Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol19.Issue1-2.Feb2001:
Journal of Asian Earth Sciences 19 (2001) 155±163
www.elsevier.nl/locate/jseaes
Mapping of shallow three dimensional variations of P-wave velocity in
Garhwal Himalaya
I. Sarkar a,*, R. Jain b, K.N. Khattri c
a
Department of Earth Sciences, University of Roorkee, Roorkee 247 667, India
b
National Geophysical Research Institute, Hyderabad 500 007, India
c
100 Rajinder Nagar, Kaulagarh Road, DehraDun 247 881, India
Accepted 23 March 2000
Abstract
A set of short aperture seismic arrays was operated in the Garhwal Himalaya close to the Main Central Thrust and recorded a large number
of small local earthquakes. This study pertains to the inversion of the body wave arrival time data of these earthquakes to construct a seismic
velocity model for the region. The analysis indicates a systematic variation in the P-wave velocities of the upper crustal rocks. We estimate (i)
signi®cantly lower seismic velocities within the Middle-Lesser Garhwal Himalaya, and (ii) higher seismic velocities in the interface zone
between the Middle-Lesser and Higher Garhwal Himalaya. Seismic activity is mostly con®ned to a relatively narrow north-east dipping zone
in the upper 4 km of the crust characterized by a relatively higher P-wave velocity. This active seismicity represents reverse thrusting along
steep north-easterly dipping parallel slip surfaces within this zone forming a ramp in the crystalline formations of Higher Himalaya. The
lower velocity zone exhibits a low level of seismicity but appears to be associated with an increased landslide hazard. q 2001 Elsevier
Science Ltd. All rights reserved.
1. Introduction
From seismological considerations, the Garhwal segment
of the Himalayan mountain chain is distinctive. Located
within a seismic gap (Khattri and Tyagi, 1983; Khattri,
1987), small and moderate magnitude earthquakes and
severe landslides have frequently occurred here in recent
times. In contrast, a total absence of large earthquake occurrence for several decades makes the region a possible zone
of high seismic risk. For that reason implementation and
construction of hydroelectric power projects of the Garhwal
Himalaya is beset with controversy.
In an effort to shed light on some of these issues, by
quantitatively assessing the seismicity and possible seismic
hazards, a systematic investigation of microseismicity was
undertaken in phases between December 1979 and June
1990 along the entire Garhwal Himalayan mountain chain,
in the vicinity of the Main Central Thrust (MCT). The
details of this investigation and the ensuing results have
already been published in the literature (Gaur et al., 1985;
Khattri et al., 1989; Sarkar et al., 1993). The following
uncertainties about the data and interpretations presented
in those studies may be recalled. (1) In the absence of an
appropriate regional travel time table or seismic velocity
* Corresponding author.
model, a homogeneous semi-in®nite earth medium has
been assumed for all hypocentral parameter estimations.
(2) For operational reasons, the time scale on analog seismic
records was compressed so that the records could be changed less frequently. As a result for all earthquakes occurring
within the recording array, while the errors of reading of the
arrival times of the direct P-phases were at the most 0.2 s
those for the direct S-phases were often more than 1.0 s (3)
Even for earthquakes whose epicentral distances exceeded
the crossover distance for the ®rst crustal layer in the region,
the ®rst P-phases had to be considered as direct due to the
constraints of the assumed earth model. These limitations
resulted in errors in estimates of epicentral locations and
focal depths, greater than 1.5 and 6.0 km respectively, for
at least 25% of all considered earthquakes.
For the present study, we winnowed the entire data set
and selected data for only those earthquakes which had (1)
occurred within or close to the recording arrays (2) been
simultaneously recorded by at least four recording stations
and (3) from previous estimations, focal depths in 0±20 km
range and estimated errors of hypocentral locations within
1.0 km. Only 166 earthquakes could meet these constraints.
The locations of these earthquake epicentres and the recording stations, which provided the related seismic wave arrival
time data, are shown in Figs. 1 and 2.
Our study is in two parts: (1) inversion of selected P and S
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00023-7
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Fig. 1. A simpli®ed geological map of the Garhwal Himalaya (adapted from Jain and Chander, 1995) showing the main tectonic units. The rectangular area
identi®es the location of Fig. 2. The closed circles identify the seismograph station locations (S Ð Sayanachatti, B Ð Barkot, D Ð Dunda, L Ð Lambgaon,
M Ð Mahidanda, Bh Ð Bhatwari). The closed hexagons mark the locations of (i) the 1991 Uttarkashi earthquake epicentre (UK) and (ii) a point 20 km west
of the 1999 Chamoli earthquake epicentre (Ch). The abbreviations in the ®gure are as follows: MBT Ð Main Boundary Thrust, MCT Ð Main Central Thrust,
VT Ð Vaikrita Thrust, JT Ð Jutogh Thrust, GT Ð Garhwal Thrust, TT Ð Tons Thrust, SAT Ð South Almora Thrust, NAT Ð North Almora Thrust, BT Ð
Bhatwari Thrust, MT Ð Munsiari Thrust, THSZ Ð Tethys Himalaya Shear Zone.
arrival times for simultaneous estimation of hypocentral
parameters of the earthquakes and the local P and S wave
velocity values and (2) re-interpretation of ®rst P-motion
data.
2. Arrival time data
2.1. Method of analysis
We used Thurber's (1983) program in three successive
iterations, in a manner explained below, to invert the arrival
time data for simultaneous and improved estimations of
hypocentral location and earth model parameters. The
input data of the ®rst iteration comprised of (1) an assumed
one dimensional two layered earth model with constant P
and S wave velocity values in each layer as estimated from
previous studies (Chander et al., 1986; Kumar et al., 1989;
Salam, 1988; (Fig. 3) and (2) the hypocentral location parameters and arrival time data pertaining to the selected 166
earthquakes (Sarkar, 1983). The earthquake epicenters
delineate a generally NW±SE oriented belt, well de®ned
in its central part over approximately (60 £ 36) 2 area but
diffuse at its two ends (Fig. 2).
During the ®rst iteration we considered a (60 £ 36 £ 20)
km 3 volume of the earth subsurface directly below the
epicentral zone and divided it into 588 rectangular elements,
each having (10 £ 6 £ 2) km 3 volume and P and S wave
velocity values at its eight nodes as speci®ed by the assumed
earth model. The inversion results from this ®rst iteration
were selectively used in the following manner as input for
the second iteration. Of the 166 earthquakes, revised
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Fig. 3. One dimensional two-layered Earth model which was the initial
input model for the simultaneous inversion scheme.
Fig. 2. The locations of the seismograph stations and earthquake epicentres
are shown. The four sections along which the estimated seismic velocities
have been projected for analysis of the results of this study are also identi®ed here.
epicenters of 138 were located within the 60 £ 36 km 2 area.
These 138 epicenters delineated a revised NW±SE directed
seismic belt whose central part, covering (42 £ 30) km 2
area, was most well de®ned. A reduced (42 £ 30 £ 20)
km 3 volume of earth subsurface directly below this epicentral zone, subdivided into 588 rectangular elements, each
with volume (7 £ 5 £ 2) km 3 and P and S wave velocity
values at its eight nodes assigned from results of the ®rst
iteration and the arrival times of these 138 earthquakes
formed the input data set for the second iteration. Similarly
during the third iteration, only those 122 of the previous 138
earthquakes whose revised epicenters were located within
the (42 £ 30) km 2 area were selected. The most well de®ned
part of the revised epicentral belt now covered an area of
30 £ 18 km 2. A volume of (30 £ 18 £ 20) km 3 of the earth
directly below this epicentral zone was divided into 588
rectangular elements, each with volume (5 £ 3 £ 2) km 3
and P and S wave velocities at its eight nodes assigned
from the second iteration results and the arrival times of
these selected 122 earthquakes formed the input data set
for this last iteration.
