Interpretation of observed ground level changes due to the 1905 Kangra earthquake, Northwest Himalaya

Tectonophysics,

289

149 (1988) 289-298

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Interpretation of observed ground level changes due to the 1905 Kangra earthquake, Northwest Himalaya RAMESH CHANDER Department (Received

of Earth Sciences, University of Roorkee, Roorkee (India)

June 12, 1987; revised version accepted

November

3,1987)

Abstract Chander,

R., 1988. Interpretation

Himalaya.

Tectonophysics,

The Kangra ground

earthquake

elevation

changes

of observed

Maximum

uplift of 14.3 cm was measured

are sufficient because

for us to estimate

at the bench

models proposed are not in accord Shahpur-Mandi smaller

km along

occurrence

extending

regarding

the bench

rather

on either

during

of the Kangra

mark

earthquake,

side of the levelling

lithospheric

rupture

buried

Northwest

the data

line near

(about

lo-15

This major

data

tectonic

of the levelling

in this study.

in a homogeneous

to area.

data having

In principle, Poisson

these

solid. But

small amounts

The observed

of uplift of rupture

elevation

changes along

can be explained

a second

Dun

if, in addition,

is assumed.

The data

to the general

of the levelling

demarcating

the geological

in the current

seismotectonics

the

can also be

strike of the Himalaya

line near Dehra

dip of about

rule out any role for the Main feature

meizoseismal

to check the reasonableness

(3) a northeasterly

to be unimportant

plate under

trending

coseismic

Saharanpur

by a single 100 km long rupture

Dehra

km) northwest

for which

line from

and also the relatively as a means

had (1) a length of 280 km parallel

the elevation

thus appears

Dun. Some details

was caused

to the strike of the Himalaya,

earthquake.

of the NW-SE

a new model independently.

However,

earthquake

The levelling

for interpretation

positions

earthquake

Dun, (2) a

5 O, and (4) a depth Boundary boundary

Thrust

between

of the region

of

in the the

and the

the mountains.

Introduction

A large part of the Northwest Himalaya and the adjoining Indo-Gangetic plains were severely shaken by the Kangra earthquake of April 4,1905. This is one of the four major earthquakes that have occurred in different parts of the Himalaya since 1897. But even the most recent of these in 1950 occurred before the modem era in instrumental seismology began with the deployment of the Press-Ewing seismographs. Any quantitative information about the source processes of these 0040-1951/88/%03.50

extremity

to use the data

Himalaya.

the earthquake

its SW edge. Finally,

of the Indian

levelling.

mark in Dehra

than to propose

up to a short distance

and the lesser Himalaya

subduction

was the first Indian

precision

of a rectangular

we have preferred

of the Northwest

of 80 km perpendicular

lo-15

at a bench

with the view that the Kangra

Kangra

Himalaya

repeat

near the southeastern

eight parameters

on other grounds

if the rupture

from around

outer

marks,

sector

rupture

explained

due to the 1905 Kangra

data for eight bench marks were considered

of some uncertainties

measured

breadth

Dun was situated

through

via Dehra

recently,

level changes

of 1905 in the Northwest were measured

Mussoorie

been published

ground

149: 289-298.

0 1988 Elsevier Science Publishers

B.V.

earthquakes that can be gleaned from the extant observations is a bonus for modelling the causes and effects of future major earthquakes in the Himalayan region. The Kangra earthquake was assigned a magnitude of 8.6 by Richter (1958). Middlemiss (1910) identified two separate zones of high intensity (Fig. l), with an intervening region of lower intensity. The Himalayan foothill towns of Kangra and Dehra Dun were in the northwest and southeast high intensity zones respectively. Both the zones overlapped parts of the outer and lesser Himalaya

_“__.._

?_?

.HPUR ANGRA

INDIA I /

\ 1

/

/’ _ L.tlIM.

1

\

f ‘,’

/O

, 1DOkm

I-

j

p*O.HIM.

/

\

GANGAR, YAMUNA R.

