NADU
-
IMPLICATIONS ON DEPOS'tTIONAL CONDITIONS AND PALEOCLIMATE by S. Ramasamy, J. Madhavaraju and J. Armstrong Altrin Sam. .Jour. Geol. Soc. India, 2000, v.56, pp.47-52. *
K.S. Subramanian, 283, 17th East Street, Karnaraj Nagar, Tiruvanmaiyur, Chennai - 600 041 comments:
The conciuding observation of the authors aptly l~ightights the importance of studies to find out whether sediments of the Lower Gondwana sequence other than the Talchir are present in the Palar basin.
In the Palar basin extending over an area of 2000 km2 and in which basement contours go down to 3000 m, Talchir sediments are exposed in the southern part and are considered to be unconformably overlain by the Upper Gondwana sediments. The moot point to consider is whether the Lower Gondwana sediments other than those of the Talchir Group were not laid down in the basin or they were deposited and were completely overlapped by the Upper Gondwana sediments. If the latter were true, it is but reasonable to expect coal-bearing sediments at depth in the Palar basin.
Boreholes drilled by the Geological Survey of India and the Exploratory Tube-well Organisation were located in the southern part of the basin, close to the Talchir-Upper Gondwana boundary. On the basis of intersection of carbonaceous shales at depth in the borehole near Serilimedu, Murthy and Ahmad (1977) opined that a part of the sediment:; might belong to the Damuda Group of the Lower Gondwana. On the basis of this observation ancl in the context of the considerable thickness of sediments encountered in the basin, it is considered that background information on the entire sequence of sediments in the basin may well come to light if drilling is carried out away from the southern boundary, in the central part of the basin. Geophysical studies may well help in the location of boreholes. The presence of Darnuda sediments at depth -hopefully with coal seams of value - may be brought out by borehole data. Even if no coal-bearing sediments are encountered, the data will significantly contribute to our better understanding of the evolution of the coastal basin, known for interdigitation of fresh and marine environments.
S. Ramasamy, J. Madhavaraju and J, Armstrong A1 trin Sam, Sedimentology Lab., Department
of Geology, University of Madras, Chennai - 600 025 reply:
DISCUSSION 68 1
boreholes in the area. The data from drilling may help to address the still poorly understood problem of the extent of marine intercalation interspersed with paralic or non-marine sequence.
References
MURTHY, N.G.K. and AHMAD M. (1977). Paleogeographic significance of Talchir in the PaIar basin near Madras, (India). Fourth International Gondwana Symposium, pp.5 15-521.
VENKATACHALA, B.S. (1977). Fossil floral assemblages in the east coast Gondwana - A critical review. Jour. Geof. Soc.
India, v.18, 110.8, pp.378-397.
DISTRIBUTION OF HEAVY MINERALS ALONG THE BEACH FROM MANDAPAM TO KANYAKUMARI, TAMIL NADU by N. Angusamy and G. Victor Rajamanickam. Jour. Geol. Soc. India, 2000, v.56, pp. 199-21 1.
A.R. Nambiar, Geological Survey of India, Op. Karnataka and Goa, Vasudha Bhavan,
Bangalore - 560 078 comments:
I. Though it is stated that subsamples were collected at every 25 crn of the 2 m long auger drill cores (p.201), there is no mention thereafter in the text on the variation of heavy mineral concentration with depth. It is not understood whether the data presented and the discussions thereon are based on the mean content of heavy minerals (HMs) for a depth of 2 m at different sample locations or they pertain only to the top 25 cm of sediments.
2. Figure 1 shows 84 samples collected from 27 locations. Is it correct to presume that 3-4 sample's shown at each location represent samples drawn from different geomorphic units of the same locality? The area of study has been divided into 5 different sectors, but the sample points which belong to each sector cannot be made out either from Figure L or from Table 1 . Though 31 places are indicated in Table 3, only 27 sample locations are shown in Figure 1. This makes it impossible to follow thedescription given in the text on the distribution of HMs at different places in different sectors.
3. Table 1 shows HM data in different size fractions of !A phi interval, except in the case of column 2, where it is shown for +45-60 (Is it 4-45 and +60?) mesh. This type of data
presentation results in erroneous interpretation.