Thus we had three sets of inversion results using relatively coarse, medium and ®ne scale grids.
seismic velocity in the earth only within 0±6 km depth.
Also the quantity and distribution of the arrival time data
of the S-phases is far less than that of the P-phases. Consequently there is poorer resolution of the S-wave velocity
variations. Only the estimated variations of the P-wave
velocity values in this depth range warrant interpretation.
Table 1 gives an overview of the comparative standard
error of the results obtained by the three successive runs of
the inversion scheme.
The standard error of the estimated P-wave velocity
values from all three iterations is small in magnitude but
those from the latter two are larger. A larger standard error
implies ®ner scale variations in estimated P-wave velocity
values. However, since the results of the second iteration
re¯ect a larger reduction in data variance when compared
with the one dimensional case, we prefer to interpret the Pwave velocity values using that iteration.
3.2. Resolving power of the results
For an overview of the resolving power of the inversion
results we conducted two separate synthetic tests. First we
considered a checkerboard resolution test (Zhao and Hasegawa, 1993; Kayal and Zhao, 1998). The checkerboard was
made by randomly assigning positive and negative perturbations of the order of 5% to P-wave velocity values at all
three dimensional grid nodes of the model space as had been
obtained from the inversion results. P-wave travel times
from the 138 earthquake hypocentres to the recording
stations were calculated using the velocity values estimated
3. Results of analysis
Table 1
3.1. The reliability of the estimated P and S wave velocity
values
Iteration Description of Decrease in data variance Standard error of
grid size
from the one-dimensional estimated P velocity
case (%)
values (km/s)
Since more than 90% of the earthquake hypocentres in
the data set are estimated to have depths of less than 6.0 km,
it has been possible to reliably resolve variations in the
1
2
3
Coarse grid
Medium grid
Fine grid
21.6
32.4
28.7
0.0276
0.0428
0.043
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Fig. 5. Projected P-wave velocity results along Section C (see Fig. 2). The
numbers along each contour indicate the corresponding estimated P-wave
velocity value there. The arrow indicates the projected position of Dunda
(DUN).
Fig. 4. (a) Projected P-wave velocity results along Section A (see Fig. 2).
The numbers along each contour denote the estimated P-wave velocity
value there. The arrow identi®es the projected position of Dunda (DUN).
(b) Projected P-wave velocity results along Section B (see Fig. 2). The
arrows indicate the intersection of the three E±W oriented sections (see
Fig. 2) and also the projected position of Sayanachatti (SAY) and Dunda
(DUN). The dots denote the selected earthquake locations projected onto
this section (see text). The numbers along each contour indicate the corresponding estimated P-wave velocity value there. The value (5.45 ^ 0.10)
on the right hand side of the ®gure, near the earthquake locations, is the
estimated average P-wave velocity at the nodes there. Because data resolution is poor here, contours have not been drawn.
at the grid nodes. Normally distributed random errors with
0.1 s standard deviation, were added to these travel times.
This error prone data was inverted while keeping the hypocentral parameters ®xed and using the checkerboard as the
initially assumed model. The image of the synthetic inversion of the checkerboard identi®ed the regions of good and
poor resolution. The errors incurred in velocity values at
nodes at depths of 0, 2 and 4 km for 7 km grid spacing
showed that the resolution was generally good over the
entire area of study but was consistently superior in its
central part. The power of resolution was highest at 0±
2 km depth but gradually diminished beyond 4 km.
However, a similar test with 10 km grid spacing indicated
that the checkerboard pattern was reconstructed with poorer
resolution. This implies that the ®ner details of the subsur-
face structure are lost in the latter case. We thus conclude
that the tomographic image obtained from our study has a
spatial resolution of at least 7 km in the upper 4 km of the
subsurface.
Next we performed a restoring resolution test (e.g. Zhao
et al., 1992). Here we used the tomographic image available
from the simultaneous inversion of our actual data as the
synthetic model, calculated travel times for this model and
introduced normally distributed random errors having standard deviation 0.1 s to these. This error prone data set was
inverted only for location parameters of the test hypocentres. The results showed that the error of locations never
exceeded 3.0 km and was less than 1.0 km for 89% cases.
The standard deviation of the distribution of the location
errors was less than 0.75 km.
The above numerical analysis provides suf®cient con®dence in the location parameters and velocity values simultaneously estimated from the actual data set.
3.3. Comments on estimates of hypocentral locations and
travel time residuals
For most of the 138 earthquakes, the estimated locations
are close to those estimated earlier (Sarkar, 1983) with shifts
at most of the order 0.5 and 1.0 km for the epicenters and
focal depths respectively. The larger shifts are always for
earthquakes located outside the array.
The travel time residuals (recorded±calculated) for all
earthquakes are positive and less than 0.20 s for 75% of
the cases. The largest residuals of the order of 0.9 s, are
contributed by earthquakes located outside the array, at
shallow depths and at smallest epicentral distances from
Dunda and Mahidanda, the southern stations of the array
(Fig. 1). This may imply that the near surface material
close to these stations has P-wave velocity values lower
than has been predicted by the computational procedure.
3.4. Variations of estimated P-wave velocity values
The estimated P-wave velocity values at different grid
nodes of the three dimensional model exhibit systematic
variations, both in depth and laterally. The depth variations
are shown in Figs. 4±6 in which velocity values have been
projected along the sections A, B, C and D (see Fig. 2).
I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
159
Fig. 6. Projected P-wave velocity results along Section D (see Fig. 2). The
numbers along each contour indicate the corresponding estimated P-wave
velocity value there. The arrow indicates the projected position of Sayanachatti (SAY).
Similarly the lateral variations at speci®c depths viz. near
the surface, 2 and 4 km below mean sea level are shown in
Figs. 7±9.
The projections along the three E±W oriented sections
shown in Figs. 4a, 5 and 6 reveal the following:
1. At any speci®c depth within 0±2 km range, the estimated
P-wave velocity values at all grid nodes generally show
negligible lateral variation. Also the rate of depth-wise
variation of these values is generally smooth. However,
compared to the velocity values at grid nodes near the
projected position of Sayanachatti, those near Dunda are
lower by several times the standard error of estimation.
This pattern of variation is manifest in the shapes of the
constant velocity contours at these two localized regions
(see Figs. 5 and 6).
2. In the 2±4 km depth range, the rate of depth-wise variation of P-wave velocity values along grid nodes in the
small localized zone below and around Dunda is reduced
Fig. 7. Isovelocity contours near the surface of the Earth. The numbers
along each contour indicate the corresponding estimated P-wave velocity
value there. The dots denote the locations of earthquake epicentres with
focal depths less than 2.0 km. The seismograph station locations (Sayanachatti Ð SAY, Barkot Ð BAR, Dunda Ð Dun, Mahidanda Ð MAH, and
Bhatwari Ð BHA) and the surface geology are also shown.
Fig. 8. Isovelocity contours at 2 km depth. The numbers along each contour
indicate the corresponding estimated P-wave velocity value there. The dots
denote the locations of earthquake epicentres with focal depths between 2
and 4 km. The seismograph station locations are also shown with open
triangles.
considerably. Also the grid nodes around the projected
position of Dunda encompassing approximately
(10 £ 10) km 2 area of the model space, have distinctly
low P-wave velocity values as compared to the surrounding nodes. This implies that a zone of signi®cantly low Pwave velocity, at least 4 km thick, may exist in the
subsurface below and around Dunda.
