JABALPUR EARTHQUAKE
I am indeed thankful to Dr. D.M. Pancholi for his useful comments on the above subject. I stand correCted on the height of the Bargi dam and the maximum head of water available therein, as stated by
n.M.
Pancholi, and also the distance from the epicentral tract to the reservoir. The error is regretted. It may be borne in mind that this hardly changes the regional seismic picture. 'Regarding the relation between the Bargi reservoir and the cause of the J abalpur earthquake, it is better to await the regional studies in the area undertaken by the Geological Survey of India and the NGRI.
Chennai - 600 004 B. RAMACHANDRAN
DISCUSSliON
"ruGH DAMS IN CENTRAL HIMALAYA IN THE CONTEXT OF ACTIVE FAULTS, SEISMICITY AND SOCIETAL PROBLEMS" by K.S. Valdiya, Jour. Geol. Soc. India, vA9, pp,479-494
K.S. Krishnaswamy, Director General, Geological Survey of India (Retd.), #409, S. Lexington Ave., White Plains, NY -10006 comments:
Dr. Valdiya has to be congratulated on the painstaking marshalling of extensive geotectonic information relating to the Central Himalayan environment of the two high dams in U.P., viz., Tehri, now under construction and Pancheswar, close to the Indo-Nepal border, which is, presumably under detailed investigation. The commentator is an engineering geologist, who had been associated, for over two decades, with all the developmental phases of several River Valley Projects proposed in the entire Himalaya from Kashmir to Nepal, as a member of the Geological Survey ofIndia and, thereafter, for a couple of years in the Arunachal and Bhutan Himalaya as a Consultant. With this background, the opinions and suggestions of the commentator may be worth the indulgent consideration of the author, both as a supplement and, as a useful complement to the views presented by him in his praiseworthy contribution in the May '97 issue of the Journal.
The arguments put forth by Valdi ya against the construction of high dams in the Central Himalaya can be grouped as Geotectonics, Seismicity, Engineering Seismology and Environmental and Societal Impacts.
Geotectonics: In dealing with the numerous thrusts and faults of the Himalaya in relation to engineering construction, one has to decide, whether or not there is adequate and measurable evidence to designate them as active.
The concept of creep-tectonics is very relevant to the seismo-tectonic investigation of Himalayan faults of the type discussed in Valdiya's paper, specially with the instrumented measurements of creep as recorded at the outcrop of the MBT in the Yamuna Hydel Project area in U.P.
It is not clear from the author's presentation of the Quaternary deposits in the Tehri area, whether or not his conclusions on the active status of the Shrinagar thrust, that is inferred to underlie the Tehri Dam, were based on sub-surface information that may have been gathered in the last 15 years or so, or, whether these are based merely on visual geomorphic evidence and by photo-geological interpretation.
The mere fact of the occurrence of the south dipping Shrinagar Thrust need not be considered a deterrent for the construction of high dams. For, the high Bhakra (concrete) dam is located close to the trace of the Bhakra Thrust in the Sutlej river section, and by virtue of its steep downstream dip, as recorded at the river level, lies a few kilometers below the dam.
Have the author's geomorphological studies established his conclusion on the re,cent strike-slip movement on the North Almora Thrust at the outlet of the Kali river gorge, by the mapping of s i 111 i I ar and identical movements on all the tributaries crossing this thrust? If not, would the author consider selective erosion by the Kali river, in the zone of sheared and pulverized rocks of the Almora thrust (as cited by the author) to be an acceptable alternative explanation for the fonnation of the cited spectacular loop?
The significant observation of the abrupt narrowing of the gorges of the Bhagirathi and the Alaknanda, where these rivers cross the Shrinagar Thrust, has been made by the author. It is not clear whether this observation is based on visual inspection and interpretation, or is supported by actual cross-sectional profiles of the cited gorges upstream and downstream of the thrust and by correlation with the erosional competencies of the rocks crossed by the river on either side of the thrust. Likewise, the cited cascading ofthe north-flowing tributary streams that join the Bhagirathi, Bhilangana and Alaknanda rivers, which are known as "nick-points" in geoseismological terminology, is very significant observation by the author, in case the existence of these nick-points has been established by actual stream-profiles of the cited reaches of at least some of the tributary streams.
The occurrence of huge fans and debris-cones of past landslides, has been pointed out by the author, which, he presumes, may have been triggered by past earthquakes. It is well known that in the Himalayan setting, large landslides can be caused by toe-erosion in flood-stage and, by planar movements in bedrock, or, by slip-circle failures in the saturated overburden materials. Therefore, it does not seem necessary to invoke earthquakes as a causative factor for all landslides.