4. The text contains many incorrect statements. A few examples are given below:
a) In Mandapam sector, except in the case of samples from one locality (SI. Nos. 81-84), HMs are concentrated in the fine sand fraction (i.e., +70 to
+
120 mesh) and not in 170-200 mesh, as stated on page 202.
b) As HMs are not analysed in the coarse sand fraction (there is no need, as HMs seldom occur in coarse sand), what made the authors to state that HMs are concentrated i n
both coarse and medium sands in Valinokkam and Tuticorin sectors (see p.202 &
204)? In fact, HMs are mostly concentrated in the fine sand fraction in both the sectors. c) Though HMs occur dominantly in 100 and 120 mesh size in samples from Kanyakumari sector (except from one locality, SI. Nos. 1-3), it is stated that 120-140 mesh size forms the primary pick of the heavy mineral population in most of the stations of this sector (p.205). Further, in the abstract, it is stated that in Kanyakumari sector, heavies
are concentrated in coarse sands, which is contrary to the statement in the text. As HM . data are avaiIabIe at 'A phi interval, the authors should have computed the mean grain size of HMs and then commented on the texture of HMs at different locations, rather than making generalized statements.
5. (i) Whether the HM count data presented in Table 2 represent the mean values obtained by
counting HMs in a11 size fractions or they represent the HMs present in any particular size fraction? This is important as different HMs in any population may have different mean sizes. For example, garnets are generally coarse grained and their content is higher in coarse size fractions, whereas zircons are fine grained, and their concentration is more in fine size fractions.
(ii) Ilmenite wt.% is shown only against the sample from Kanyakumari. Does this mean that samples from other places are devoid of ilrnenite or has ilmenite not been estimated? (iii) The data presented raise serious doubts on identification of minerals and the accuracy
of grain counting. A few examples are cited: (a) absence of sillirnanite in samples of
Mandapam, Tuticorin and Kanyakumari and of monazite except in Kanyakumari, (b) very high concentration of topaz in Tuticorin and Manappad samples, ( c ) very high percentage of chlorite in Manappad and (d) presence of glaucophane in Valinokkam and Kanyakumari samples. It is not'necessary to evoke derivation of sediments from greenschist facies rocks, particularly when such low grade rocks do not occur in the southern Peninsular Shield, to explain the presence of good amount of biotite and muscovite in Mandapam setor.
6. Location-wise distribution of HMs in different sectors is shown in Table 3. Are the figures
the mean values of samples from different geomorphic units of a locality and obtained from Table 1 ? If so, the data presented in the two tables are at variance. To cite an example, the three samples from Vattakottai (Samples 4-6) have HM wt.% of 73.00, 46.64 and 65.74 (Table I), and in Table 3, the wt. of HMs at this, locality shown is 86.92%.
7. To draw frequency distribution curves, one should use the same interval along an axis, which has not been followed i n Fig.5. 'A phi interval has been used for size fractions finer than 60 mesh and % phi interval for fractions coarser than 60 mesh. This may result in erroneous interpretation of data and it is almost certain that the primary or secondary modes that occur around 40/60 mesh size in almost all frequency distribution curves (Fig. 4) have resulted because of this.
N. Angusarny and G. Victor Rajamanickam, Department of Industries and Earth Sciences, Tamil University, Thanjavur - 61 3 005 reply:
We thank Mr. AIR. Narnbiar for critically going through our pqper and welcome his comments
and queries. Specific clarifications to the queries are as follows:
1. The heavy mineral concentration shown in the respective station is not for the top 25 cm. It is for the entire auger core collected within the depth of 2 rn.
2. Fig.1 shows 3-4 stations at each location. Each station represents one geornorphic unit, generally accommodating low tide, high tide, berm and dune zones, wherever required (p.201). The area of study has been divided into five sectors. But the sample points in each sector could not be shown either in the figure or in the table because of referees' suggestions.
DISCUSSION 683
Vdinokkam sector, namely at K.R. Puram, have not been shown in Figure 1. Heavy mineral concentration is very low and heavy mineral analysis of these samples has been done in three different combined size grades of -45 +80, -80 +140, -140 +230. Because the heavy mineral separation has not been done fraction-wise, as in the case of other samples, the results are not shown in Table 1, which includes fraction-wise distribution. As the heavy mineral concentration values are very less, it is felt that if these stations are shown in Table
3, it may provide a better understanding of significant shift within the studied area.