IN.GA.PL.

1

Fig. 1. General setting of the region under consideration. The right-hand figure is based partly on fig, 46 of Middlemiss (191O).The double line represents the levelling line schematically. The dashed lines enclose the regions of high intensity according to Middiemiss. The dotted fine denotes the SW limit of the Himalaya that is marked by a series of thrusts. The abbreviations used are:

R-river,

MBT-Main

Boundary

Thrust, MCT-Main

Central Thrust, LHIM. -lesser

laya, 0. HIM. -outer

Himalaya, IN. GA. PL. -Indo-Gangetic

eastward via Dehra Dun in the Himalayan foothills to Mussoorie, on a frontal range of the lesser Himalaya (Figs. 1 and 2). The line had been previously levelled between Saharanpur and Dehra Dun in 1861-62 and between Dehra Dun and Mussoorie in 1903-04. A summary of elevation changes (short for coseismic ground elevation changes) deduced from these observations was reported by Middlemiss (1910). Recently, Rajal et al. (1986) have given more details about the levelling observations. This has afforded us an opportunity to interpret the elevation change observations and infer some facts about the causative rupture of the Kangra earthquake. Th45Ol.y

Hima-

plains.

and thus straddled the Main Boundary Thrust (Figs. 1 and 2). The Kangra zone had a larger area and higher intensities than the Dehra Dun zone. From our standpoint, an interesting aspect of the inv~tigations following the earthquake was the relevelling, during 1905-1907, of a line from Saharanpur on the Indo-Gangetic plains north-

t+(x,,

Fig. 2. Schematic cross-section along the Saharanpur-Dehra Dun line of Fig. 1. The topography above the line 00

We interpret the elevation change data using a formula derived by Mansinha and Smylie (1971, p. 1437) to predict the long time response of a Poisson solid half-space to dip-slip displacement U along a finite rectangular shaped rupture on a buried fault plane dipping at an angle 8 (Fig. 3). Let our 0123 coordinate system have its 1 axis along the intersection of the fault plane and the free surface (short for free surface of the halfspace), its 2 axis in the free surface along the true dip direction, and its 3 axis normal to the free surface and pointing into the half-space. Let 5 be the perpendicular distance of a general point in the fault plane from the 1 axis. A point (5i, 5) in the fault plane has coordinates ($i, 5 cos 8, [ sin 6) in the 0123 system. Let the rectangular rupture in the fault plane be delineated by -L G t16L and d
is

x2,0> =f(L, -f(-L,

drawn notionally. The lower line marks the top of the subduct-

where

ing Indian plate and the detachment surface of Seeber and

fG7 0

Armbruster (1981) in cross section. The left portion of this line shown dashed is drawn to correspond schematically to the basement of Raiverman et al. (1983). The middle solid part represents the ruptured portion of the detachment according to Model 3. The crosses mark the position of the belt of intermediate magnitude thrust earthquakes (iVi and Barazangi, 1984). The vertical scale on the rigbt applies betow the line 00

d)

D) +f(-J,

=(Usin8/2~)[-x,t;sine/R(R+x,-E,) -tan-‘((x,

-[,)(x,-5

cos 8)

/(~+~sine)(~+~)~ +i.5 tan-i((x,

only. The dotted line indicates the span of the levelling line segment considered here.

D) -f(L,

/RX, sin e}]

- 5i j(+

cos e-s>

d)

(1)

291

h2 =X;

+ t2 -

2x25

COS 8

plane

free surface

of the half-space

above theory applies.

and:

simulate

elevation

to which the

Hence we did not attempt

changes

at these bench

to

marks,

R2 = (x1 - .$,)’ + h2

all of which are to the north of the Main Boundary

The formula

Thrust

(1) agrees with that given by Dragoni

et al. (1985). In order necessary rupture

to use eqn.

(1) to predict

to assume values of rupture breadth

strike azimuth dinates

(Figs. 1 and 2).