The Almora nappe, as it exists now after erosion, is rootless, as has been vividly brought out by the author in Fig.2. There are several kilometers of outcrop of this thrust-bound nappe, as erosionally carved out at the present time. These avenues for movement can be considered adequate to release accumulated strain on the thrust, even assuming it to be seismogenic, more by tectonic or by non-tectonic creep at the outcrops than by sudden failures along the thrust. The mere occurrence of the rootless Shrinagar Thrust under the Tehri Dam, or, of the Almora Thrust under the Pancheswar Dam, need not be considered as possible loci of future earthquakes of a magnitude that can endanger the safety of the dams.
Seismicity: As a geologist, the commentator is unable to convince himself, as some seismologists have done, that a "seismic gap", as hypothesized, can encompass earthquake events that relate to two different tectonic features on which strain accumulated probably not at the same rate, for release as one major earthquake. However, such a "seismic gap" is an acceptable hypothesis, if it relates to the same tectonic feature even though such a hypothesis may not be convincing to some seismologists, in the I ight of the successive magnitudes of earthquakes as recorded on some of the major earthq uake-generating faults of the world, such as the San Andreas Fault of California. In the case of the "seismic gap" cited by the author, the seismic events of Kangra and Bihar-Nepal epicentres, seem to relate to two different tectonic features, viz., the HFF and the Patna Fault under the Indo-Gangetic alluvium and hence the "seismic gap" seems to be more apparent than real.
If one were to look ':it Fig.6 and at the cluster of earthquakes portrayed therein, even in the area of the so-called "seismic gap", it seems possible that Mother Nature has been releasing accumulated strain in a number of smaller magnitude earthquakes and that it is
not necessary to invoke the prospect of an 8M+ earthquake for the release of the accumulated strain. The author himself seems to have accepted this possibility; but he quotes Khattri's conclusions of 1995 that the "gap-filling earthquake will at best be delayed but not forestalled". The commentator is unable to share this pessimism as predicted on the basis of the cited seismic gap area.
Whether or not the high seismicity of the Dharchula-Bajrang sector is really attributable to the author's postulated northward extension of the Moradabad fault as mapped under the Indo-Gangetic alluvium by aeromagnetic surveys and, cutting right across two contrasted tectonic units, viz., the foredeep and the overthrust belt, a feature which is uncommon, or, whether or not this is related to the MBTIMCT extensions at depth as well as to some of the tear faults that displace these features is a moot question.
Engineering Seismology: As a geologist, the commentator fears to tread this ground that is unfamiliar to him. However, one has to bear in mind that in the last 50 years or so of modern aseismic design of high dams there is to the best of his knowledge, not a single total fai1ure of a dam that can be related to earthquake acti vity, although reports of damage by cracking and settlement of structures exist. One good example of this is the Koyna Dam in Maharashtra, which lay right at the doorstep of a moderate sized earthquake and yet did not fail disastrously. This, the commentator feels is a testimony to the hard -rock and relati vel y low seismogenic environment of the Peninsula, as well as to the advances made in the field of engineering seismology.
The author makes certain observations that related to the hypothesis of reservoir and settlement load and seismicity. There has been no unquestioned acceptance of this hypothesis among both geoscientists and engineering seismologists. In this regard. the commentator belongs to the dissenting group of geoscientists. It is significant that the author, while quoting the dissenting views of Gupta and Rajendran, published by the Seismological Society of America in 1986. which rule out the possibility of reservoir induced seismicity in the thrust-bound Himalayan terrain, still prefers to invoke the cited hypothesis. While considering the reservoir and sediment load as affecting the pre-existing stress fields in the rocks at depth. thereby tilting the balance of stresses to result in the induced release of the strain through earthquakes, one has to contend with the engineering concept of dispersal of load at divergent angles and, therefore. over a larger area with depth. This would reduce the unit load over the area at the depth of a few km to a very small value, not adequate to imbalance the pre-existing stress conditions at depth. If this concept is accepted, there seems to be no great consequence to the reservoir and sediment load behind the two high dams of Tehri and Pancheswar.
Environmental and Sociological Impacts: While analyzing the prospect of heavy landslides and rapid siltation of reservoirs, due to the heavy sediment loads carried by the Himalayan rivers, it seems worthwhile to temper the conclusions made, with the knowledge already obtained on the functioning of the reservoirs behind the high dams already constructed in the Himalaya, viz., the Bhakra, Beas and Ramganga reservoirs. during the last 15 to 25 years of their existence. Does the author's knowledge in this regard support the view that large landslides can be caused by the Tehri and Pancheswar reservoirs? The information available to the commentator is 15 years old and in case this is trustworthy, the information does not permit such a conclusion.
The proven high siltation rates behind the dams in the Himalaya is indeed one of the reasons why, low dams, as postulated by the author, may not be found acceptable in all cases. High dams become an inescapable necessity.