3. TabIe I shows different size fractions at quarter phi interval. We thank Mr. Nambiar for
having pointed out the typographical error. The column 2 should have been -45 to'+6O instead of +45 tcp -60.
4. (a) In Mandapam sector, the observation made by Mr. Nambiar is acceptable to us. But, when we add all the four stations which have not been shown in Table 1, the dominant presence of heavy minerals in many samples is in the range of -170 to +230 mesh sizes. Since these data are not provided in TabIe 1, probabIy such a misunderstanding has arisen.
(b) The views of Mr. Nambiar regarding the texture of sands as coarse and medium are fully agreeable to us. Yes, the heavy minerals are chiefly segregated in fine sands only. (c) The whole paper discusses the heavy mineral distribution primarily for -45 to +230
mesh size fractions and also wherever appreciable heavies are noticed in very fine (-230 mesh size) fractions. It is known that the entire heavy mineral distribution is primarily in
the'fine sand category. Whatever we have used in. our study as coarse and medium sands, are only relative terms within the range of grain size spread (-45 to +230 mesh). Regarding the suggestion of computation of mean grain size of heavy minerals, it is well taken. As there is no significant shift in the mean values of the heavies in the different
sectors, we have attempted to use relative terms, coarse, medium etc. within the fine sand category, just to draw attention of the reader to the subtle differences we could observe.
5. (i) The suggestions on heavy mineral distribution are already well known.
It
is true that each heavy mineral has certain affinity to certain grain size. So, in order to avoid the fundamental differences in the distribution of heavy minerals in different fractions, we have prepared heavy mineral sections for each fraction and obtained the average only after adding the percentages obtained from all fractions.(ii) We are surprised to note such an observation. In Table 2, wherever the samples are devoid of ilmenites, we have shown those places by dashed lines.
(iii) Minerals like monazite, sillimanite, topaz etc. are very distinct in their characters and there is little possibility of the identification being wrong. However, after Mr. Narnbiar's doubts over the identification, representative slides have been rechecked and found to be correct. The samples are available in the Department fox verification.
6. We have presented the view that biotite and muscovite are likely to have been supplied from greenschist facies source area. Unless detailed studies are carried out, it is not possible to pinpoint the source area.
7. No, it is not the direct mean values of Table 1. The wt.% given in Table 3 are of the samples collected in the different geomorphic units around a locality, whereas the wt.% given in Table 1 represents different samples collected in traverse at a station. Table 1 is to project the concentration of heavies in different size fractions, whereas Table 3 is to project the
total concentration of heavies just to assess the possible economic feasibility. The differences in the wt.% of the heavies are on account of sampling.
DISCUSSION 685
8. We are fully in agreement with the suggestion of Mr. Nambiar. As there is not much variation in the distribution of heavies within :45 to +60, in order to avoid much reduction in the presentation: we have jumped with half phi interval from 45 to 60 mesh. We wanted to project the prolific distribution zone in prominence. So we maintained !A phi interval beyond 60 mesh. Likewise, where there is very low percentage of heavies, we have shifted knowingly, the scale in y-axis, so that the reader can follow the variations in the frequency curves in a better perspective despite its lower percentage. However, as suggested by Mr. Nambiar Fig.4 is redrawn and we find that there is no drastic shift as apprehended, because whatever we have shown in the paper is basedon actual results. Now, in theredrawn curves, maintaining x-axis at !A phi interval and wt.% the same interval in y-axis, we notice that the peaks match well with the published results (p.202, 204,205).
EVOLUTION OF QUATERNARY SEDIMENTS ALONG THE COAST
BETWEEN VEDARANYAM AND RAMESHWARAM, TAMIL NADU by P.M. Mohan, K. Shepard, N. Angusamy, M. Suresh Gandhi and G.V. Rajamanickam. Jour. Geol. Soc. India, 2000, v.56, pp.271-283.
A.R. Nambiar, Geological Survey of India, Op. Karnataka and Goa, Vasudha Bhavan, BangaIore - 560 078 comments:
I . It is difficult to understand the author's statement "when the mean values of different environmental stations are plotted separately in Fig.2, it is found that coarser particles shifted from north to south, in such a way that the existing trend is seen in TH and DE for offshore environments. The same trend is noticed for the beach in SM and ML. If one moves further inland, a trend almost similar to that of the offshore and the beach is observed in the station VL, which is almost parallel to the MI area of the present day (pp.274-275)."