The conditions

(D - d),

slip (U),

of the rupture

of some specific point

plane,

u3, it is

ground

depart

length (2L),

the remaining

dip (0)

graphical

and

and the coor-

on the rupture.

bench

Rajal bench

Rajal et al. (1986) reported elevation changes due to the Kangra earthquake at 17 bench marks on the Saharanpur-Dehra Dun-Mussoorie level-

to the flatness

of the

marks

effect is definitely

also, but much

at

the topo-

less serious

in

their cases than it was for the above seven. et al. (1986) listed only distances

marks along the levelling

(also unspecified)

Data on elevation changes

in regard

from those of the above theory

of the

line from its start

in Saharanpur.

We used topo-

graphical maps to estimate the coordinates of the bench marks as required in the above theory. We were aided by the fact that the positions of two

ling line. Seven of these bench marks (Rajal et al.‘s

bench marks could be determined unambiguously. Positions of six other bench marks could then be estimated to perhaps within a kilometre. We felt

serial numbers soorie highway

that the positions of two other bench marks (Rajal et al.3 serial numbers 7 and 10) could not be

18-24) on the Dehra Dun-Musand in Mussoorie lie on the side of

a steep mountain range, which becomes increasingly knife-edge like with elevation above sea level.

estimated to this approximation, data were not utilized also.

The topographical situation in the case of these bench marks is very different from the infinite

eight bench

TABLE

Rajal

et al.‘s (1986) elevation marks

and hence change

whose locations

data

could

their for

be ob-

1

Coseismic elevation change data * ID

Sample no.

Bench mark description

(RajaI et al.,

Distance

Coseismic

(km)

elevation

1986) SP

1

MD

3

Remark ’

change (cm) GTS Saharanpur BM, 1904

0.00

0.0

a.1.

44.05

10.3

r.1.

Stone bench mark; Mohabawala

60.17

11.4

r.1.

GTS iron plug; GB office,

71.26

13.7

a.1.

Stone bench mark; Dehra Dun Saharanpur Road, Mohand

Mw

6

IP

9

Dehra Dun Rl

13

Cut on stone on Rajpur Road

72.81

14.3

a.1.

R2

14

Cut on stone on Rajpur Road

78.33

12.3

a.1.

RP

15

Cut on stone at Rajpur

79.19

11.5

a.1.

RB

16

Cut on plinth of a house

81.87

10.2

a.1.

at Rajpur Bazar * First four entries in column 5: level c. 1905 to 1907-level c. 1861 to 1862. Last four entries in column 5: level c. 1905 to 1907-level c. 1903 to 1904. GTS = Grand triangulation survey. ’ a.1. = approx. location, r.1. = reliable location

Fig. 3. A schematic embedded

figure

to represent

in a solid half-space.

only a part of the half-space plane as well as the rupture

a rectangular

The dashed

rupture

lines indicate

that

is shown. The ,$ axis is in the 23 plane.

tamed thus are reproduced in Table 1. The points are spread unevenly over a distance of 82 route kilometres. They lie more or less along a straight line normal to the general NW-SE strike of the Himalaya. Bench marks RP and RB are close to the outcrop of the Main Boundary Thrust.

LO

Fig.

4. Comparison

changes.

Uplift

horizontal 29 O55’N deviation

of

observed

is shown

positive

axis is a SW-NE and

77 “30/E.

values.

TABLE

2

Rupture

models

Rupture

length 2 L

Rupture

breadth

D-d

Slip on rupture

U

Dip of rupture

9

Sense of dip-slip

movement

on rupture

Depth of rupture SW edge NE edge Rupture

location

position

calculated

elevation

on the vertical

(left-right)

line

axis. The

with

estimated position

line terminates

Solid triangles,

calculated

values

for

represents

the calculated

(dyn cm)

for of

observed

solid circles, and open circles represent Models

1, 2 and

3. The

dotted

line

results for Model 4.