The low velocity zone is evident in Fig. 4b, the N±S
oriented projection also. Further this ®gure indicates that
the zone is localized in the southern part of the array. We
are of the opinion that this lateral limit is real and is not a
computational manifestation of poor seismic ray coverage.
For more than 75% of the 138 earthquake hypocentres with
(i) revised locations well within the array, between 0 and
6 km depth and (ii) travel time residuals less than 0.2 s, are
situated outside and north of the low velocity zone (Fig. 4b).
However, our results cannot provide any reliable lateral
limit for the southern end of this zone. Another interesting
feature evident in Fig. 4b is that earthquakes in the 0±2 km
depth range are generally located a little (less than 3 km)
south of those in the 2±4 km depth range indicating a north
dipping zone of earthquake distribution.
The isovelocity contours in Figs. 7±9 also provide insight
into the pattern of lateral variations in P-wave velocity at
shallow depths. Fig. 7 shows that near the ground surface
the estimated P-wave values at grid nodes close to the
projected position of Dunda are comparatively lower than
those at the surroundings nodes; in contrast, nodes near the
projected position of Sayanachatti have values comparably
higher than the surrounding nodes. The pattern of change of
the estimated velocity values along adjacent contours
reaf®rms the patterns in the constant velocity layers around
Dunda and Sayanachatti as was evident in the projections
along the E±W oriented sections (Figs. 4a, 5 and 6). The
difference between the velocity values in these two regions
is more than the standard errors of estimation. Fig. 8
exhibits similar variation patterns at 2 km depth although
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
migmatites and gneisses are present. The Higher Himalayan
crystalline rocks are separated from the metamorphosed
sedimentary Lesser Himalayan rocks here by the northerly
dipping MCT and its splays, the Bhatwari Thrust and
Munsiari Thrust. Although the exact ground location of
MCT is liable to varied interpretation, it is reported to
be evident at outcrop about a kilometer southwest of
Sayanachatti.
We infer the following from the variations in P-wave
velocity values of rocks lying in the shallow subsurface
below the array spread
Fig. 9. Isovelocity contours at 4 km depth. The numbers along each contour
indicate the corresponding estimated P-wave velocity value there. The
seismograph station locations are also shown with open triangles.
the differences between the velocity values surrounding
Dunda and Sayanachatti are reduced and appear less significant when compared with the standard errors of estimation.
At 4 km depth (Fig. 9) the contours are diffuse and suggest
further minor variation of P-wave velocity around Dunda
and Sayanachatti. In regions beyond 4 km depth, data resolution is poor. However Figs. 7, 8 and 9 provide evidence
that at least down to 4 km depth, the region around Sayanachatti has signi®cantly higher velocity as compared to
that around Dunda.
The areas of lower and higher velocity at the shallowest
level (Fig. 7) display a NW±SE trend with Sayanachatti and
Bhatwari forming two end points of the high velocity zone
and Dunda forming a single center of low velocity. The
epicenters are generally located in the transition zone
between these two zones. In the next deeper level of 2 km
(Fig. 7) the lowest velocity value center shifts northward
from Dunda, while the higher velocity zone now shows a
single high point between Sayanachatti and Bhatwari. The
transition zone shows a much more rapid rate of change of
velocity compared with the shallower level. The epicentres
occur slightly to the north of the zone of transition at this
level. At the 4 km depth level, the lower velocity zone
becomes narrower and better de®ned while the high velocity
zone also becomes a little bit tighter.
3.5. Interpretations of the variations in velocity
The six stations of the seismic array, whose recorded data
is considered here, straddles an area where rock units are
distinct and bounded by tectonic features identi®ed from
surface geology (Fig. 1). Dunda, Mahidanda and Lambgaon
lie in the inner Lesser Himalaya where Proterozoic quartzite, slate and limestone are identi®ed; Barkot lies in the
middle Lesser Himalaya amidst predominantly argillocalcareous metasediments. The south dipping North Almora
Thrust has been geologically identi®ed to separate these two
belts of Lesser Himalaya. Sayanachatti and Bhatwari are
situated in the Higher Himalaya where essentially schist,
1. At shallow depths generally the rocks are horizontally
strati®ed in thin sub-parallel layers.
2. The upper crustal rocks of inner Lesser Himalaya near
Dunda and Mahidanda have signi®cantly lower P-wave
velocity. In contrast, the upper crustal rocks of Higher
Himalaya near Sayanachatti have comparatively higher
P-wave velocity.
3. The higher velocity zone coincides with the surface
exposure of crystalline formations of the Higher Himalaya, which at its southern margin are demarcated by the
MCT (Fig. 4b). Estimated P-wave velocities in such
rocks are generally (5.3 ^ 0.3) km/s (see, for example,
Birch, 1942). In the exposed Proterozoic formations
between Dunda and MCT and the under-thrusted
argillo-calcareous metasediments of the Middle-Lesser
Himalaya, exposed SW of Dunda, estimated P-wave
velocities are generally (4.3 ^ 0.4) km/s (see, for example, Birch, 1942). It is suggested that the lower velocity
zone is located within these formations.
4. The majority of the small earthquakes occurred in the
higher velocity material and very few occurred in the
lower velocity material at these depths. Thus the small
earthquake activity in the region is mostly con®ned to the
higher velocity material, near Sayanachatti.
5. Because the surface trace of the MCT is identi®ed to be
very near Sayanachatti we consider this small earthquake
activity to be indicative of an active MCT zone.
4. First motion data
We winnowed the total P-wave polarity data from the 138
earthquakes considered for the arrival time inversion
program and selected only those readings, which were
most reliable and de®nitely unambiguous. Ninety-eight
polarity readings from 37 earthquakes were thus available
for this study. Of these hypocentres, 32 had revised locations within the higher P-wave velocity rock material identi®ed above and contributed 89 of the polarity readings.
4.1. Fault plane solutions
We projected all polarity data in a composite plot using
an equal area upper hemisphere projection (Fig. 10). The
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
the Yamuna-Bhagirathi river valleys must imply that this
region is tectonically different. However ®eld visits and
satellite imagery do not show any special physiography to
support such a view.
Hence we infer that the small earthquake activity, identi®ed in the higher P-wave velocity zone, amidst the upper
crustal rocks of Garhwal Higher Himalaya, is a result of
active deformation due to reverse faulting along steep
north-easterly dipping planes.
5. Discussion
Fig. 10. Composite fault plane solution using equal area upper hemisphere
projection. The open circles denote dilatations while the closed circles
denote compressions. The solid curves identify the nodal planes of the
preferred reverse thrust solution (see text for details).
projected data are distributed non-uniformly over the
projected focal sphere with more than 40% occurring near
the periphery. This is because the earthquake hypocentres
occur in a comparatively small volume of rock and are
mostly located directly below some recording station. The
nodal planes are poorly constrained and yield two equally
possible solutions. These are shown in Table 2.