2. Statements such as ( 1 ) "This skewness is attributed to the deposition of different kinds of sediments from different depositional conditions that existed from time to time in the different environments" (p.275) and (2) "Based on the foregoing inferences, it could be conveniently concIuded that the sediments available in the KI area have locally got redistributed by the existing waves and currents" (p.274) do not convey any d e a r meaning. Waves and currents play dominant role in sediment deposition not only in KI area, but everywhere in a nearshore marine environment.
3. The authors have wrongly used the term coastal landforms in place of marine landforms under the head 'Geomorphology' (p.271). Further, the aeolian landforms also should include coastal dunes.
4. The samples studied have been grouped into offshore, beach and inland samples. Though not mentioned in the text, it appears from Fig.] that the inland samples represent sediments from different environments like river, delta and coastal plain (including dunes?). Grouping sediments of different environments into a single category and analyzing depositional environments on the basis of textural parameters is not proper.
5. Inferences, in many cases, are drawn without any supportive evidence. For example, only
on the basis of skewness of sediments, the present inland area has been inferred to be a beach of the past (p.275).
6. Fig.4 does not indicate the discriminant line separating the beach and river sediments after Moiola and Weiser (1968).
7. The authors on the basis of linear discriminant analysis (Sahu, 1964) infer the offshore sediments to be derived from fluvial source (p.280). In a near offshore area, as the sediments are mostly brought in by rivers, such inferences do not have any validity. Linear discriminant analysis, in the present case, has simply failed to distinguish shallow marine environment from fluviatile environment.
8. Though there is no mention in the descriptive part of the text, in the abstract it is written that grain size analysis indicates the coastal plains to extend 25 km inland from the present shoreline. Further, what made the authors to consider the thickness of sediments as 10 m in the inland area?
9. The computation made by the authors in arriving at the figure of 1,50,000 years as the time taken for development of 25 km stretch of land is not understandable. It is stated that for an average depth of 10 m, the land-forming process takes an average of 60 years for an area of 10 sq. cm. What does this mean? Further to get the figure of 1,50,000 years, the following calculation is given: (100xl000x2500x60)/(lOOx1000). What do these numbers represent?
P. Mohan, K. Shepard,
N.
~ n ~ u s a r n ~ , M. Suresh Gandhi and G.V. Rajamanickam, Departmentof Industries and Earth Sciences, Tamil University-, Thanjavur - 61 3 005 reply:
We thank the cornmentator for his critical observations. The rep1 y to his comments is given as
under:
1. We fully appreciate the point raised by the cornrr~entator. The original manuscript submi tted contained elaboration on the place and parameter. On the advice of the referees, we had abbreviated the point, which we felt was nevertheless understandable.
2. The statement ( I ) "This skewness is attributed to
...
different environments" is nothing but a prelude before explaining the fact in detail afterwards. If one reads the full matter that follows, one may be able to understand the explanation given. Regarding the statement (2) we clarify that normally one expects in the inner shelf and that too close to shore line, the presence of medium to coarse sand. But in the site of KI (Kodiyakkarai), fine sediments arepresent. They are attributed to the action of waves and currents, and the type of sediments. available at that place. It means that the type of currents or waves that existed there could
not bring any coarse sediments, as noticed in ather stations like Manamelkudi (MI). The direct impact of waves and currents could not reach this point due to the presence of barriers like spit, etc. Moreover, low energy conditions of waves and currents in the KI. area is pointed out by the statement "the existing waves and currents".
3. The terminology that we have used for coastal landforms is correct and has been taken from Dornkamp and King (1970), Bird (1984, pp.277-296) and Hart (1986, p.71). Marine landforms are different from coastal landforms and we differ with the commentator's suggestion for inclusion of coastal dunes in the aeolian landforms. Even though coastal dunes are aeolian in origin, they are included in coastal landforms. There are also inland dunes which may not appear in coastal landforms.
offshore, beach and inland for grouping the respective sub-environments. We have grouped some related micro-environments like low tide, high tide, etc., with the beach environment. We did not group upstream and beach environments together.