square error erms was defined as: e rms=

$(

l/-7 1

o b served - calculated) elevation change at i th bench mark} */8

Model 1

Model 2

Model 3

Model 4

1OOkm

50 km

280 km

280 km

84 km

80 km

80 km

30 km

3m

0.42 m

5m

3m

5O

5O

5O

so

thrust

thrust

thrust

thrust

15 km

10 km

10 km

15 km

22.2 km

17 km

17 km

17.5 km

24kmNE

3kmNE

4kmSW

1 kmNE

223 km NW 1028

25kmSE 6.7 x 1026

15.3 km NW 4.5 x 1028

11.7 km NW

of bench mark IP in Dehra Dun:

SE edge Seismic moment

at

is within

just short

of the thick solid line represent

relative to estimated

SW edge

zero

NW-SE

of a bench mark from this line is 4 km. However,

seven out of eight bench marks the estimated the MBT. Vertices

A straightforward trial-and-error procedure was adopted for the interpretation of observations for two reasons. Firstly, we had only eight observations of elevation changes and there are eight unknowns in eqn. (1). Secondly, the need to estimate bench mark positions from topographical maps, using road distances, added an element of uncertainty in the observations. Still, a root mean

and

The maximum

2 km of the line. The horizontal

R6W4ltS

~3’

WST km

1028

293

TABLE

3

Comparison

of observed

and computed

Bench

Observed

mark

change

ID

(cm)

SP

0.0

MD

10.3

elevation

changes

Calculated

*

change

for

model 1

model 2

model 3

model 4

(cm)

(cm)

(cm)

(cm)

-0.1

0.2

1.5

1.0

(5.9)

-0.2

1.4

6.1

5.3

Mw

11.4 (11.4)

-0.2

5.1

12.2

11.2

IP

13.7 (11.6)

-0.2

12.8

12.6

12.8

Rl

14.3

-0.2

14.3

14.4

14.4

R2

12.3

-0.2

12.2

13.9

12.0

RP

11.5

-0.2

11.5

13.9

11.5

RB

10.2

-0.2

10.7

13.6

10.4

erms

11.4

3.9

2.3

1.8

erms (corrected)

10.7

2.8

1.7

0.6

* Positive readings

and negative in parentheses

values in first eight rows indicate and e_

ground

uplift

and subsidence

respectively.

See text for explanation

of

(corrected).

to maintain a measure of objectivity in choosing between models. Parameters of four rupture models, out of more than 50 considered, are given in Table 2. Computed elevation changes for the eight bench marks are given in Table 3. They are shown graphically in Fig. 4. erms, calculated with respect to the observed elevation changes of Table 1, are given in the last but one row of Table 3. The four rupture models are compared in a map view in Fig. 5.

graphical features encountered as one proceeds from the Indo-Gangetic plains towards the Himalaya along a line normal to the strike of the mountains. The data are given in Table 4 and illustrated in Fig. 2. Raiverman et al. (1983) gave a geological cross-section roughly along the route of the levelling line. It shows that the Tertiary and Recent terrigenous sediments rest on an undulating pre-Tertiary basement, the depth of which between Mohand (Fig. 2) and a point near Dehra Dun increases monotonically, reaching a maximum value of about 10 km below the ground surface.

Before discussing the above results we provide a resume of the pertinent geological and topo-

Assessment

0

,

50 t

km

Fig. 5. Comparison numbers

correspond

represent

the bench

.

of rupture to model

models

in a map

numbers.

mark positions,

The

dots

SP

view. The at right

three of which are identi-

fied by the ID letters of Table 1, column

1.