Our preference is for the reverse thrust solution (solution
2) with the nodal plane NP1 as the fault plane. We give the
following reasons for our selection. Two composite fault
plane solutions for small upper crustal earthquakes, locally
recorded during December 1979 to June 1990 and located in
two speci®c regions between the Tons-Yamuna and Bhagirathi-Alkananda river valleys are available in literature
(Khattri et al., 1989; Sarkar et al., 1993). The two regions
¯ank the hypocentral zone of the present 37 earthquakes
from either side and are located close to it. Both composite
solutions are well-constrained and exhibit thrust mechanisms. It has been argued elsewhere (Sarkar et al., 1993)
that upper crustal earthquakes in the Garhwal Himalaya
generally occur due to thrusting. A strike slip fault model
for earthquakes occurring in the intervening area between
The data relate to small earthquakes whose hypocentres
were generally well distributed within a 170 £ 30 £ 20 km 3
volume of upper crustal Garhwal Himalayan rocks, extending from west of the Tons river valley to the Alaknanda river
valley and to epicentres distributed on either sides of the
MCT (Gaur et al., 1985; Khattri et al., 1989; Sarkar et al.,
1993). It may thus appear surprising that the data we used
for this study, the highest quality subset of the original data
set, relate only to those earthquakes which occurred in and
around the Yamuna-Bhagirathi river valley. This is possibly
because (i) in the entire eleven year period of ®eld recordings, the portable arrays operating in and around the
Yamuna-Bhagirathi river valleys were densest and most
well distributed in azimuth and (ii) in terms of epicentral
distances, these arrays were ideally suited for more reliable
P-wave arrival time recordings.
Our analysis has discovered the presence of a shallow
zone of low P-wave velocity and a low level of small earthquake seismicity, latitudinally con®ned in the Lesser Himalaya, to the Yamuna-Bhagirathi river valley region. Our data
do not reveal its longitudinal boundaries. However, we are
of the opinion that since the analysis pertains to earthquakes
which belong to the major continuous small earthquake
cluster (Gaur et al., 1985; Khattri et al., 1989; Sarkar et
al., 1993), it is most probable that this low velocity zone
actually extends in lateral directions throughout the entire
Lesser Garhwal Himalaya, from the Tons river valley to the
Alaknanda river valley. Further, due to constraints on the
resolution of data because of uneven distribution of the focal
depths, we are unable to provide estimates for the maximum
depth of this zone. Results of two systematic magnetic
surveys in the Ganga-Yamuna river valley have been interpreted to identify a narrow highly conductive zone in Garhwal Lesser Himalaya, in the same localized region as this
low P velocity zone but at 15±20 km depth (Arora and
Table 2
NP1
NP2
Solution
Strike
Dip
Slip
Strike
Dip
Slip
1
2
N 1728
N 1338
808 to the east
498 in N438 direction
88
888
N 2648
N 1238
888 to the south
428 in N 2148 direction
728
848
162
I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Reddy, 1995). Due to the present status of available ®eld
data, no reliable correlation between these two distinct
zones Ð one of low P-wave velocity and the other of
high conductivity Ð can be attempted. Further local earthquake investigations, audio magneto-telluric surveys and
geothermal investigations should prove useful in resolving
this issue.
Our study has also indicated that the upper crustal rocks
in the Yamuna-Bhagirathi river valley, lying close to the
MCT zone and to its immediate north, have higher seismic
velocities and show a high level of small earthquake seismicity. The composite fault plane solution imply that these
small earthquakes indicate thrust motion on high angle slip
surfaces. The occurrence of these earthquakes, a part of the
major small earthquake cluster (Gaur et al., 1985; Khattri et
al., 1989; Sarkar et al., 1993), is not con®ned to the area but
rather a part of the ongoing crustal deformation in the entire
Garhwal Higher Himalaya. We suggest that a crustal shear
zone with a card deck mechanism of simple shear exist in
the interface zone between Higher and Lesser Garhwal
Himalaya. Within this zone there are numerous parallel
slip surfaces, dipping steeply northeastwards, across which
reverse dip slip motion occurs causing small earthquakes. It
has already been established that the Higher Himalaya and
Lesser Himalaya are both rising but the Higher Himalaya
rises more relative to the Lesser Himalaya. This uplift of the
Higher Himalaya has been linked to the convergence of the
Indian and Eurasian plates (e.g. Seeber and Gornitz, 1983).
We suggest that this uplift is associated with the small earthquake activity within this upper crustal ramp zone.
The observed anomalous low velocity structure may be
caused by a variety of geological situations. For example
such a variation can result due to (i) a lateral change of
facies, from a higher carbonate content facies to a higher
arenaceous content facies, from south to north, (ii) a
temperature elevation caused by a deeper seated magma
intrusion or (iii) thrusting in a ramp thrust environment.
Although presently we have no de®nite evidence, because
of the general tectonic style of the region this later model is
considered to be quite plausible. However, V. Raiverman
(Oil and Natural Gas Commission, India) is of the opinion
that the low velocity is possibly related to elevated temperatures at depth in the region (personal communication).
The presence of lower velocity rocks in the Garhwal
Lesser Himalaya may have direct bearing on the possible
natural hazards of the region. For higher incidences of landslide hazards are generally to be expected within such rock
materials. Landslide susceptibility zoning in different river
catchment areas of Garhwal Himalaya, using geological and
geomorphological data, have indicated a high probability of
major mass movement activity in Garhwal Lesser Himalaya
(Pachauri et al., 1998). It is pertinent to mention here a
systematic ground and satellite imagery survey conducted
immediately after the occurrence of the Chamoli earthquake
(mb 6.3) on March 29, 1999 in the Garhwal Higher Himalaya. The survey showed that although damage to buildings
and ground was maximum in regions near and around the
MCT zone, almost all major landslides, caused or reactivated by the earthquake, occurred in the Lesser Himalaya
(Sarkar and Saraf, 2000; Sarkar et al., 2000).
6. Conclusions
1. A shallow, low P-wave velocity zone, exhibiting minor
seismic activity, extending from the valley of Tons river
to Alaknanda river in Garhwal Lesser Himalaya has been
mapped. The presence of this low velocity material may
generally be associated with the zones of landslides in the
region.
2. A crustal shear zone in the interface zone between the
Higher and Lesser Garhwal Himalaya with considerably
high P-wave velocity and pronounced seismic activity is
identi®ed. In this zone, there are numerous parallel slip
surfaces, steeply dipping to the northeast, along which
reverse thrust motion occur to cause these small earthquakes. We propose that this activity is associated with
the uplift of the Higher Himalaya relative to the Lesser
Himalaya and is a consequence of the general plate
convergence process of the region.
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belt of Garhwal Himalaya and their seismotectonic signi®cance. In:
Gupta, H.K., Gupta, G.D. (Eds.), Uttarkashi Earthquake. Memoirs of
the Geological Society of India, Bangalore, pp. 109±124.
Birch, F. 1942. Handbook of Physical constants. Geological Society of
America. Special Paper 36.
Chander, R., Sarkar, I., Khattri, K.N., Gaur, V.K., 1986. Upper crustal
compressional velocity in the Garhwal. Tectonophysics 124, 133±150.
Gaur, V.K., Chander, R., Sarkar, I., Khattri, K.N., Sinvhal, H., 1985. Seismicity and the state of stress from investigations of local earthquakes in
the Kumaon Himalaya. Tectonophysics 118, 243±251.
Jain, A.K., Chander, R., 1995. Geodynamic models for the Uttarkashi earthquake of October 20, 1991. In: Gupta, H.K., Gupta, G.D. (Eds.), Uttarkashi Earthquake. Memoirs of the Geological Society of India,
Bangalore, pp. 225±233.
Kayal, J.R., Zhao, D., 1998. Three dimensional seismic structure beneath
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Khattri, K.N., 1987. Great earthquakes, seismicity gaps and potential for
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plate boundary and identi®cation of areas of high seismic potential.
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seismological results on the tectonics of the Garhwal Himalaya.
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Sciences) 98, 91±109.
Kumar, S., Chander, R., Khattri, K.N., 1989. Compressional wave speed in
the second crustal layer in Garhwal Himalaya. Journal of the Association of Exploration Geophysicists 8, 219±225.