5. Inferences are drawn from the results that we have obtained. We have attempted our level best to supplement the inferences from the other sources e.g. Bruckner (1988).
6. Moiola and Weiser ( 1 968) have not developed any discriminant line for skewness vs. standard deviation. So, it is not included in Fig.4.
7. We have clearly pointed out in the text that there are only 1 I samples which indicate the positive marine influences and the rest of 46 samples are of fluviatile origin. By virtue of this finding, we cannot say that the discriminant analysis failed to distinguish shallow marine environment from fluviatile environment. The deposited fluviatile sediments do not undergo the change in signatures related to shallow marine environment all of a sudden, and changes take place only after some time. The discriminant analysis could not therefore reflect the nature of sediments i n the transformation process.
8. The extent of coastal plain is indicated only from the analyses of samples taken by the authors. The thickness of 10 rn coastal sediments is inferred from the average thickness noticed in local domestic wells and groundwater bodies.
9. Taking the latest data available on the rate of sedimentation in Palk Strait, the extent of area is considered for the existing locations. The computation is made as a hypothetical case to understand the time taken to develop this extent. Since our attempt of computation matches with Bruckner's (1 988) age of radiometric dating, we thought that it will be of interest to researchers. The numbers used in the calculations are as follows:
(1OOx 1OOOx2500x60)/(1OOx1000) = 1,50,000 years 100 = I cm width x 100 cm depth
25x I OOx 1000 = 25,00,000 cm or 25 km length across 60 = 60 years
(To convert cm to km, the total distance is divided by 100x1000)
Otherwise, the straight way of calculation can also be carried out in the following manner:
25x 100x 1000 cm (length across) divided by the rate of sedimentation of 17 cm per year may give the result of (25,00,000)/17 = 1,47,058 years. This may be rounded off to 1,50,000 years.
References
RRUCKNER, D.H. (1988). Indicators for formerly higher sea levels along the e a t coast of India and on the Andaman Islands. Hamburgel- Geographische Studien, v.44, pp.47-72.
BIRD, E.C.F. (1984). An Introduction to Coastal Geomorphology, 3'Edn., Basil Blackwell, Oxford. HART, M.G. (1986). Geomorphology - Pure and Applied. George Allen and Unwin, London, 228p.
DORNKAMP. I. C. and KING, C.A.M. ( 1 970). Numerical Analysis in Geornorphology - An Introduction. St. Martin's Press, New York, pp.207-272.
MOIOLA, R.1. and WEISER, D. ( 1 968). Textural parameters: An evaluation. Jour. Sed. Petrol., v.38, pp.45-53,
SAWU, R.K, (1964). Depositional mechanism from the size analysis of clastic sediments. Jour. Sed. Petrol., v.34, pp.73- 83.
STRUCTURE
OF MANGANESE-BEARING METASEDIMENTARY ROCKS OFTHE
SAUSAR GROUP AROUND KANIDRI MINES, NAGPUR DISTRICT, MAHARASHTRA by S. Mohanty and P.K. 1Nayak. Jour. Geol. Soc. India, 2000,v.56, pp.79-87.
R.K.
Bopche, Sr. Lecturer, CED, KITS, Ramtek, Nagpur (MS) and I.A. Siddiqi, Sr. Manager(Geol.), Kandri Mines, Rarntek, Nagpur (MS) comment:
1. We would like to differ with authors on their observations regarding the quartzite of Chorbaoli Formation. The authors have called (he rock as conglomerate-quartzite. We both visited the site (distance post no.113619 and 1137 on Ramtek-Kandri branch railway line as described earlier by Mohanty, 1993, Jour. Geol. Soc. India, v.42, pp.55-60) and found that the conglomerate described by the authors actually represents fractured and broken pieces of quartzite on the top of the hill situated northwest of new IT1 building. We could not fine
a single piece of conglomerate in the outcrops.
2. Terminology used by the authors is highly confusing, as they have mentioned that the lithology of conglomerate-quartzite-marble grades into one another. The depositional environments of conglomerate and marble are poles apart. The facies cannot vary so abruptly, that immediately after conglomerate, marble would start deposition without any depositional break.
3. Whatever is found in and around Mansar-Kandri mines is dolomitic marble, not quartzite- marble, as confirmed by data from several quarries and bore wells.