of levelling data

Some doubts have been raised about the reliability of elevation change data, the interpretation of which is the aim of this paper. According to Seeber and Armbruster (1981), “The measured change in elevation (- 13 cm) is above random noise but close to conceivable error from poorly calibrated rods.” Middlemiss (1910) included a summary of elevation change data with the following declamations: “The following details kindly furnished me by Lt. Col. S. Burrard.. . The amounts of elevation being so slight (only a matter of a few inches) in such a long and steep course, the statements are given with all reserve.. . It may be mentioned that the greatest care was taken in

levelling

over such a long and mountainous

the reliability

them with a standard being

tract,

of the staves and the comparison

duly

of

steel unit kept at Dehra Dun

attended

to. It is also necessary

emphasize

the fact that in the last levelling

the whole

line

the results

agree

to

along

so closely

over

much of it.. .” Rajal et al. (1986) also argued the reliability Another

of the data. criticism

assumption

Saharanpur

did not undergo

that

the models

considered

change

change

the bench elevation

ing the pre- and post-earthquake For

mark

levelling

Dun.

by assuming

at

epochs.

by us, the predicted

at Saharanpur

1) in Dehra

committed

data

change dur-

is small

11% in the worst case) in comparison maximum change of 14.3 cm at bench (Table

Hence,

(about

with the mark Rl

while error was

no elevation

change

at

Saharanpur, the data may still be interpreted at least for order of magnitude estimates of rupture parameters.

proposed Kangra

three

first on some common

to the general strike of the Himalaya every case. This is in accord that P wave focal mechanism shallow

earthquakes dips

(e.g.

rupture

based

Mode1 1, with a rupture to the strike

levelling in which stricted

located

rupture between

for the

data.

Our

length of 100 km parallel

of the Himalaya

line, conforms

models

on intensity

of the (1987)

and a southeastern

223 km northwest

of the

to that mode1 of Molnar

is suggested Shahpur

to have

and Mandi

the NW zone of high intensity

been

(Fig.

identified

re1) in

by Mid-

dlemiss (1910). Calculated elevation changes for this model indicate that ground subsidence of a few millimetres would have taken place, whereas uplift of up to 14.3 cm was actually observed. Hence this rupture observations.

mode1 is inconsistent

with the

Mode1 2 represents a rupture of length 50 km parallel to the strike of the Himalaya and situated approximately symmetrically with respect to the line (Fig. 5). It was considered

following

Molnar’s (1987) suggestion that rupture during the Kangra earthquake may have been in two parts, features

of

all the four models. Dip-slip thrust motion on a fault dipping at 5 o along the NE direction normal

Himalayan

possible

earthquake

levelling Discussion of models We comment

such as “SW edge of rupture”

We turn next to some special features models listed in Table 2. Recently, Molnar

edge of rupture

of the elevation

is the tacit

elevation

for

use phrases

Fitch

is assumed

in

with (1) the result solutions for many

indicate 1970,

thrust

faulting

Chandra,

at

1978),

predominantly normal to the strike of the Himalaya; (2) the view (e.g. Valdiya, 1986) that major thrusts observed in the Himalaya flatten at depth; (3) the value of 3” adopted by Lyon-Caen and Molnar (1983) for dip of the underthrusting Indian plate to simulate gravity observations in the Himalaya. Also, a rigidity value of 4 x 10” Nm-2 was assumed following Kasahara (1981, p. 97). This quantity is not directly required in order to use eqn. (l), but seismic moment MO has become an important descriptor of the source of a large earthquake, and the rigidity value is needed in order to compute MO from rupture length, breadth and slip. As the ruptures considered are rectangular planes with northeasterly dips, it is convenient to

viz. a major rupture between Shahpur and Mandi in the northwest and a minor rupture around Dehra Dun in the southeast. As just pointed out the northwest rupture would have produced negligible ground subsidence around Dehra Dun and thus only the smaller rupture would have to be modelled from the levelling data. Model 2 is picked from a large suite. It can explain observed elevation changes at six out of eight bench marks very closely, the overall erms being 3.9 cm. The seismic moment of 6.7 x 10z6 dyn cm for the rupture of Model 2 is a small fraction of the probable seismic moment for the Kangra earthquake seen

that

Molnar’s

of about 10z8 dyn cm as a whole. It may be

two-rupture

mode1

is mod-

erately consistent with the observations, but the predicted slip on the rupture around Dehra Dun is only 42 cm. The rupture length is 280 km parallel to the general strike of the Himalaya in both Models 3 and 4. This length is the map distance between Kangra and Debra Dun, and hence is an approximate measure of the separation between northwestern and southeastern zones of high in-