Pachauri, A.K., Gupta, P.V., Chander, R., 1998. Landslide zoning in a part
of Garhwal Himalaya. Environmental Geology 36, 325±334.
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Salam, A. 1988. Teleseismic P-wave travel time residuals and crustal structure of Roorkee. MTech dissertation (unpublished) University of Roorkee, India.
Sarkar, I. 1983. On seismological investigations of the Kumaon Himalaya
using local earthquake data. PhD dissertation (unpublished), University
of Roorkee, India.
Sarkar, I., Saraf, A.K., 2000. Some observations of the Chamoli earthquake-induced damage using ground and satellite data. Current Science
78, 91±97.
Sarkar, I., Chander, R., Khattri, K.N., Sharma, P.K., 1993. Evidence from
small magnitude earthquakes on the active tectonics of northwest Garhwal Himalaya. Journal of Himalayan Geology 4, 279±284.
Sarkar, I., Pachauri, A.K., Israil, M., 2000. On the damage caused by the
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indicators of active tectonics. Tectonophysics 92, 335±367.
Thurber, C.H., 1983. Earthquake locations and three-dimensional crustal
structure in the Coyote Lake area, Central California. Journal of
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Zhao, D., Hasegawa, A., 1993. P wave tomographic imaging of the crust
and upper mantle beneath the Japan Islands. Journal of Geophysical
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Zhao, D., Hasegawa, A., Horiuchi, S., 1992. Tomographic imaging of P and
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www.elsevier.nl/locate/jseaes
Mapping of shallow three dimensional variations of P-wave velocity in
Garhwal Himalaya
I. Sarkar a,*, R. Jain b, K.N. Khattri c
a
Department of Earth Sciences, University of Roorkee, Roorkee 247 667, India
b
National Geophysical Research Institute, Hyderabad 500 007, India
c
100 Rajinder Nagar, Kaulagarh Road, DehraDun 247 881, India
Accepted 23 March 2000
Abstract
A set of short aperture seismic arrays was operated in the Garhwal Himalaya close to the Main Central Thrust and recorded a large number
of small local earthquakes. This study pertains to the inversion of the body wave arrival time data of these earthquakes to construct a seismic
velocity model for the region. The analysis indicates a systematic variation in the P-wave velocities of the upper crustal rocks. We estimate (i)
signi®cantly lower seismic velocities within the Middle-Lesser Garhwal Himalaya, and (ii) higher seismic velocities in the interface zone
between the Middle-Lesser and Higher Garhwal Himalaya. Seismic activity is mostly con®ned to a relatively narrow north-east dipping zone
in the upper 4 km of the crust characterized by a relatively higher P-wave velocity. This active seismicity represents reverse thrusting along
steep north-easterly dipping parallel slip surfaces within this zone forming a ramp in the crystalline formations of Higher Himalaya. The
lower velocity zone exhibits a low level of seismicity but appears to be associated with an increased landslide hazard. q 2001 Elsevier
Science Ltd. All rights reserved.
1. Introduction
From seismological considerations, the Garhwal segment
of the Himalayan mountain chain is distinctive. Located
within a seismic gap (Khattri and Tyagi, 1983; Khattri,
1987), small and moderate magnitude earthquakes and
severe landslides have frequently occurred here in recent
times. In contrast, a total absence of large earthquake occurrence for several decades makes the region a possible zone
of high seismic risk. For that reason implementation and
construction of hydroelectric power projects of the Garhwal
Himalaya is beset with controversy.
In an effort to shed light on some of these issues, by
quantitatively assessing the seismicity and possible seismic
hazards, a systematic investigation of microseismicity was
undertaken in phases between December 1979 and June
1990 along the entire Garhwal Himalayan mountain chain,
in the vicinity of the Main Central Thrust (MCT). The
details of this investigation and the ensuing results have
already been published in the literature (Gaur et al., 1985;
Khattri et al., 1989; Sarkar et al., 1993). The following
uncertainties about the data and interpretations presented
in those studies may be recalled. (1) In the absence of an
appropriate regional travel time table or seismic velocity
* Corresponding author.
model, a homogeneous semi-in®nite earth medium has
been assumed for all hypocentral parameter estimations.
(2) For operational reasons, the time scale on analog seismic
records was compressed so that the records could be changed less frequently. As a result for all earthquakes occurring
within the recording array, while the errors of reading of the
arrival times of the direct P-phases were at the most 0.2 s
those for the direct S-phases were often more than 1.0 s (3)
Even for earthquakes whose epicentral distances exceeded
the crossover distance for the ®rst crustal layer in the region,
the ®rst P-phases had to be considered as direct due to the
constraints of the assumed earth model. These limitations
resulted in errors in estimates of epicentral locations and
focal depths, greater than 1.5 and 6.0 km respectively, for
at least 25% of all considered earthquakes.
For the present study, we winnowed the entire data set
and selected data for only those earthquakes which had (1)
occurred within or close to the recording arrays (2) been
simultaneously recorded by at least four recording stations
and (3) from previous estimations, focal depths in 0±20 km
range and estimated errors of hypocentral locations within
1.0 km. Only 166 earthquakes could meet these constraints.
The locations of these earthquake epicentres and the recording stations, which provided the related seismic wave arrival
time data, are shown in Figs. 1 and 2.
Our study is in two parts: (1) inversion of selected P and S
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1367-912 0(00)00023-7
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Fig. 1. A simpli®ed geological map of the Garhwal Himalaya (adapted from Jain and Chander, 1995) showing the main tectonic units. The rectangular area
identi®es the location of Fig. 2. The closed circles identify the seismograph station locations (S Ð Sayanachatti, B Ð Barkot, D Ð Dunda, L Ð Lambgaon,
M Ð Mahidanda, Bh Ð Bhatwari). The closed hexagons mark the locations of (i) the 1991 Uttarkashi earthquake epicentre (UK) and (ii) a point 20 km west
of the 1999 Chamoli earthquake epicentre (Ch). The abbreviations in the ®gure are as follows: MBT Ð Main Boundary Thrust, MCT Ð Main Central Thrust,
VT Ð Vaikrita Thrust, JT Ð Jutogh Thrust, GT Ð Garhwal Thrust, TT Ð Tons Thrust, SAT Ð South Almora Thrust, NAT Ð North Almora Thrust, BT Ð
Bhatwari Thrust, MT Ð Munsiari Thrust, THSZ Ð Tethys Himalaya Shear Zone.
arrival times for simultaneous estimation of hypocentral
parameters of the earthquakes and the local P and S wave
velocity values and (2) re-interpretation of ®rst P-motion
data.
2. Arrival time data
2.1. Method of analysis
We used Thurber's (1983) program in three successive
iterations, in a manner explained below, to invert the arrival
time data for simultaneous and improved estimations of
hypocentral location and earth model parameters. The
input data of the ®rst iteration comprised of (1) an assumed
one dimensional two layered earth model with constant P
and S wave velocity values in each layer as estimated from
previous studies (Chander et al., 1986; Kumar et al., 1989;
Salam, 1988; (Fig. 3) and (2) the hypocentral location parameters and arrival time data pertaining to the selected 166
earthquakes (Sarkar, 1983). The earthquake epicenters
delineate a generally NW±SE oriented belt, well de®ned
in its central part over approximately (60 £ 36) 2 area but
diffuse at its two ends (Fig. 2).