S. Mohanty, Department of Applied Geology, Indian School of Mines, Dhanbad
-
826 004 replies:The following clarifications are offered against the points raised:
1. The conglomerate bed was reported by Mohant:y (1993, Jour. Geol. Soc. India, v.42, pp.55-
601, along with the photographs and reference to locality. The outcrops are located on the low ground and on the hill slopes on either side of the distance posts mentioned (not at the top of thk hill). The conglomerate of the area is not a fractured rock with only broken pieces of quartzite. It is polymictic in character. The correlation of the conglomerate-quartzite unit with the Chorbaoli Formation is, however, debatable. Mohanty (1993) has demonstrated this unit to be older than Mansar Formation. This can also be correlated with the Sitasaongi Formation.
2. The environmental model for conglomerate ancl marble considered by Bopche and Siddiqi is speculative. Associations of conglomerate and limestonelmarble are found in many stratigraphic records of the Precambrian of India (e.g. Aravalli Supergroup and Penganga Group).
3. The point raised regarding the nature of marble around Mansar and Kandri Mines is irrelevant.
DISCUSSION 689
RUPTURE MECHANISM OF CHAMOLI EARTHQUAKE ON 29 MARCH 1999 AND ITS IMPLICATIONS FOR SEISMOTECTONICS OF GARHWAL HIMALAYA by P.L. Narula, Ravi Shanker and S. Chopra. Jour. Geol. Soc. India, 2000, v.55(5), pp.493-503.
B.K.
Rastogi, National Geophysical Research Institute, Hyderabad - 500 007 comments:1 was happy to read the idea that transverse faults have controlled ruptures of 1991 Uttarkashi and 1999 Chamoli earthquakes and the recommendation, "It is therefore, necessary that the location of the fundamental transverse features is properly constrained by multidisciplinary inputs". Narula et al. have missed some of the earlier work published on the rupture processes in the Himalaya. Earlier ideas on the topic under discussion should have been properly credited with appropriate references. The constraints provided by other studies and models should have been considered. If the ideas are not acceptable, they should have been adequately debated, at least for their earlier appearance in the literature. Mentioned below are three main points:
1. Narula et al. write in their conclusion "It is now accepted that the Himalayan arc has been segmented by transverse tectonic surfaces". This idea put forward earIier by Valdiya (1973) and later expanded by Rastogi (198 1) has not been referred to. I quote from p.57 of the iater
reference
'.
.
.
,. . .
Faults transverse to Himalayan arc have been given significant importance by several geologists (Valdiya, 1973)...
A plausible explanation of the transverse faults in the collision model is that thrusts result from shortening and non-consumption on collision while the transverse faults are essentially due to later block adjustments'.2. For the 1999 Chamoli earthquake, westward migration of the rupture along the detachment surface was recorded by Rastogi (1999). It is reassuring to read the idea of Narula et al. that the rupture has migrated towards south and west. This matches with our unpublished model of strong motion data.
3. Narula et al. mention that rupture for 1991 Uttarkashi earthquake has migrated to up-dip and
towards east along the detachment surface. The basis for this assertion is not given and is contrary to the finding by Cotton et al. (1995). They modeIIed the strong motion data and worldwide teleseismic data. The major rupture migration is' up-dip and towards west and minimal towards east. About these findings Professor Pradeep Tal wani of University of South Carolina commented 'demonstration of seismicity on a shallow thrust, the nature of slip, model studies relating to rise-time to the nucleation process are exciting results'. The details of this study are as follows:
The Uttarkashi earthquake of 20 October (19 October UTC) 1991 was associated with the Main Central Thrust of Himalaya (just south of the Vaikrita Thrust). With a
Mw
6.6,Ms
7.0 (USGS) and intensity VIII, this event is characteristic of the present-day motion on the thrust-fault system. Study of this earthquake with different data sets yielded better understanding of the faulting process of a major earthquake in the Himalayan frontal arc region. Firstly, modelling of the centroid moment tensor solution with teleseismic data indicated a shallow (between '10 and 15 krn) depth, low angle thrust event consistent with the fault-plane solution from first motion phase data. Secondly, forward modelling of the strong motions recorded at six stations (run by Roorkee University) helped in confirming the epicenter and showed that the rupture propagated towards west and south. Inversion of the accelerograms gave distribution of the slip on the fauIi plane that is inferredto have a dimension of about 36 km x 48 km (Fig. 2 ) . the maximum slip (1.5 rn) occurred 10 km west and 15 krn southwest of the hypocenter (Fig.2). The slip source-time function obtained with near-field data is similar to the function obtained from teleseismic records and shows a low moment
DISCUSSION
-
12 distance from hypocenter km. 'Q,
C
t
0
O 0
=
Slip, rnC
NORTH
E
e 1.50
.c 1.25
8 l2 1.00
C a75
e
a5 oa 24
\
36 24 12 0 -12
distonce from hypocenter km.