295

tensity as well as the length of the meizoseismal area of the earthquake. The rupture breadth is 80 km normal to the strike of the Himalaya in Model 3. The length and breadth of rupture in this model conform to the corresponding figures in the last of the three models of Molnar (1987), as also to the rupture length deduced by Seeber and Armbruster (1981). With an assumed slip of 5 m on the fault, the seismic moment is 4.5 X lot8 dyn cm. The area (280 km x 80 km) and seismic moment value for this model are in accord with the observations for large interplate earthquakes plotted on Kanamori and Anderson’s curve quoted by Kasahara (1981, p_ 141). A stress drop of 30 bar is thus deduced for the Kangra earthquake. The calculated elevation changes agree with the observed ones, with an erms of 2.3 cm. Model 4 was actually deduced independently. It was picked from a suite in which every member yielded a seismic moment of 102* dyn cm, a value obtained using Brune’s (1968) curve for log-seismic moment versus magnitude, and a magnitude of 8 assigned to this earthquake by Kanamori (1977). The main differences between Models 3 and 4 are in regard to the breadth of rupture and its depth (see Table 2). Model 4 is superior to Model 3 in terms of erms. Fault area and seismic moment values for this model are also in accord with Kanamori and Anderson’s curve mentioned earlier. In short, the non-u~queness of rupture models deducible from observed elevation changes is manifest. Model 3, although only the second best from the standpoint of erms, is preferable because it has additional inputs from considerations of observed intensities due to the earthquake and general seismicity of the Himalaya (Molnar, 1987) as well as reports of landslides, foreshocks, etc. (Seeber and Armbruster, 1981). Effect of ~eotectonic e~euotio~changes Rajal et al. (1986) also reported the results of relevelling along the Saharanpur-Dehra DunMussoorie line during 1926-1928 and 1974-1975. Still based on the assumption that the bench mark at Saharanpur remained static, these authors made out a case for neotectonic uplift in the region at the rate of 1 mm/yr. Referee R.A. Shay has

questioned this assumption. The point is well taken, but the data to quantify the vertical movements of the Saharanpur bench mark are not available to us and probably do not exist. At the same time, this bench mark is located in the Indo-Gangetic plains at a distance of about 44 km from the southwestern boundary of the outer Himalaya. It is possible that the effect of the neotectonic uplift of the outer and lesser Himalaya may not be significant at this reference bench mark. It is seen from Table 3 that the maximum individual disagreement between the observed and calculated elevation changes is for bench mark MD at Mohand. We tried to ascribe the disagreement to neotectonic changes. As this bench mark was surveyed along with MW and IP (Table 1) in 1861-1862, corrections were attempted for all three of them. We took the observed post-seismic elevation changes occurring at these bench marks between the 1926-28 and 1974-75 levelling epochs (Rajal et al., 1986) as estimates of the neotectonic elevation changes during the 43 years preceding the earthquake. The revised estimates of coseismic elevation changes at bench marks MD, MW, and IP are shown in parentheses in column 2 of Table 3. erms after the corrections is listed in the last row of Table 3 and shows considerable improvement. The correction was not applied to the remaining bench marks because the pre-seismic survey was carried out only two years prior to the earthquake. The relative standing of Models l-4 is not disturbed by the correction as far as enns is concerned. Effect of sediments on observed elevation changes All the bench marks whose data were considered in this study were situated on a sediment cover of variable thickness over the pre-Tertiary basement. Some idea of the effect of sediments on elevation changes due to thrust faulting in the basement may be obtained from numerical and laboratory modelling carried out by Rodgers and Rizer (1981). The difference. in elevation changes when the sediment cover is absent or present can be small or dramatic, depending upon the position of the observation point relative to the buried

296

rupture.