The sociological issues that have been raised by the author, in tenns of the large submergence
areas and the destruction of the environment, as well as the uprootal of the people, are indeed issues, not special to Central Himalaya, but relevant to all big reservoirs proposed in hilly terrain..;, in any partofIndia, where forestry and population related problems are significant. The commentJtor is not aware of the provisions made in the environmental impact chapters of the project reporl.~. of the Tehri and Pancheswar Dams. However, he agrees with the author, that before clearing the projects, these reports should be open for pu blic hearing and clearance, as is the case now with the highly evolved democratic society ofthe USA, and which probably, in course of time, would also prevail in India, as the country progresses with its laudable experiments in democracy. It seems, however, that since omelettes cannot be made without breaking eggs, and if the omelette of better opportunities for living and for raising the standards of living of the people has to be made, the agony of breaking the eggs may not be avoidable. Even so, the author deserves to be praised for bringing out, so ably and so forcibly, not only the implications of the seismo-tectonic environment of the Central Himalaya for the two high dams, but alsD drawing attention to the cDnstraints posed by the natural environment and, by the sociological issues that arise from the construction of large reservoirs in the Himalaya.
K.S. Valdiya replies:
I am grateful to Shri. V.S. Krishnaswamy for very thought-provoking and informative comments on high dams in the Himalaya.
Having given profuse proofs.of activeness of many Lesser Himalayan faults including the Shrinagar and Almora Thrusts in a number of papers, I dwelt on this matter deliberately briefly in this paper on impacts of high dams. Geodetic measurements made by the Survey .of India scientists demonstrated continuing movements on the Shrinagar Thrust. The movements on fault\) are partly creep, but dominantly episodic in a very large number of cases. Since requisite subsurface information on dam sites is not available to the public at large, my deductiDn was perforce based on ge.omDrphic and structural evidence. Quaternary movements did affect the fluvial terraces in the Garhwal rivers crossed by the Shrinagar Thrust. On the basis of studies carried out elsewhere in many places I am convinced that the strike-slip faults coinciding with the boundary thrusts are quite active. This is the case in the Kali valley, in the Saryu between Haldughat and Rautgara and between Raintoll and Naichaum, NW of Rameshwar, and also in the stretch of the Kali river NE of
Tanakpur in the Siwalik. .
The abrupt narrowing of the otherwise wide valleys after crossing the active faults and the cascading of transversely flowing tributaries joining the Bhagirathi, Alaknanda, Bhilangana and Saryu rivers are based on visual inspection as well as on drawing cross-s.ectional profiles across the main valleys and along tributary stream channels in a few selected areas, including in the Ladhiya Valley in SE Kumaun.
Considering the extraordinary large dimension (each stretching over 6-8 km) and fan-like shape of the huge debris deposits only in that stretch of the Bhagirathi Valley which is cut by Shrinagar Thrust, I am inclined to attribute their formation to enormous landslides triggered by major earthquakes. Undeniably, toe erosion must have prepared the ground for slope failure.
The Almora Thrust delimiting the "rootless" synclinal Almora Nappe is active in both the flanks. Nowhere have I suggested that the Shrinagar and Almora Thrusts beneath the high dams would become the loci of great earthquakes of magnitude +8. All I said, and am restating, is that the Tchri-Uttarkashi and Dharchu]a-Bajang regions in Central Himalaya which are cut by active faults and thrusts and are under considerable tectonic strain, are potential sites of future great earthquakes and that in events of severe seismicity the dams located close to the active faults would face serinus problems. I do subscribe to the validity of "seismic gap", and feel that the small and moderate
earthquakes that happen recurrently do not adequately release the strain build-up in the compressed and faulted crust in the two regions. There is quite a substantial amount of strain yet to be released. Only mega events can do that. Irrespective of whether a great earthquake will or will not occur, the very fear of its occurrence is agonizing and tormenting. No one, I am sure, would want those people in the catchment areas behind the high dams to live in the dread of disasters.
It was commentator himself who had first pointed to the dangerous implications of the hidden Muradabad Fault in the Ganga Basin. This fault, in my opinion, influences the tectonics of northeastern Kumaun. The great loads of water and sediments behind dams will surely influence the stress regime of the already over-stressed and faulted crust made up of dominantly brittle rocks. Consequently, the frequency of natural hazards is bound to increase. Admittedly, in-depth studies need to be carried out in support of my postulation. A project on this aspect is being initiated.
I am sure that there are minimal chances of failure of the dams being built. It is not the safety of the dams that bothers me, it is the well-being, security and safety of the people who will be uprooted due to submergence of their lands and habitats and who will live in the constant fear of natural hazards that stir me intensely and excruciatingly.
True enough, one cannot make omelettes without breaking eggs, but the share of the omelettes should also go to the hapless hill people who will be paying unaccountably heavy price of the egg-breaking (development). Has this ever happened anywhere in the country of ours? As a matter of fact, there is not even a promise of benefit-sharing in the project proposals on dams.