Fig.2.
- -
det0chme.t Indian cruet lllph Hlmaloyo A e ~ r e t i o n pr1.l.O S t o t t o n r
03
Seismic rnomenl: 1.5 10++19~ - m
r I
0
Seismic moment: 1.2 10++19~ rn
time , sec time
,
secMost of the seismic moment was released between 2 and 8 sec with total rupture duration of 9 to
1 1 sec (Fig.3). The relation between the slip distribution obtained by inversion, isoseismals, mapped faults and the aftershock locations indicated that the Uttarkashi earthquake probably occurred along the detachment surface, which coincides with !.he upper surface of the subducting Indian lithosphere. This detachment surface gently dips under the Lesser Himalaya and south of the Vaikrita thrust. The Vaikrita thrust marks the line separating the very shallow-dipping detachment (along which earthquakes like the Uttarkashi earthquake could occur) from the steeper-dipping, aseismic basement thrust further north. This observation is important for anappropriate estimate of seismic hazard in the Uttarkashi region.
RL.
Narula, Ravi Shanker and S. Chopra, Geologic:al Survey of India, Lucknow reply:We appreciate that the commentator agrees with our idea of transverse faults which have controlled the ruptures of 1991 Uttarkashi and 1999 Chamoli earthquakes and our recommendations
for location of the fundamental transverse features. Rastogi in his second comment also writes, "It is reassuring to read the idea of Narula et al. that the rupture has migrated towards south and west. This matches with our modelling of the strong motion data*'. The authors would like to convey their appreciation for going through our paper and generating a healthy discussion and making these observations supporting the contention of the authors. Point-wise reply to the different comments is given below:
1. Though his observation that some of the earlier references in this regard such as Valdiya (1973) and Rastogi (1981) have not been referred to is correct, there are some more references as well on the subject and if these could not be included for whatever reason, we would like to clarify that there was no intention on the part of the authors to take away the credit of the earlier workers.
2. Regarding the second comment for not referring to the work of Rastogi (1999), the authors would like to clarify that the manuscript of the paper was submitted to the Journal in May, 1999, whereas the said workshop was held only in October, 1999.
3. Regarding his third comment on the rupture migration direction for 1991 Uttarkashi earthquake, the authors would like to state that thrxe observations are based on the pattern of isoseismals which were delineated on the basis of actual field observations. No seismological modelling has been done to amve at the source mechanism of Uttarkashi earthquake, but the energy attenuation, as shown by isoseismal patterns, suggests that the rupture propagation was in southeasterly direction. For more details the commentator may refer to GSI Spec. Publ. on the Uttarkashi Earthquake (1995) and paper by Narula et al. (1995) published in the Geol. Soc. India Memoir no.30. We would no1 like to enter into further discussion on this point as Uttarkashi earthquake does not form the topic of discussion of our paper.
References
COTTON, E., CAMPILLO, M., DESCHAMPS, A. and RASTOGI, B.K. (199ti). Rupture history and seismotectonics of the 1991 Uttarkashi, Himalaya earthquake. Tectonophysics, v.258, pp.35-5 1.
RASWI, B.K. (1981). Fault plane solutions and tectonics of Himalilya. Himalayan Geology Seminar, Geol. Sum. India Misc. Publ., 110.41, pt.IV, pp.51-60.
RASMGI, B.K. (1999). Seismotectonic inferences from the study of Chamoli earthquake of March 29, 1999. Workshop on Chamoli Earthquake and its Impact. (WOCEI-99). WIHG, Debra Dun, October 22-23, p.45 (abs).