During the ®rst iteration we considered a (60 £ 36 £ 20)
km 3 volume of the earth subsurface directly below the
epicentral zone and divided it into 588 rectangular elements,
each having (10 £ 6 £ 2) km 3 volume and P and S wave
velocity values at its eight nodes as speci®ed by the assumed
earth model. The inversion results from this ®rst iteration
were selectively used in the following manner as input for
the second iteration. Of the 166 earthquakes, revised
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Fig. 3. One dimensional two-layered Earth model which was the initial
input model for the simultaneous inversion scheme.
Fig. 2. The locations of the seismograph stations and earthquake epicentres
are shown. The four sections along which the estimated seismic velocities
have been projected for analysis of the results of this study are also identi®ed here.
epicenters of 138 were located within the 60 £ 36 km 2 area.
These 138 epicenters delineated a revised NW±SE directed
seismic belt whose central part, covering (42 £ 30) km 2
area, was most well de®ned. A reduced (42 £ 30 £ 20)
km 3 volume of earth subsurface directly below this epicentral zone, subdivided into 588 rectangular elements, each
with volume (7 £ 5 £ 2) km 3 and P and S wave velocity
values at its eight nodes assigned from results of the ®rst
iteration and the arrival times of these 138 earthquakes
formed the input data set for the second iteration. Similarly
during the third iteration, only those 122 of the previous 138
earthquakes whose revised epicenters were located within
the (42 £ 30) km 2 area were selected. The most well de®ned
part of the revised epicentral belt now covered an area of
30 £ 18 km 2. A volume of (30 £ 18 £ 20) km 3 of the earth
directly below this epicentral zone was divided into 588
rectangular elements, each with volume (5 £ 3 £ 2) km 3
and P and S wave velocities at its eight nodes assigned
from the second iteration results and the arrival times of
these selected 122 earthquakes formed the input data set
for this last iteration.
Thus we had three sets of inversion results using relatively coarse, medium and ®ne scale grids.
seismic velocity in the earth only within 0±6 km depth.
Also the quantity and distribution of the arrival time data
of the S-phases is far less than that of the P-phases. Consequently there is poorer resolution of the S-wave velocity
variations. Only the estimated variations of the P-wave
velocity values in this depth range warrant interpretation.
Table 1 gives an overview of the comparative standard
error of the results obtained by the three successive runs of
the inversion scheme.
The standard error of the estimated P-wave velocity
values from all three iterations is small in magnitude but
those from the latter two are larger. A larger standard error
implies ®ner scale variations in estimated P-wave velocity
values. However, since the results of the second iteration
re¯ect a larger reduction in data variance when compared
with the one dimensional case, we prefer to interpret the Pwave velocity values using that iteration.
3.2. Resolving power of the results
For an overview of the resolving power of the inversion
results we conducted two separate synthetic tests. First we
considered a checkerboard resolution test (Zhao and Hasegawa, 1993; Kayal and Zhao, 1998). The checkerboard was
made by randomly assigning positive and negative perturbations of the order of 5% to P-wave velocity values at all
three dimensional grid nodes of the model space as had been
obtained from the inversion results. P-wave travel times
from the 138 earthquake hypocentres to the recording
stations were calculated using the velocity values estimated
3. Results of analysis
Table 1
3.1. The reliability of the estimated P and S wave velocity
values
Iteration Description of Decrease in data variance Standard error of
grid size
from the one-dimensional estimated P velocity
case (%)
values (km/s)
Since more than 90% of the earthquake hypocentres in
the data set are estimated to have depths of less than 6.0 km,
it has been possible to reliably resolve variations in the
1
2
3
Coarse grid
Medium grid
Fine grid
21.6
32.4
28.7
0.0276
0.0428
0.043
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Fig. 5. Projected P-wave velocity results along Section C (see Fig. 2). The
numbers along each contour indicate the corresponding estimated P-wave
velocity value there. The arrow indicates the projected position of Dunda
(DUN).
Fig. 4. (a) Projected P-wave velocity results along Section A (see Fig. 2).
The numbers along each contour denote the estimated P-wave velocity
value there. The arrow identi®es the projected position of Dunda (DUN).
(b) Projected P-wave velocity results along Section B (see Fig. 2). The
arrows indicate the intersection of the three E±W oriented sections (see
Fig. 2) and also the projected position of Sayanachatti (SAY) and Dunda
(DUN). The dots denote the selected earthquake locations projected onto
this section (see text). The numbers along each contour indicate the corresponding estimated P-wave velocity value there. The value (5.45 ^ 0.10)
on the right hand side of the ®gure, near the earthquake locations, is the
estimated average P-wave velocity at the nodes there. Because data resolution is poor here, contours have not been drawn.
at the grid nodes. Normally distributed random errors with
0.1 s standard deviation, were added to these travel times.
This error prone data was inverted while keeping the hypocentral parameters ®xed and using the checkerboard as the
initially assumed model. The image of the synthetic inversion of the checkerboard identi®ed the regions of good and
poor resolution. The errors incurred in velocity values at
nodes at depths of 0, 2 and 4 km for 7 km grid spacing
showed that the resolution was generally good over the
entire area of study but was consistently superior in its
central part. The power of resolution was highest at 0±
2 km depth but gradually diminished beyond 4 km.
However, a similar test with 10 km grid spacing indicated
that the checkerboard pattern was reconstructed with poorer
resolution. This implies that the ®ner details of the subsur-
face structure are lost in the latter case. We thus conclude
that the tomographic image obtained from our study has a
spatial resolution of at least 7 km in the upper 4 km of the
subsurface.
Next we performed a restoring resolution test (e.g. Zhao
et al., 1992). Here we used the tomographic image available
from the simultaneous inversion of our actual data as the
synthetic model, calculated travel times for this model and
introduced normally distributed random errors having standard deviation 0.1 s to these. This error prone data set was
inverted only for location parameters of the test hypocentres. The results showed that the error of locations never
exceeded 3.0 km and was less than 1.0 km for 89% cases.
The standard deviation of the distribution of the location
errors was less than 0.75 km.
The above numerical analysis provides suf®cient con®dence in the location parameters and velocity values simultaneously estimated from the actual data set.
3.3. Comments on estimates of hypocentral locations and
travel time residuals
For most of the 138 earthquakes, the estimated locations
are close to those estimated earlier (Sarkar, 1983) with shifts
at most of the order 0.5 and 1.0 km for the epicenters and
focal depths respectively. The larger shifts are always for
earthquakes located outside the array.
The travel time residuals (recorded±calculated) for all
earthquakes are positive and less than 0.20 s for 75% of
the cases. The largest residuals of the order of 0.9 s, are
contributed by earthquakes located outside the array, at
shallow depths and at smallest epicentral distances from
Dunda and Mahidanda, the southern stations of the array
(Fig. 1). This may imply that the near surface material
close to these stations has P-wave velocity values lower
than has been predicted by the computational procedure.
3.4. Variations of estimated P-wave velocity values
The estimated P-wave velocity values at different grid
nodes of the three dimensional model exhibit systematic
variations, both in depth and laterally. The depth variations
are shown in Figs. 4±6 in which velocity values have been
projected along the sections A, B, C and D (see Fig. 2).
I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
159
Fig. 6. Projected P-wave velocity results along Section D (see Fig. 2). The
numbers along each contour indicate the corresponding estimated P-wave
velocity value there. The arrow indicates the projected position of Sayanachatti (SAY).
Similarly the lateral variations at speci®c depths viz. near
the surface, 2 and 4 km below mean sea level are shown in
Figs. 7±9.