It is our qualitative

on fig. 3 of Rodgers sediments

interpretation,

based

and Rizer, that the effect of

may be comparatively

small

at bench

marks RB, RP, R2, RI, IP, and MW because were favourably this regard,

placed

relative

they

to the rupture

and at MD and SP because

in

the sedi-

ment cover was thinner.

is the upper surface of the subducting and the lower surface sediments

and

2). The rupture be proposed regarded surface

Consequences

of the models

Models 3 and 4, and others with rupture by us, indicate

lengths

that observed

4 and

Fig.

that may

for the Kangra

earthquake,

may be

statements

about

the

the NW Himalaya.

2 and

detachment

Thus, the depth

in any of these models of the depth

Models of 280 km considered

Models

(Table and others

under

estimate

metasediments

plate

terrigenous

2-4,

as

of rupture

Indian

of the overlying

would

of the detachment

3 are attractive

depth

of the southwestern

would

agree closely

because

edge

be an surface.

a 10 km

of the rupture

with the depth

of the base-

elevation changes can be simulated in these cases only if the levelling line is not directly above the

ment near Dehra Dun in the geological cross-section of Raiverman et al. (1983): An 80 km across

postulated

strike extent

rupture,

i.e. if the southeastern

rupture

is some distance

levelling

line (Fig. 5). The distance

and

edge of

to the northwest

11.7 km in the case of the Models

respectively.

of the

is actually

The result may be expressed

15.3

northwest

of the Indian

Eurasian under

plate. the

According

outer

spasmodically

and

during

along a detachment

plate

to them,

lesser large

minor contradiction with the detachment model, in that the detachment would then be not at the sediment-basement boundary but at some depth within the basement. A possible resolution of the contradiction would be that the detachment may not be a razor-sharp surface but may be a few kilometres wide zone. The Main Boundary Thrust (MBT) and the

under

Himalaya

or decollement

the

subduction

earthquakes

occurs by slip

surface, which

TABLE 4 Some geological information regarding the region Geographical unit

Nature of rocks

Indo-Gangetic plains

Terrigenous sediments of

Outer Himalaya > Tertiary and Quatemary ages ____I ________ Mai,, Boundary -st (MBT) __ _______ -_Lesser Himalaya ____---_-_---Main

of the width of the detachment surface. is given to Model 4, then 15 km depth

altema-

of where it actually

lithospheric

becomes

of the

was. Seeber and Armbruster (1981) proposed a model for the occurrence of large Indian earthquakes within an overall scheme for the subduction

in these models

3 and 4

tively thus: if a single large rupture were to be responsible for the Kangra earthquake, then much larger elevation changes would have been observed if the levelling line had been situated a few tens of kilometres

an estimate If credence

of rupture

Unfossiliferous metasediments Central kst(MCT) -------------

High Himalaya

Crystallines and high grade meta-

Tethys Himalaya

Fossiliferous marine Paleozoics

morphics and Mesozoics \ _________-_______Indus SutureZone .__I_-. ----------

Main

southwestern

Central

Thrust

edge

(MCT)

of rupture

implies

are, on surface

a

geo-

logical evidence, major tectonic features of the Himalaya (Gansser, 1964). However, their roie in the neotectonics and the current seismotectonics of the region is a matter of debate. For example, according successive

to Le Fort (1975), MCT and MBT are tectonic boundary thrusts. MBT is the

current boundary of the continental convergence zone, while MCT is a less active or dormant feature. Seeber and Armbruster (1981) argued that MBT and MCT are contemporaneous and MCT is currently active. Ni and Barazangi (1984) stated that the MBT, together with nearby surface and blind thrusts, rather than the MCT, are the most active structures in the Himalayan arc currently. Gaur et al. (1985) also found evidence for the dormancy of MCT. Middle~ss (1910) while considering the cause of the Kangra earthquake, drew attention to the fact that the two zones of high intensity lay approximately along the MBT. HOWever, he attributed the earthquake to a sudden rupture among or below the folded outer

297

Himalayan stated

that

formations.