"ANALYSES OF GROUND MAGNETIC ANOMALIES OVER A PART OF VINDHYAN BASIN AROUND DAMOR AND JABALPUR DISTRICTS, MADHYA PRADESH" by VP. Singh, O.P. Singh and C.L. Singh, Jour. GeoI. Soc. India, v.50, pp.4l-48
A.P. Agrawal, No.1 MIG, Shiv Nagar, Jabalpur - 482 002 and V.K. Khanna, Department of Geology, Government Science College, Jabalpur - 482 001 comment:
It appears that the authors are unaware of the detailed geology of the area. Further their interpretation is based on small-scale geological map published by GSI in 1993. As such there are some factual omissions and misinterpretations that might create confusion regarding the geologica] set up of the area. Following are such omissions and misinterpretations.
1. The southern and southeastern boundary of the Vindhyans in the said area is marked by a deep seated fault known as Son-Narmada n<;>rth fault. The fault is present close to Patan and Katangi and its trend is NE-SW. 2. To the south and southeast of the fault, rocks of Mahakoshal Group or thin alluvial cover over Mahakoshal
Group are present.
3. Deccan Trap is exposed in the southern most part of the area as a thin fringe. They invariably overlie Upper Gondwana and Lameta, which in turn overlie the Mahakoshal Group. Point Dis atleast 15 km north of the nearest Deccan Tmp outcrop.
4. Due to above mentioned omissions the subsurface interpretation along profiles BB', ce' and DD' appear to
be misleading.
V.P. Singh, O.P. Singh and C.L. Singh reply:
We thank A.P. Agrawal and VK. Khanna for their interest.
) . Our interpretation is based on the map published by GSI (1993), The demarcation of Vindhyans is as per this map.
2. Magnetic data interpretation is based on the assumption of susceptibility contrast. The susceptibility contrast between Vindhyans and crystalline basement as well as between the Mahakoshal group and the crystalline basement are expected to be of the same order; firstly, because the amplitude of the anomalies over the two formations does not change and secondly. both Vindhyans and Mahakoshal group are of sedimentary origin. For interpretation of profiles over sedimentary rocks average susceptibility contrast has been assumed. Hence the thickness of the formations derived here are correct.
3. Geologically the Mahakoshal group has been mapped at the surface, but the extent below the Vindhyans is not known. By magnetic method also the extent of the two grades of sediments cannot be derived specially when the susceptibility contrast is negligible. Therefore, the interpretation of underground geology along the profiles BB' , CC' and DO' are not misleading. The thickness of the sediments is not going to change in Figs. 6, 7 and 8.
4. Kaila et a1. (1989, Tectonophysics. v.162, pp.227-289) have given a detailed analysis of crustal structure along Hirapur-Mandala DSS profile. They reported crystalline basement beneath Jabalpur and Katangi on the basis of velocity distribution. However, in the absence of any well data, it is difficult to say that below Deccan Trap (Fig.S), Gondawana or Mah~oshal group are present.
"RECORD OF BENTONITIC CLAY NEAR GARJIA;RAMNAGARDISTRICT NAINITAL, UTTAR PRADESH" by R. Shukla and S.K. Kar, 10ur.Geol.Soc.lndia, v.50,
pp. 107-10~ .
Gopendra Kumar, 48, Pandariba, Lucknow - 226 004 comments:
I happened to examine the 'clay bed' along with the authors. The clay bed occurs in a tectoni~ zone - the Foot Hill Fault, between the Middle Siwalik and the rudaceous facies of the Varanasi Alluvium, the latter dipping at about 30° towards north. Unfortunately, the contact with the clay bed is concealed and nowhere is it found to occur at the base of the Varanasi Alluvium.
In the light of my observations, the correlation of the day bed with the Toba Ash, is hasty. Bentonitic clays are well known from the Siwalik sediments occurring at the contact of Middle and Upper Siwalik groups.
The authors in an earlier publication of 1994 have classified these Quaternary sediments as Varanasi Older Alluvium and Ramnagar Alluvium. They have not given any reason1Vjustifications for changing the nomenclature. Moreover, the use of name 'Ramnagar' for both the litho-units is not permitted as per the code of stratigraphic nomenclature.
R. Shukla and S.K. Kar, reply:
The sole objective of the present study was to record the occurrence of Bentonitic clay bed from the Quarternary alluvial sequence of Ramnagar area. The lithosection, under reference, indicate the disposition of the clay bed at the base of a gravel sequence which mayor may not be
the part of the rudaceous facies of the Varanasi Older Alluvium. The scope of regional correlation was limited jn the present study and hence a local informal stratigraphic nomenclature was adopted. Correlation with the Toba Ash was attempted on the basis of the compositional similarity as described in the paper.