The projections along the three E±W oriented sections
shown in Figs. 4a, 5 and 6 reveal the following:
1. At any speci®c depth within 0±2 km range, the estimated
P-wave velocity values at all grid nodes generally show
negligible lateral variation. Also the rate of depth-wise
variation of these values is generally smooth. However,
compared to the velocity values at grid nodes near the
projected position of Sayanachatti, those near Dunda are
lower by several times the standard error of estimation.
This pattern of variation is manifest in the shapes of the
constant velocity contours at these two localized regions
(see Figs. 5 and 6).
2. In the 2±4 km depth range, the rate of depth-wise variation of P-wave velocity values along grid nodes in the
small localized zone below and around Dunda is reduced
Fig. 7. Isovelocity contours near the surface of the Earth. The numbers
along each contour indicate the corresponding estimated P-wave velocity
value there. The dots denote the locations of earthquake epicentres with
focal depths less than 2.0 km. The seismograph station locations (Sayanachatti Ð SAY, Barkot Ð BAR, Dunda Ð Dun, Mahidanda Ð MAH, and
Bhatwari Ð BHA) and the surface geology are also shown.
Fig. 8. Isovelocity contours at 2 km depth. The numbers along each contour
indicate the corresponding estimated P-wave velocity value there. The dots
denote the locations of earthquake epicentres with focal depths between 2
and 4 km. The seismograph station locations are also shown with open
triangles.
considerably. Also the grid nodes around the projected
position of Dunda encompassing approximately
(10 £ 10) km 2 area of the model space, have distinctly
low P-wave velocity values as compared to the surrounding nodes. This implies that a zone of signi®cantly low Pwave velocity, at least 4 km thick, may exist in the
subsurface below and around Dunda.
The low velocity zone is evident in Fig. 4b, the N±S
oriented projection also. Further this ®gure indicates that
the zone is localized in the southern part of the array. We
are of the opinion that this lateral limit is real and is not a
computational manifestation of poor seismic ray coverage.
For more than 75% of the 138 earthquake hypocentres with
(i) revised locations well within the array, between 0 and
6 km depth and (ii) travel time residuals less than 0.2 s, are
situated outside and north of the low velocity zone (Fig. 4b).
However, our results cannot provide any reliable lateral
limit for the southern end of this zone. Another interesting
feature evident in Fig. 4b is that earthquakes in the 0±2 km
depth range are generally located a little (less than 3 km)
south of those in the 2±4 km depth range indicating a north
dipping zone of earthquake distribution.
The isovelocity contours in Figs. 7±9 also provide insight
into the pattern of lateral variations in P-wave velocity at
shallow depths. Fig. 7 shows that near the ground surface
the estimated P-wave values at grid nodes close to the
projected position of Dunda are comparatively lower than
those at the surroundings nodes; in contrast, nodes near the
projected position of Sayanachatti have values comparably
higher than the surrounding nodes. The pattern of change of
the estimated velocity values along adjacent contours
reaf®rms the patterns in the constant velocity layers around
Dunda and Sayanachatti as was evident in the projections
along the E±W oriented sections (Figs. 4a, 5 and 6). The
difference between the velocity values in these two regions
is more than the standard errors of estimation. Fig. 8
exhibits similar variation patterns at 2 km depth although
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
migmatites and gneisses are present. The Higher Himalayan
crystalline rocks are separated from the metamorphosed
sedimentary Lesser Himalayan rocks here by the northerly
dipping MCT and its splays, the Bhatwari Thrust and
Munsiari Thrust. Although the exact ground location of
MCT is liable to varied interpretation, it is reported to
be evident at outcrop about a kilometer southwest of
Sayanachatti.
We infer the following from the variations in P-wave
velocity values of rocks lying in the shallow subsurface
below the array spread
Fig. 9. Isovelocity contours at 4 km depth. The numbers along each contour
indicate the corresponding estimated P-wave velocity value there. The
seismograph station locations are also shown with open triangles.
the differences between the velocity values surrounding
Dunda and Sayanachatti are reduced and appear less significant when compared with the standard errors of estimation.
At 4 km depth (Fig. 9) the contours are diffuse and suggest
further minor variation of P-wave velocity around Dunda
and Sayanachatti. In regions beyond 4 km depth, data resolution is poor. However Figs. 7, 8 and 9 provide evidence
that at least down to 4 km depth, the region around Sayanachatti has signi®cantly higher velocity as compared to
that around Dunda.
The areas of lower and higher velocity at the shallowest
level (Fig. 7) display a NW±SE trend with Sayanachatti and
Bhatwari forming two end points of the high velocity zone
and Dunda forming a single center of low velocity. The
epicenters are generally located in the transition zone
between these two zones. In the next deeper level of 2 km
(Fig. 7) the lowest velocity value center shifts northward
from Dunda, while the higher velocity zone now shows a
single high point between Sayanachatti and Bhatwari. The
transition zone shows a much more rapid rate of change of
velocity compared with the shallower level. The epicentres
occur slightly to the north of the zone of transition at this
level. At the 4 km depth level, the lower velocity zone
becomes narrower and better de®ned while the high velocity
zone also becomes a little bit tighter.
3.5. Interpretations of the variations in velocity
The six stations of the seismic array, whose recorded data
is considered here, straddles an area where rock units are
distinct and bounded by tectonic features identi®ed from
surface geology (Fig. 1). Dunda, Mahidanda and Lambgaon
lie in the inner Lesser Himalaya where Proterozoic quartzite, slate and limestone are identi®ed; Barkot lies in the
middle Lesser Himalaya amidst predominantly argillocalcareous metasediments. The south dipping North Almora
Thrust has been geologically identi®ed to separate these two
belts of Lesser Himalaya. Sayanachatti and Bhatwari are
situated in the Higher Himalaya where essentially schist,
1. At shallow depths generally the rocks are horizontally
strati®ed in thin sub-parallel layers.
2. The upper crustal rocks of inner Lesser Himalaya near
Dunda and Mahidanda have signi®cantly lower P-wave
velocity. In contrast, the upper crustal rocks of Higher
Himalaya near Sayanachatti have comparatively higher
P-wave velocity.
3. The higher velocity zone coincides with the surface
exposure of crystalline formations of the Higher Himalaya, which at its southern margin are demarcated by the
MCT (Fig. 4b). Estimated P-wave velocities in such
rocks are generally (5.3 ^ 0.3) km/s (see, for example,
Birch, 1942). In the exposed Proterozoic formations
between Dunda and MCT and the under-thrusted
argillo-calcareous metasediments of the Middle-Lesser
Himalaya, exposed SW of Dunda, estimated P-wave
velocities are generally (4.3 ^ 0.4) km/s (see, for example, Birch, 1942). It is suggested that the lower velocity
zone is located within these formations.
4. The majority of the small earthquakes occurred in the
higher velocity material and very few occurred in the
lower velocity material at these depths. Thus the small
earthquake activity in the region is mostly con®ned to the
higher velocity material, near Sayanachatti.
5. Because the surface trace of the MCT is identi®ed to be
very near Sayanachatti we consider this small earthquake
activity to be indicative of an active MCT zone.
4. First motion data
We winnowed the total P-wave polarity data from the 138
earthquakes considered for the arrival time inversion
program and selected only those readings, which were
most reliable and de®nitely unambiguous. Ninety-eight
polarity readings from 37 earthquakes were thus available
for this study. Of these hypocentres, 32 had revised locations within the higher P-wave velocity rock material identi®ed above and contributed 89 of the polarity readings.
4.1. Fault plane solutions
We projected all polarity data in a composite plot using
an equal area upper hemisphere projection (Fig. 10). The
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I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
the Yamuna-Bhagirathi river valleys must imply that this
region is tectonically different. However ®eld visits and
satellite imagery do not show any special physiography to
support such a view.
