slip rates has not been properly attempted. Nevertheless, it is hoped that this Atlas would stimulate interest to unravel still younger tectonic surfaces and deformation styles which, in turn, would help in properly understanding the contemporary tectonic adjustments as well as in building models for understanding the source mechanism of earthquakes in the region.

Part-II of the Atlas comprises isoseismal maps and crisp write ups of the damaging earthquakes of the North lndia and thus brings the complete database on macroseismic study of different earthquakes at one place. In order to have an idea oflocation of different earthquake epicenters vis-a-vis the major tectonic elements of Northern India, Part-II of the document opens with a map having epicentral locations of the earthquakes, which originated in North India or adjoining regions. There is, however, omission of few earthquake epicenters simply because they originated way outside the Indian boundaries.

Though an unauthentic record of an earthquake occurrence in Rajasthan in 26 B.C. is available, the historical record of the earthquakes in North India dates back to 15th July, 1505. The first detailed survey of a damaging earthquake was made by Thomas Oldham of the Geological Survey of India for the Cachar (Assam) Earthquake of 1869 (Oldham \882). He also prepared and published the first known Catalogue of the Indian Earthquakes in 1883 (Oldham 1883). The classic, comprehensive account of the Great Assam Earthquake of 1897 by RD. Oldham including the delineation of isoseismals, laid the foundation of modern Seismology in India (Oldham 1899). Since the period of Oldham, valuable contributions have been made by various workers of the Geological Survey of India on the documentation of the effects and probable causes of important earthquakes. Such documentations are present in the Records, Memoirs and Special Publications of the Geological Survey of India and other national and international publications. Beside these a few earthquakes have also been investigated by other agencies and researchers. This document, therefore, is an attempt to bring the North India in perspective vis-a-vis the earthquakes by putting the scattered information on the earthquakes, which have either originated in or affected the Northern India, at one place. The compilation would be of immense use in reviewing different earthquakes and their source characteristics, which eventually would help in the evaluation of seismic hazard in different regions on macro as well as micro scales.

Since the time the earliest systematic earthquake studies were carried out, a number of intensity scales, which measure the violence of the ground motion on the basis of the effects on structures and the human perceptions, have been suggested. The Rossi-Forel (RF) scale (1883) is one of the oldest one which was utilized for quite some time. Though the 10-degree Mercalli Scale (1897) had also come into practice, R.D. Oldham, keeping regional suitability in view, propounded his own scale for the study of the Great Assam Earthquake of 1897. Subsequently came the Mercalli- Cancani-Sieberg scale (1917), which was followed by the most widely utilized modified Mercalli (MM) scale (1931). All these scales giving the sliding effects of earthquake motions were based on qualitative damage descriptions and effects on human being and other objects. The scale which has had wide acceptance during the last 4 decades is the Medvedev-Sponheuer-Kamik (MSK) scale (1964). This scale incorporates greater details about the grades of damage as well as the types or categories of structures and thus lessens the subjectivity. The MSK scale has been updated in the form of European Macroseismic Scale (EMS-1992), wherein certain inconsistencies of the MSK scale regarding types of constructions, effects on terrain, etc. have been rationalized to remove some more elements of subjectivity. In India, too, the macroseismic studies of different earthquakes have been carried out by using the then state-of-the-art intensity scales. However, in retrospect, this has introduced a certain amount of incompatibility among the studies of different earthquakes, which can be minimized by bringing the database to the same format or intensity scale.

Oldham (1899) suggested a conversion table for correlating his scale with R F scale, whereas Richter (1958) proposed a table for converting the RF scale to MM scale. Lately, many other conversion tables have been proposed, which could be utilized for standardization of the database. But re-designation of the isoseismals from these tables could be erroneous as in comparison the upper and lower bounds of intensities of different scales do not correspond exactly. The compilers of this document have, therefore, kept the scales described by various workers to maintain the originality of the database, and for realistic assessment of ground response to seismic waves. However, for a few major events for which sufficient database was available, corresponding intensity on MSK scale, presently in vogue, has been described in the text. A table comparing different intensity scales is given here (Table-I) and could be utilized by the readers as a ready reference. The original isoeismal maps have been plotted on a geological base which will be of great utility for the user to comprehend the possible source mechanism as well as the contribution of geological conditions for the attenuation/accentuation of ground motions. Thus, the endeavor of the compilers has been to enhance the quality of the database, by supplementing it with the state-of-the-art knowledge while maintaining its originality.