Krishnaswamy

coseismic

slip was not

MBT or any other thrust

et al. (1970) observed

in the region.

on

They also

observed

at

(1) The rupture the

which are southwest

Himalaya.

subparallel

to it (the thrusts

lie along

the dotted

line in the map of Fig. 1). The elevation listed in Table 1 constitute matter.

critical

evidence

changes on the

In all, there are three views to be sorted

out. The first is that slip during quake

occurred

imply,

through

the eight bench

only

the Kangra

on the MBT.

eqn. (1) substantial marks

where uplift

This

quake involved slip on the MBT and a deep-seated fault jointly. The uplifts observed at the eight bench marks can be explained if the slip on the deep-seated rupture was sufficiently large to counteract the subsidence engendered by concomitant slip on the MBT. But then, just north of the MBT, the uplift would have to be the sum of the individual uplifts due to these two faults. Although we excluded from consideration the observations from seven bench marks north of the MBT because of the influence of topography, the reported uplifts are so small (Rajal et al., 1986) that, in our opinthis view is negated

that the Kangra

earthquake

also. The third

view is

was due to slip on a

deep-seated rupture alone. The models discussed above are in accord with this view, which was also propounded

the rupture

by Seeber et al. (1981) on the basis of

the analogy between the uplifts observed for the Kangra earthquake and the Alaska earthquake of 1964. In short, the observed elevation changes rule out the involvement of MBT in the occurrence of the Kangra earthquake. We believe that these are strong arguments for the dormancy of even the MBT with regard to the current seismotectonics of the Himalaya and the subduction of the Indian plate under the mountains. Conclusions The following conclusions can be drawn from the above analyses of the elevation changes induced by the Kangra earthquake of 1905, and

the line

area. not confined

of

the

around

Dehra

Dun.

to

Northwest

changes can be explained

40 cm. Such a rupture rupture

Dehra

along levelling

with

The slip on

would have to be not much more than

the latter’s influence

served. Hence the view must be held to be untenable. The second view is that the Kangra earth-

ion,

a small rupture

would

has been ob-

sector

(2) The elevation

earthat

marks

was definitely

Shahpur-Mandi

larger

subsidence

bench

Dun-Mussoorie

at the SE edge of the meizoseismal

argued that the earthquake occurred due to slip on buried portions of Satlitta and Kalagarh thrusts, of the MBT and have strikes

eight

Saharanpur-Dehra

Dun

would

be subsidiary

in the Shahpur-Mandi on elevation

would

changes

be negligible

distance. (3) The elevation

change

to a

sector,

but

around

on account

of

data can also be ex-

plained by a rupture extending over a distance of the order of 280 km from the northwest to the southeast high intensity zone parallel to the general strike of the Himalaya. But the southeastern edge of rupture

would lie lo-15

km northwest

the levelling line near Dehra Dun. (4) The rupture causing the Kangra was concealed.

Its depth,

according

of

earthquake

to the models

obtained, would be about lo-15 km. (5) The extent of rupture to the northeast

of

about 80 km suggested by Molnar (1987) is not inconsistent with the elevation change data. (6) The available amenable

elevation

to simulation

change

data

being

with more than one rup-

ture model, we can conclude at least that Seeber and Armbruster’s (1981) detachment model for large Himalayan earthquakes is not inconsistent with the elevation changes observed. (7) It is improbable

that the MBT was involved

in the occurrence of the Kangra earthquake. This implies that this tectonic feature is no longer active in the seismotectonics of the Himalayan convergence zone in the broader framework of the global plate tectonics. Acknowledgements The author is indebted to Professors K.N. Khattri, P.S. Moharir and A.K. Jain of this Department for reading the manuscript critically and making perceptive comments. The help of Professor Jain in furnishing some literature and clarifying some points acknowledged.

of geology

is also

gratefully

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