Hence we infer that the small earthquake activity, identi®ed in the higher P-wave velocity zone, amidst the upper
crustal rocks of Garhwal Higher Himalaya, is a result of
active deformation due to reverse faulting along steep
north-easterly dipping planes.
5. Discussion
Fig. 10. Composite fault plane solution using equal area upper hemisphere
projection. The open circles denote dilatations while the closed circles
denote compressions. The solid curves identify the nodal planes of the
preferred reverse thrust solution (see text for details).
projected data are distributed non-uniformly over the
projected focal sphere with more than 40% occurring near
the periphery. This is because the earthquake hypocentres
occur in a comparatively small volume of rock and are
mostly located directly below some recording station. The
nodal planes are poorly constrained and yield two equally
possible solutions. These are shown in Table 2.
Our preference is for the reverse thrust solution (solution
2) with the nodal plane NP1 as the fault plane. We give the
following reasons for our selection. Two composite fault
plane solutions for small upper crustal earthquakes, locally
recorded during December 1979 to June 1990 and located in
two speci®c regions between the Tons-Yamuna and Bhagirathi-Alkananda river valleys are available in literature
(Khattri et al., 1989; Sarkar et al., 1993). The two regions
¯ank the hypocentral zone of the present 37 earthquakes
from either side and are located close to it. Both composite
solutions are well-constrained and exhibit thrust mechanisms. It has been argued elsewhere (Sarkar et al., 1993)
that upper crustal earthquakes in the Garhwal Himalaya
generally occur due to thrusting. A strike slip fault model
for earthquakes occurring in the intervening area between
The data relate to small earthquakes whose hypocentres
were generally well distributed within a 170 £ 30 £ 20 km 3
volume of upper crustal Garhwal Himalayan rocks, extending from west of the Tons river valley to the Alaknanda river
valley and to epicentres distributed on either sides of the
MCT (Gaur et al., 1985; Khattri et al., 1989; Sarkar et al.,
1993). It may thus appear surprising that the data we used
for this study, the highest quality subset of the original data
set, relate only to those earthquakes which occurred in and
around the Yamuna-Bhagirathi river valley. This is possibly
because (i) in the entire eleven year period of ®eld recordings, the portable arrays operating in and around the
Yamuna-Bhagirathi river valleys were densest and most
well distributed in azimuth and (ii) in terms of epicentral
distances, these arrays were ideally suited for more reliable
P-wave arrival time recordings.
Our analysis has discovered the presence of a shallow
zone of low P-wave velocity and a low level of small earthquake seismicity, latitudinally con®ned in the Lesser Himalaya, to the Yamuna-Bhagirathi river valley region. Our data
do not reveal its longitudinal boundaries. However, we are
of the opinion that since the analysis pertains to earthquakes
which belong to the major continuous small earthquake
cluster (Gaur et al., 1985; Khattri et al., 1989; Sarkar et
al., 1993), it is most probable that this low velocity zone
actually extends in lateral directions throughout the entire
Lesser Garhwal Himalaya, from the Tons river valley to the
Alaknanda river valley. Further, due to constraints on the
resolution of data because of uneven distribution of the focal
depths, we are unable to provide estimates for the maximum
depth of this zone. Results of two systematic magnetic
surveys in the Ganga-Yamuna river valley have been interpreted to identify a narrow highly conductive zone in Garhwal Lesser Himalaya, in the same localized region as this
low P velocity zone but at 15±20 km depth (Arora and
Table 2
NP1
NP2
Solution
Strike
Dip
Slip
Strike
Dip
Slip
1
2
N 1728
N 1338
808 to the east
498 in N438 direction
88
888
N 2648
N 1238
888 to the south
428 in N 2148 direction
728
848
162
I. Sarkar et al. / Journal of Asian Earth Sciences 19 (2001) 155±163
Reddy, 1995). Due to the present status of available ®eld
data, no reliable correlation between these two distinct
zones Ð one of low P-wave velocity and the other of
high conductivity Ð can be attempted. Further local earthquake investigations, audio magneto-telluric surveys and
geothermal investigations should prove useful in resolving
this issue.
Our study has also indicated that the upper crustal rocks
in the Yamuna-Bhagirathi river valley, lying close to the
MCT zone and to its immediate north, have higher seismic
velocities and show a high level of small earthquake seismicity. The composite fault plane solution imply that these
small earthquakes indicate thrust motion on high angle slip
surfaces. The occurrence of these earthquakes, a part of the
major small earthquake cluster (Gaur et al., 1985; Khattri et
al., 1989; Sarkar et al., 1993), is not con®ned to the area but
rather a part of the ongoing crustal deformation in the entire
Garhwal Higher Himalaya. We suggest that a crustal shear
zone with a card deck mechanism of simple shear exist in
the interface zone between Higher and Lesser Garhwal
Himalaya. Within this zone there are numerous parallel
slip surfaces, dipping steeply northeastwards, across which
reverse dip slip motion occurs causing small earthquakes. It
has already been established that the Higher Himalaya and
Lesser Himalaya are both rising but the Higher Himalaya
rises more relative to the Lesser Himalaya. This uplift of the
Higher Himalaya has been linked to the convergence of the
Indian and Eurasian plates (e.g. Seeber and Gornitz, 1983).
We suggest that this uplift is associated with the small earthquake activity within this upper crustal ramp zone.
The observed anomalous low velocity structure may be
caused by a variety of geological situations. For example
such a variation can result due to (i) a lateral change of
facies, from a higher carbonate content facies to a higher
arenaceous content facies, from south to north, (ii) a
temperature elevation caused by a deeper seated magma
intrusion or (iii) thrusting in a ramp thrust environment.
Although presently we have no de®nite evidence, because
of the general tectonic style of the region this later model is
considered to be quite plausible. However, V. Raiverman
(Oil and Natural Gas Commission, India) is of the opinion
that the low velocity is possibly related to elevated temperatures at depth in the region (personal communication).
The presence of lower velocity rocks in the Garhwal
Lesser Himalaya may have direct bearing on the possible
natural hazards of the region. For higher incidences of landslide hazards are generally to be expected within such rock
materials. Landslide susceptibility zoning in different river
catchment areas of Garhwal Himalaya, using geological and
geomorphological data, have indicated a high probability of
major mass movement activity in Garhwal Lesser Himalaya
(Pachauri et al., 1998). It is pertinent to mention here a
systematic ground and satellite imagery survey conducted
immediately after the occurrence of the Chamoli earthquake
(mb 6.3) on March 29, 1999 in the Garhwal Higher Himalaya. The survey showed that although damage to buildings
and ground was maximum in regions near and around the
MCT zone, almost all major landslides, caused or reactivated by the earthquake, occurred in the Lesser Himalaya
(Sarkar and Saraf, 2000; Sarkar et al., 2000).
6. Conclusions
1. A shallow, low P-wave velocity zone, exhibiting minor
seismic activity, extending from the valley of Tons river
to Alaknanda river in Garhwal Lesser Himalaya has been
mapped. The presence of this low velocity material may
generally be associated with the zones of landslides in the
region.
2. A crustal shear zone in the interface zone between the
Higher and Lesser Garhwal Himalaya with considerably
high P-wave velocity and pronounced seismic activity is
identi®ed. In this zone, there are numerous parallel slip
surfaces, steeply dipping to the northeast, along which
reverse thrust motion occur to cause these small earthquakes. We propose that this activity is associated with
the uplift of the Higher Himalaya relative to the Lesser
Himalaya and is a consequence of the general plate
convergence process of the region.
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